Introduction

Metal matrix composites are usually comprised of a metallic matrix phase and at least one more constituent phase for reinforcing purpose. The reinforcement can be ceramics, carbon materials, or other metallic materials, in the form of particulates, platelets, whiskers, or fibers. For decades of years, MMCs have attracted considerable attention in aerospace, automotive, and other structural applications, as a result of their cost-effectiveness, enhanced mechanical and physical properties, as well as property-tailoring capability, compared to conventional monolithic metal alloys [1, 2]. The interest in MMCs began in the late 1950s, with limited development and applications in the aerospace field. In the 1960s, ceramic whiskers as discontinuous reinforcement in metals were studied for high-temperature applications in aircraft engines. Later in the 1970s and 1980s, automotive industries started to take MMCs into applications extensively [3]. In the past two decades, MMCs have experienced unprecedented growth thanks to the developments in materials and manufacturing technologies. Meanwhile, extensive research has been conducted to characterize the mechanical behaviors and corresponding mechanisms of MMCs that lead to significantly enhanced strength, stiffness, and weight saving. Various combinations of reinforcements and matrix materials have been reported in the literature, while the focus has been on aluminum-, copper-, iron-matrix, nickel- and titanium-matrix composites, and the most frequently adopted reinforcement materials include carbides, nitrides, oxides, and so forth [4]. Without any doubt, MMCs still possess enormous potentials for scientific research as well as commercial applications.

Traditional manufacturing is often based on multi-stage processes, with the initial stage dealing with the rough part creation and the subsequent stages for material removal operations. Creating a rough metallic part from the raw material and later removing most of its volume may not be cost-effective. Moreover, the various stages of manufacturing often take place in different locations, which creates logistics issues and incurs more energy consumption. On the other hand, additive manufacturing (AM) technologies are able to build fully functional parts in a single operation, with minimum waste of materials. AM technologies also empower design engineers with more freedom to create the complex geometrical entities without worrying about the manufacturability issue. The Wohlers Report 2019 [5] indicates that the revenue of metal AM continues a 5-year streak of more than 40% growth each year. It also forecasts that the revenue of entire AM market will climb to $23.9 billion in 2022 and $35.6 billion in 2024. Industry is becoming aware of the benefits of producing metal parts by additive manufacturing.

In the past three decades, laser-assisted AM techniques have drawn significant attention from both academia and industry. They are now regarded as increasingly important manufacturing methods with growing niche areas of applications. A typical laser-assisted AM technique employs a laser beam to manufacture a part by fusing metal powders in a layer-by-layer manner based on the sliced 3D digital data containing the part geometry. Due to the market strategies of various companies in the additive manufacturing field, there exist many different names used for very similar additive manufacturing processes. Based on the terminologies defined by ASTM Standard F2792 [6], the metal AM processing methods can mainly be classified into two categories. The first type is directed energy deposition (DED), in which focused thermal energy is used to fuse materials by melting as the materials are being deposited. DED bears several names, such as LENS (laser engineered net shaping) and LMD (laser metal deposition). The DED-type additive manufacturing owns the advantages of fast build rate, flexible powder composition, and capability of deposition on curved surfaces, and so on. The second type is powder bed fusion (PBF), in which thermal energy is used to selectively fuse powders in a powder bed. PBF includes several variants such as selective laser sintering (SLS), direct metal laser sintering (DMLS), and selective laser melting (SLM). The main advantages of PBF include high accuracy, surface quality, and often high mechanical properties. It is generally accepted that those advantages are attributed to small laser focus spot and the resulted fine microstructure and high resolution [7]. In this review, research works are organized according to the material classification of MMCs. In each group of MMCs, the laser-assisted AM techniques that have been adopted and performance enhancements that have been achieved are summarized and analyzed. Furthermore, the challenges and the future research directions on MMCs using laser-assisted AM are pointed out.

Aluminum-based metal matrix composites

Aluminum (Al) based materials are the second most used engineering material group after steels [8]. The low melting temperature of aluminum and its good bonding between a wide variety of reinforcements make Al-based alloys an ideal type of metal matrix material. Extensive studies on Al/Al alloy matrix-based composites have been reported in the literature, and the investigated reinforcement materials mainly include ceramic and carbon particles. Most of the existing studies on using laser-assisted AM to fabricate Al-based MMCs adopt the external second phase ceramic particles as reinforcements, which are economically viable. Generally, the external second phase particles bring certain strengthening effect but sacrifice in material ductility, due to dispersion difficulty, detrimental phase formation, and reduced powder flowability [9]. The investigation of these fundamental issues has been insufficient. On the other hand, the incorporation of carbon nanomaterials, such as carbon nanotube or graphene, can significantly increase the material strength due to their excellent load-transfer effect [10]. However, the effective preservation of the structural integrity of carbon materials is a major challenge as the carbon elements tend to react with the metal matrix to form carbide, especially in the laser-induced high-temperature molten metals [11].

Reinforced by SiC particles

Ghosh et al. [12] use the DMLS method to manufacture SiC-reinforced Al–4.5Cu–3Mg MMC. The bonding between SiC particulates and the matrix alloy is smooth and free of cracks, as shown in Fig. 1. Also, the addition of 300 mesh size particles brings the optimum density, while the addition of 1200 mesh size particles generates the optimum microhardness. Also, the wear characteristics of DMLS-processed Al-MMCs are investigated under various combinations of size and volume fraction of SiC particles. It is found that the wear resistance of synthesized materials drops when the size of reinforcement particles increases, and crack density due to thermal stress of contraction stress increases after the volume percentage of reinforcement reaches 15%. Simchi and Godlinski [13] show that the densification rate of Al–7Si–0.3Mg/SiC composite in the laser sintering process obeys first-order kinetics and is significantly affected by SiC fractions. Also, Al4SiC4 is formed as a result of reaction between the aluminum melt and reinforcement particles. Similarly, in the DMLS process of Al–7Si–0.3Mg, Manfredi et al. [14] adopt two different ceramic reinforcement particles, namely, 10 wt% SiC and 1 wt% nano-sized MgAl2O4. The results show that in the case of SiC-reinforced MMC, the well-dispersed SiC particles completely react with the matrix and form aluminum carbides, which lead to the hardness increase by 70%. However, in the case of nano-MgAl2O4-reinforced MMC, the hardness decreases by 11% owing to the high inhomogeneity of nano-reinforcement particles and high residual porosity. Another study also indicates that SiC particles react with AlSi10Mg matrix, and this leads to an in situ formation of micron-sized Al4SiC4 [15].

Figure 1
figure 1

Al–4.5Cu–3Mg composite reinforced by SiC in the SLS process: a machine setup schematic, b as-built composite, and c SEM image showing the SiC particles [12]

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Reinforced by TiC particles

In reinforcing AlSi10Mg with nano-TiC particles [16], the microhardness of reinforced AlSi10Mg can increase by up to 30% compared with the unreinforced AlSi10Mg with proper parameter settings of the SLM process. It is found that the TiC particles tend to aggregate in the interdendritic regions as shown in Fig. 2. When a higher LEPUL (laser energy per unit length) is used, the nanoparticles tend to form a ring-like structure because of the coarsening of dendrites. Meanwhile, some TiC particles react with the Al matrix to form Al9Si and Mg2Si, as shown by the XRD spectrum. In a study on SLM-produced nano-TiC particle-reinforced AlSi10Mg MMC [17], the addition of TiC reinforcement significantly lowers the coefficient of friction (COF) and wear rate. Balling phenomenon on the material surface and the generation of detrimental residual stress are believed to be associated with insufficient laser energy density. Laser deposition method has also been reported to produce TiC particles-reinforced aluminum matrix composite [18], and the wear performance of obtained composite shows a 10-factor improvement, i.e., the wear rate reduces from 7.6 × 10−3 to 0.88 × 10−3 mm3 m−1.

Figure 2
figure 2

TiC/AlSi10 Mg MMCs prepared by SLM: a–d microstructures under laser energy densities of 160 J mm−3, 200 J mm−3, 240 J mm−3, and 280 J mm−3, respectively, and e XRD spectra showing the constituent phases of SLM-processed Al-based nanocomposites [16]

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Reinforced by other types of ceramic particles

Other types of ceramic particles, such as Fe2O3, Al2O3, AlN, TiB2, and SiC, have also been used to reinforce Al-based MMCs. Dadbakhsh and Hao [19] discover that HIP (hot isostatic pressing) does not efficiently densify the SLM-processed Al/5–15 wt% Fe2O3 material due to the existence of thick oxide bands formed between solids during the SLM process. The hardness also drops after HIP processing due to the annealing effect on material microstructure. A follow-up study is performed to investigate the influence of laser power and scanning speeds on the mechanical properties of the same composite material [20]. The results indicate the laser power is positively related to the surface roughness, but the relation of material density to laser powder is dependent upon the weight percentage of reinforcement. The addition of 4 vol% Al2O3 into Al during SLM significantly improves the yield strength by 36.3% and the hardness by 17.5% [21], as well as the ductility by more than 100%, as shown in Fig. 3. Sercombe et al. [22] use the SLS technique to fabricate an aluminum alloy preform, which is subsequently debound and infiltrated with a second aluminum alloy. The partial transformation of aluminum powders into a percolating AlN structure by reaction with nitrogen is achieved to form a rigid skeleton in the SLS process. This provides the structural support during the infiltration stage. A similar process is attempted by Yu and Schaffer [23], in which an aluminum alloy preform is first prepared by SLS, and then nitrided to form Al-AlN structure. The composite preform is then pressurelessly infiltrated by AA 6061 to achieve 100% improvement in density. For an AlN/AlSi10Mg composite system, the decrease in LEPUL from 1800 to 450 J/m results in a homogenous distribution of AlN particles [24]. Gas atomization is implemented to produce nano-TiB2 decorated AlSi10Mg composite powder from a TiB2/AlSi10Mg master casting ingot. The SLM-produced nTiB2/AlSi10Mg exhibits superior strength and ductility that are higher than most conventionally fabricated wrought and tempered Al alloys [25]. In a SiC/AlSi10Mg composite fabricated by SLM [26], it is found that excessive filler content (over 10 wt%) leads to the prevalence of pores in the composite and triggers the formation of carbides.

Figure 3
figure 3

Tensile performance of Al–Al2O3 composite fabricated by SLM, a tensile curves and b fractured specimens. [21]

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Reinforced by carbon materials

Recently, carbon nanomaterials have also become an attractive option to reinforce aluminum and its alloys. The low processing temperature of aluminum helps preserve the structural integrity of carbon materials. For instance, carbon nanotubes (CNTs) have been used as reinforcement to strengthen the SLM-processed AlSi10Mg. With 1 wt% addition of CNTs, the hardness and the tensile strength increase by 10% and 20%, respectively [27]. As shown in Fig. 4, although some CNTs react with the AlSi10Mg matrix phase to generate aluminum carbides, many CNTs are found intact in the matrix and contribute to the high strength of the composite. The electrical resistivity is found to be significantly lower without the formation of common brittle Al4C3 phase [28]. Wang and Shi [29] apply SLM to synthesize graphene-reinforced AlSi10Mg composites. The strengthening effect of graphene is estimated to be over 60 MPa. However, high porosity is observed in the composite due to the adverse effect of graphene nanoplates on powder flowability. Table 1 summarizes the materials, techniques, and experimental parameters of additively manufactured Al-based MMCs.

Figure 4
figure 4

CNTs and reaction product in the SLM-produced AlSi10Mg. a, b TEM of CNTs in the test parts, wherein the yellow arrow indicates Al4C3 and the red indicates CNTs. c, d The high-resolution TEM images of CNTs in the test parts [27]

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Table 1 Literature on Al/Al alloy matrix-based composites fabricated by laser-assisted additive manufacturing
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Copper-based metal matrix composites

Thanks to excellent high-temperature mechanical properties and thermal conductivity, many copper alloy materials have been widely used for critical components serviced in high temperature environments. The high chemical stability of copper also makes it an ideal material for laser-assisted additive manufacturing. Copper-based matrix composites have been fabricated with various reinforcing materials. Tungsten and tungsten carbides are the major type of reinforcing materials to reinforce copper in AM processes, mainly because of good chemical stability between copper and tungsten. Thanks to the high hardness of tungsten, this type of composite generally exhibits significantly enhanced hardness and wear performance. However, inferior ductility is obtained because of the brittle interface between copper and tungsten formed during rapid cooling [32]. In this regard, effective engineering of the matrix/reinforcement interface could potentially lead to a major breakthrough. Additionally, a wide variety of copper matrix composites can be manufactured through post-infiltration using molten copper, due to the relatively low melting point of copper. This method brings better matrix/reinforcement interface quality due to controllable slow cooling, but it requires a green part to be built first and then go through costly post-processing steps. Thus, it is believed that the efficient green part design and the development of fast infiltration technology are necessary in the endeavors of further exploring the potential of copper-based composites.

Reinforced by tungsten/tungsten carbide

Pintsuk et al. [33] develop W/Cu functionally graded composite materials through laser metal deposition process and observe well-distributed W particles within Cu matrix. The product property is comparable to that obtained from another additive manufacturing method—plasma spraying. Gu and Shen [34] fabricate submicron WC–Co particulate-reinforced Cu matrix composite material by DMLS. Phase characterization of WC–Co particles indicates that reinforcement particles are partially dissolved and smoothened. Further research of the same material is carried out to understand the sintering and reinforcement mechanism during DMLS process [35], as well as the effects of DMLS process parameters and the weight fraction of WC–Co reinforcement. Interestingly, the influence of rare earth (RE) material addition on WC particle reinforce Cu composite by the DMLS process is also studied [36]. It is found that the addition of RE–Si–Fe significantly improves the mechanical properties of product, homogenizes the reinforcement particles by significantly reducing the surface tension of the melts in the pool and pining effect of liquid/solid-phase boundary. Similar findings are reported in the investigation of adding rare earth oxide La2O3 to DMLS-processed WC–Co-reinforced Cu material [37], as shown in Fig. 5.

Figure 5
figure 5

Addition of rear earth (RE) element effectively promotes the dispersion of WC powder in Cu matrix, a without RE addition and b with RE addition [37]

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Reinforced by nickel

Nickel is another efficient reinforcement for copper because of its good bonding wettability on copper. In this regard, nickel particles are usually mixed with the copper matrix particles to achieve strengthening effects. Agarwala et al. [38] produce bronze–nickel parts by SLS process, in which the raw martial used are bronze and nickel powder mixture. Ni particles are well preserved in the SLS green part. Liquid phase sintering as a post-processing is used on the SLS green part to increase the density and strength. The addition of Ni is found helpful to mitigate the high porosity issue in the composite material. Similarly, the DMLS process is employed to reinforce Cu matrix by Ni particles [39]. Full melting of matrix material and partial melting of reinforcing particles are the main sintering mechanism during the DMLS process. It is also discovered that the addition of phosphorus element acts as deoxidizer that limits the formation of copper oxide, and thus enhancing the liquid–solid wettability. In addition, Dürr et al. [40] fabricate EDM electrodes based on bronze–nickel powder mixture by means of SLS. The SLS EDM electrodes are infiltrated with silver, which leads to the improvement of wear performance by 30%.

Reinforced by in situ formation of ceramics

In situ formation of TiC is observed in the SLM process of Cu–Ti–C powder mixtures [41]. By adding Ni into the powder mixture, it is found that both microstructure and surface quality of SLM-produced product is improved. Similarly, in situ formation of TiB reinforcing particles is observed in the SLM process of Cu–Ti–B4C powder mixture [42]. In the SLM process, the in situ reaction results in the formation of TiB2 and non-stoichiometric TiC1−x particles in the Cu matrix.

Reinforced by carbon materials

A graphene-copper nanocomposite is fabricated by Hu et al. [43], in which the graphene is found to be intact in the copper matrix and the reinforced copper shows increases of 17.6% and 50% in average modulus and hardness, respectively. More recently, a feasibility study is carried out on additive manufacturing of metal-based cutting tool with incorporated diamond particles [44], in which Cu–Sn–Ti alloy powders with 10–20 vol% Ni-coated diamonds are SLM-processed. It is found that the majority of the diamonds can survive the SLM process and the diamond particles are well bonded to the matrix material.

Reinforced by infiltration materials

Cu/Cu alloy matrix-based composites can be obtained from a two-step approach, with the first step being laser-assisted additive manufacturing, and the second step being the post-processing to infiltrate a new type of material. In the study of Liu [45], the mixture of electrolytic Fe powder, deoxidized Cu powder, carbonyl Ni powder, graphite powder, and polymer binder powder is indirectly laser-sintered to obtain Fe–Cu–Ni–C alloy. After being infiltrated in molten Cu, the Cu matrix Fe–Cu–Ni–C alloy composite is obtained and the relative density of the product is significantly increased. Pressure infiltration is adopted to reduce the porosity of bronze–nickel injection molds fabricated by DMLS [46]. The infiltrant used is Sn60PbAg. It is discovered that residual porosity rises at higher infiltration temperatures because of the reduced capillary forces as well as the decreased infiltration depth, as shown in Fig. 6. Surface tension and pressure difference in the pores are found to be the most critical factors that impact infiltration performance. Meanwhile, due to its high viscosity and good wettability to metals, epoxy is adopted to post-treat the Cu-based alloy fabricated by laser-assisted additive manufacturing processes [47, 48]. Significant improvements on material hardness, surface roughness, and average density are obtained. For the materials, techniques, and experimental parameters adopted in the above-mentioned studies on Cu/Cu alloy composites, we refer readers to Table 2 for further information.

Figure 6
figure 6

Sn60PbAg used as infiltrate material for bronze–nickel alloy produced by DMLS [46]

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Table 2 Literature on Cu/Cu alloy matrix-based composites fabricated by laser-assisted additive manufacturing
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Iron-based metal matrix composites

Iron is the most used metal material in the industry due to its low cost, stable chemical properties, balanced mechanical properties, and high compatibility with a wide range of alloying elements. Compared with nonferrous metals, iron-based alloys are relatively easier to produce due to the improved interaction with laser beam, higher energy absorptivity, lower reflection, and comparatively lower thermal conductivity. Thus, various types of iron-based alloys, as well as the associated MMCs, have been prepared successfully using additive manufacturing methods. Incorporation of ceramic particles is the most common way to strengthen iron-based alloys, and most studies show improved mechanical properties. However, the existing studies generally focus on static mechanical properties, including tensile, compression, and hardness. Fatigue properties of iron-based MMCs are much less studied, which continues to create challenges for the application of such MMCs in the real load-bearing scenarios.

Reinforced by TiC particles

TiC particle is the mostly adopted reinforcement in producing iron-based matrix composite. Higher microhardness and enhanced modulus of elasticity are observed when TiC is added into FeAl alloy during the SLM process [49]. The results show that TiC-reinforced FeAl intermetallic matrix composite bulk parts present a relatively smooth and dense melted microstructure, giving rise to the improved wear performance. Emamian et al. [50] study the temperature/microstructure relation in the laser cladding processed Fe–TiC MMC. It is found that low cooling rate could lead to a less uniform distribution of TiC particles in the matrix, while high cooling rate may become the source of micro-crack formation across the deposited layer. Later, such process is studied by a simulation approach [51], and the cooling rate and peak temperature during the process are predicted. The morphology of TiC particles and material microstructure can be tailored by controlling cooling rate. Nano-TiC and nano-TiB2 particles can be employed to reinforce 316L stainless steel during the SLM process, and improvements on hardness and anti-oxidation property are achieved [52]. A numerical analysis model is developed and verified by the nanoindentation results, which is able to optimize the controlling parameters in the SLM process. Gåård et al. [53] fabricate TiC particle-reinforced Fe–Ni alloy by DMLS. The dissolution of TiC promotes the FCC to BCC transition of the matrix phase. When the TiC addition is lowered to 30%, the bending strength, hardness, and wear performance of the composite are found inferior to conventionally manufactured hard metals. A few simulation studies have been performed to study the manufacturing process of TiC-reinforced iron MMCs. In the work of Amano et al. [54], a thermodynamics- and fluid dynamics-based numerical model is established to investigate how the processing parameters would affect the structure of nickel-coated TiC-reinforced 316L steel prepared by the LENS process. Besides, Alimardani et al. [55] develop numeric model to study the effect of temperature on the Fe–TiC composite by laser cladding process, and the simulation results help to optimize the processing parameters. With the addition of 9 vol% nano-TiC particles and followed by LSP at optimized parameters, the laser-sintered material shows a hardness increase of 61% as compared with the laser-sintered pure iron.

Reinforced by SiC particles

Ramesh et al. [56] reinforce the iron matrix by nickel-coated SiC particles using DMLS and characterize the mechanical properties of the formed composites. The improvements in microhardness and abrasive wear resistance are observed in the composite material. A follow-up study [57] is performed to investigate the effect of SiC content and manufacturing parameters on the mechanical properties of a SiC/iron composite by DMLS. A negative correlation relation between the scan speed and hardness, and a positive correlation between the SiC content and coefficient of friction are observed. Similarly, Song et al. [9] employ the SLM process to synthesize Fe/SiC composite. It is discovered that the addition of SiC promotes the formation of martensite and pearlite in the matrix.

Reinforced by other ceramic materials

Other types of ceramic materials, such as TiN, WC, TiB2, have also been used to reinforce steels fabricated by laser-assisted AM processes. For instance, the fatigue performance of DMLS-produced iron matrix composite reinforced by nano-TiN particles is investigated by Lin et al. [58]. The addition of TiN nanoparticles helps to increase the dislocation density during LSP (laser shock peening) and pin the dislocation propagation, thus improving residual stress stability. WC particles are found either partially or fully melted in the steel matrix during SLM, but a much higher wear resistance can be achieved [59]. Cold spray technique and SLM are combined to produce WC-reinforced MS300 steel, and improved wear resistance is observed [60]. TiB2 particle is used to reinforce 316L stainless steel in the SLM process and the composite exhibits good combination of compression strength and ductility. It is found that the addition of 15 vol% reinforcement leads to a significantly lowered wear rate [61]. Moreover, calcium silicate (GaSiO3) is used as reinforcement in SLM of 316L stainless steel [62]. The obtained composite is classified as ductile material, and the fracture surface shows both ductile and brittle characteristics. The addition of GaSiO3 is found to improve corrosion resistance.

Reinforced by infiltration materials

Due to the large differences between iron and copper, as well as good bonding between them, copper is usually used as an infiltration material to make the AM-produced iron-based green parts fully dense. Dewidar et al. [63] investigate the processing parameters (e.g., laser power, scan spacing, and scan speed) on the mechanical properties (e.g., density, flexural strength, and flexural modulus) of high-speed steels by the SLS process. It is found that after infiltration with bronze, the flexural strength and modulus are significantly enhanced. In addition, Kumar and Kruth [64] investigate the infiltration of bronze particles into laser-sintered iron-based products and obtain a modest increase (around 5%) in the material density. Some studies also report using epoxy resin to infiltrate the iron green parts. In the work of Yan et al. [65], green parts are obtained by SLS of nylon-12 coated carbon steel powders and infiltrated by epoxy. Compared with the green parts, the infiltration yields improvements in material bending strength, tensile strength, and impact strength for more than 40 times, 7 times, and 2 times, respectively. In a similar fashion, Liu et al. [66] use the mixture of epoxy resin/iron powder as the starting material for SLS and infiltrate the green part by bronze after debinding process. It is observed that the pores are almost filled up in the infiltrated parts and significant increase in strength is also obtained.

Reinforced by carbon materials

There are a few studies on additive manufacturing of iron-based MMCs with carbon materials as reinforcements. The direct laser-sintered iron–graphite composite is found to consist of multiple phases, such as ferrite, martensite, carbide, and ledeburite [67]. Graphite plays an important role in densifying the final composite, because of the increased laser absorptance, the reduction of the melt pool surface tension, as well as the decrease in oxygen. A graphene oxide (GO)/iron MMC coating is fabricated using the SLS process [68]. The evaporation of solution agent promotes the vertical alignment of GOs in the matrix, as shown in Fig. 7. There is a coherent bonding between GOs and iron matrix, and the GOs are completely stretched due to directional vaporization of binder material. The application of such a GO/iron coating on stainless steel specimen improves the surface hardness and the fatigue performance (Table 3).

Figure 7
figure 7

Laser deposition of GO–Fe nanocomposite layer, a schematic of coating process, b schematic of laser sintering process, c microstructure of as-built layer, d EDS mapping of carbon, and (e) EDS mapping of iron [68]

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Table 3 Literature on Fe/Fe alloy matrix-based composites fabricated by laser-assisted additive manufacturing
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Nickel-based metal matrix composites

Nickel-based alloys have good high-temperature mechanical properties so that they are widely used for structural loading components in the high temperature environments of aerospace, energy, metallurgy industries. Additive manufacturing of Ni-based alloy has drawn much attention due to the decent laser absorption as well as the hard-to-machine nature of nickel-based alloys. Many studies have been carried out on fabricating nickel-based alloys through additive manufacturing methods. Due to similar physical properties between iron and nickel, reinforcement materials used in iron-based MMCs are generally suitable in nickel-based MMCs as well. Among them, TiC particle is probably the most adopted reinforcing material. The addition of ceramic particles during AM of nickel-based MMCs has been reported to effectively enhance various mechanical properties, such as hardness, strength, and wear performance. However, the major gap of existing efforts lies in the insufficient investigation about high-temperature mechanical properties, given that nickel-based superalloy is widely used for high-temperature critical components. Therefore, the fundamental research about the reinforcement/matrix interaction of AM-built nickel-based MMCs during high-temperature service carries significant scientific merit.

Reinforced by TiC particles

The strengthening effects of three most deployed ceramic particles, namely SiC, Al2O3, and TiC, are compared in the LMD-produced Inconel 625 matrix composites [69]. The most outstanding mechanical properties (high hardness at uncompromised material ductility and integrity) are brought by the addition of TiC particles. The addition of TiC particles is also reported to benefit the anti-oxidation performance [70] and to help refine the grain size in LMD-processed Inconel 625 [71]. Several works investigate the parameter optimization of TiC-reinforced Inconel 718 [72] [73]. It is found that the increase in laser power density results in a smoothed structure of reinforcement particle, but excessive laser power density may cause the coarsening of reinforcement and thus lead to weak interfacial layer. Nearly fully dense Inconel 718/TiC composite is obtained under optimized processing parameters in the SLM process [74]. TiC particles have also been used for the development of functionally gradient metal matrix composites (FGMMC) by LMD process [75]. The TiC particles exhibit a uniform and gradual distribution in the FGMMC, as shown in Fig. 8. Over 30 vol% of TiC leads to a columnar to equiaxed transition and enhanced wear performance. Moreover, Cui et al. [76] fabricate the Ni–Ti–C composite coating on cast iron substrate using laser cladding technique. The starting material is a mixture of nickel, titanium and graphite, and the in situ formation of TiC is characterized. The formation mechanism of TiC is determined as nucleation growth. The hardness and wear performance of the TiC-reinforced coating are significantly enhanced. Zheng et al. [77] test both uncoated and Ni-coated TiC particles to fabricated Inconel 625 composite using the LENS technology. The results indicate that Ni coating of TiC can effectively improve the manufacture process in terms of flowability of melt pool, the integrity of interface, and interaction between laser and TiC particles. Also, a higher density of dislocation is observed around reinforcing particles due to the large difference of CTE (coefficient of thermal expansion) between TiC and matrix. Wang et al. [78] perform heat treatment on nano-TiC-reinforced Inconel 718 made by SLM, and results show that nano-TiC particles play an important role in strengthening the material by suppressing microstructure coarsening during post-solution treatment.

Figure 8
figure 8

Micrograph of a TiC/In690 functional gradient material [75]

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Reinforced by other materials

Other than TiC, the use of reinforcement including WC, TiB2, and graphene to strengthen nickel-based alloy has also been reported. For instance, wear performance is usually the focus when WC particles are used as reinforcement. In the study of Rong et al. [79], a very low and stable COF (about 0.35) and wear rate (2.5 × 10−4 mm−3 N−1 M−1) are achieved by adding 25 wt% WC particles into Inconel 718. The gradient interface between WC particles and matrix plays an important role in improving the wear performance by hindering the propagation of cracks, as shown in Fig. 9. Similar WC particle-reinforced Inconel 718 is reported by Nguyen et al. [80]. It is found that WC particles have a hindering effect on the columnar growth of matrix grains and thus refine the microstructure. Moreover, a drop of strength will happen if the amount of WC reaches 15 wt%. TiB2 is used to reinforce Inconel 625 superalloy, and an in situ interfacial layer rich in Mo and Ti is formed between the reinforcement particle and matrix, which is believed to prevent crack formation [81]. Up to 1 wt% graphene nanoplatelets are added into Inconel 718 superalloy by using the SLM process [82, 83]. There is a significant improvement of strength and Young’s modulus but accompanied by a considerable drop in material ductility. Meanwhile, the wear performance significantly improves thanks to a self-lubricating effect of graphene nanoplatelets. Graphene nanoplatelets are found to suppress the microstructure from coarsening during post-solution treatment of graphene/Inconel 718 composite [84] (Table 4).

Figure 9
figure 9

Schematics showing how graded interface improves the wear performance a without a graded interface, b with graded interface [79]

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Table 4 Literature on Ni/Ni alloy matrix-based composites fabricated by laser-assisted additive manufacturing
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Titanium-based metal matrix composites

Titanium-based materials have wide advantages such as high strength-to-weight ratio, good high-temperature performance, and good biocompatibility. Therefore, they have been widely used in aerospace, biomedical, and automotive industries. Laser-assisted additive manufacturing of Ti-based MMCs has been extensively investigated in recent years. The strengthening effects of various types of reinforcement have been evaluated for the titanium-based MMC system, and TiC is again among the most adopted reinforcements. Unlike other metals, the choice of reinforcement materials for titanium-based MMCs is relatively limited, mainly due to the reactive nature of titanium. Titanium matrix can react with many external phases under high temperature and oxidation is highly likely to occur, promoting the formation of various carbide, oxide, and intermetallic phases, which are detrimental to the overall properties [85]. Thus, the trial-and-error cost of synthesizing titanium-based MMCs is high. Due to this reason, the use of thermo-dynamic calculation theories, such as calculation of phase diagram, is believed to expedite the design of titanium-based MMCs through AM.

Reinforced by TiC particles

TiC is extensively adopted as the reinforcing material in the processing of Ti alloy matrix composites via laser-assisted AM processes. Gu et al. [86] use mechanically milled Ti–Al–graphite powder mixture as the starting material to obtain TiC-reinforced Ti(Al) matrix composite through the SLM process. The major matrix phase of SLM-produced composite is TiAl3 and the minor phase is Ti3Al2, while TiC is in situ formed as the reinforcing phase. To obtain the in situ formed TiB and TiC, Zhang et al. [87] adopt laser deposition to process the powder mixture of Ti6Al4V titanium alloy and B4C. Microstructure observations show that the formed TiB appears to be needle like and prismatic, while TiC tends to be granular. However, the tensile property of the composite is limited due to the existence of some unreacted B4C. Gu et al. [88] focus on similar in situ formation process of TiC during the laser deposition, where Ti and SiC are used as starting material to form TiC-reinforced Ti5Si3 matrix composite. Laser energy density is varied to see its effect on product quality. Ni-coated TiC powder is directly mixed with Ti6Al4V powder, and the blended powder mixture is used as the starting material for the LMD process [89]. Melting and re-solidification of TiC particles in the matrix are observed during the LMD process. This results in fine microstructure and strong interface bonding between TiC particles and matrix, as well as improved tensile strength. Mahamood et al. [90] adopt the same process of manufacturing Ti6Al4V/TiC composite material. Laser scanning velocity is varied to study its effect on microstructure and mechanical behavior of the composite, and the optimal scanning speed is obtained. A similar work on studying the effect of laser scanning speed can be seen in Gu et al.’s study [91], in which TiC reinforcement particles are mixed with Ti particles for the SLM process. TiC particles are added in Ti to make composite coating by LMD process [92], and the wear performance of the TiC-reinforced composite coating is investigated. It is found that microhardness can be significantly improved by adding 40 vol% TiC particles, a 15-fold wear rate reduction is observed as well. Besides that micro-sized TiC particles are extensively adopted as reinforcement in laser-assisted AM, the use of nano-sized TiC particles is also addressed in the literature [93]. As shown in Fig. 10, the wear test indicates that the adhesive wear mechanism is dominant when filler content is below 17.5 wt%. As the content of nano-TiC exceeds this amount, the TiC particles lose nanostructure and become coarsened, leading to a decrease in wear performance.

Figure 10
figure 10

Morphologies of worn surfaces of nano-TiC-reinforced Ti MMCs at various content of reinforcement, a 7.5 wt%, b 12.5 wt%, c 17.5 wt%, and d 22.5 wt%. [93]

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High-temperature creep performance of 10.8 vol% TiC particulates-reinforced TA15 alloy fabricated by LMD is investigated at 873 K and 923 K by Liu et al. [94]. The microstructure of as-deposited MMC mainly exhibits near-equiaxed and coarsened dendritic morphology. It is shown that the TiC-reinforced composite exhibits superior creep resistance compared to the monolithic titanium alloy. TiC particles play an important role in increasing the creep rupture performance due to the load transfer from the matrix and refinement of the Widmanstätten matrix. The laser-deposited TiC/TA15 composite possesses superior creep performance as compared to a similar composite fabricated by the traditional method, as shown in Fig. 11. In the work of Zhang et al. [95], functional gradient Ti-matrix composite with 0–40 vol% TiC reinforcement is laser-deposited by varying the powder feed rate. It is found that the increase in TiC addition leads to the increase in the hardness and tensile strength of composite material but a decrease in the ductility. Meanwhile, the partially melt and solidified TiC particles act as the microstructure refiner. Based on the similar concept of functional gradient composite material, Ti6Al4V and Ti6Al4V/TiC dual material transition joints are manufactured using the LMD method [96]. Different designs of material transitions, such as butt joint, gradient joint, and interlocking joint, are fabricated by controlling the ratio of TiC to Ti6Al4V during the deposition process. The results indicate that the interlock design yields best tensile performance and all the joints survived in the three-point loading test. Shishkovsky et al. [97] produce the Ti–TiB2 gradient composite material and show that the microhardness increases as the content of TiB2 increases. The XRD spectrum suggests that although some peaks of TiB2 still exist, most TiB2 particles disappear after laser treatment, as shown in Fig. 12.

Figure 11
figure 11

Laser-deposited TiC/TA15 composite show superior creep performance as compared to similar composites fabricated by traditional methods [94]

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Figure 12
figure 12

Microhardness and phase constituents of a Ti–TiB2 functional gradient composite material produced by SLM [97]

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Reinforced by other materials

In the literature, it is reported that Ti or Ti-based alloys are also reinforced by particles other than TiC in laser-assisted AM processes. TiB-reinforced Ti matrix composites are synthesized through SLM process of Ti–TiB2 powder mixture [98, 99]. TiB is formed through the reaction between Ti and TiB2, and the in situ formed TiB appears to be needle shaped, and the microstructure of Ti matrix is significantly refined. The composites show superior mechanical properties compared with the material obtained by the powder metallurgy process. Farayibi et al. [100] investigate the corrosion resistance of WC-reinforced Ti6Al4V alloy fabricated by laser cladding. Thanks to the uniformly distributed reaction products such as nanoscale TiC and W, the composite with 76 wt% WC particles exhibits up to 13 times better erosion resistance compared with wrought Ti6Al4V, as shown in Fig. 13 (Table 5).

Figure 13
figure 13

Comparison of erosion rate between WC reinforced and Ti6Al4V and pure Ti6Al4V obtained under plain water jet (PWJ) pressure of a 275 MPa, b 345 MPa [100]

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Table 5 Literature on Ti/Ti alloy matrix-based composites fabricated by laser-assisted additive manufacturing
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Other metal matrix composites

Beside those frequently used metals/alloys as mentioned in previous sections, there are also a few AM studies that use other metals and alloys as the matrix materials. For example, Sun et al. [101] prepare a ZrB2-reinforced Zr matrix composite by SLS. The starting powders consist of ZrB2 and 30–50 wt% Zr. The sintered material exhibits good density and hardness value. Meanwhile, TiC-reinforced Zr matrix composite manufactured by LMD is presented by Ochonogor et al. [102]; Zr powders are mixed with 10–30 wt% TiC powders. It is found that the Zr metal reacts with the TiC to form ZrC, leaving Ti in solid solution. A few studies focus on Mg-based MMCs for better electrical performance. For instance, in the study of Zhang [103], LaNi5–Mg2Ni composite, an electrode material with high hydrogen storage capability, is fabricated by SLM. With the weight percentage of Mg2Ni varying from 10 to 30%, several phases are detected in the sintered material (i.e., LaNi5, LaNi4Mg, and LaMg2Ni9), but with significant variation in amount. This leads to different electrochemical performances, in terms of discharge capabilities and the number of cycles for activation. In addition, Mg–20 wt% LaNi5 composite is obtained by using the same method [104]. Phases identified within the material are Mg, Mg2Ni, and LaMg12. It is also indicated that Mg particles are surrounded by the network consisting of eutectic Mg + Mg2Ni layers and small LaMg12 blocks, which favors the improvement of activation property of electrodes.

Challenges and Research Opportunities

General challenges in metal AM

Additive manufacturing is a technology that has been enthusiastically pursued in recent years. Despite the advantages of metal additive manufacturing, there exist challenges that need to be addressed before the potential of AM can be fully explored. Those challenges occur at different levels, starting with the initial design to the final manufacturing stage, including a wide array of issues such as raw material qualification, process control, quality assurance, and industry standards. For instance, residual stress is a major obstacle in achieving consistent geometry in laser-assisted AM processes. It is usually generated due to the high thermal gradients formed in rapid heating and cooling cycles. The temperature differences in the irradiated region produce transient thermal strains at the different surface and depth locations, and such strain is retained after heat is removed [105, 106]. Solid-phase transformation during the complex thermal history is also reported to induce significant residual stress [107]. Another major challenge of laser-assisted AM lies in the fact that only limited materials have appropriate interaction with high energy beam. For example, AM of pure copper has tremendous challenges due to copper’s high thermal conductivity. The melting area experiences rapid heat dissipation and high local thermal gradients, resulting in delamination, layer curling, and part failure. For aluminum alloys, the high reflectivity of the laser source, high oxidization tendency, and the high thermal conductivity of aluminum result in significant obstacles for additive manufacturing [108]. Besides, the relatively poor surface finish achievable with laser-assisted AM constitutes a major challenge. The high surface roughness and existence of open pores on the surface deteriorate both the fatigue properties and the corrosion resistance of AM processed metallic parts. In addition, for laser-assisted AM, the large variations in microstructure may lead to the inconsistency in mechanical properties. As such, addressing those general challenges represents the exciting research opportunities for AM-produced MMCs.

Defect mitigation and control

Although various AM-produced MMCs have been demonstrated in the literature, the performance of the MMCs in general is not on par with that of traditional MMCs, mainly due to the natural weaknesses of the laser-assisted AM processes. For instance, the laser-assisted AM processes often result in a degree of porosity, which generally is detrimental to the mechanical properties and service life of components. It is well understood that the existence of pores deteriorates material strength, ductility, and fatigue properties [109]. If no compression is used, laser-sintered materials are usually porous [110]. Controlling parameters of the laser-assisted AM process, such as scan speed (exposure time) [111] and laser power [112], are found to significantly affect the porosity of products. In the literature, optimization of AM process parameters for regular metal alloys is well documented. However, such effort has been lacking for AM-produced MMCs. One should expect that the optimal process parameters for a metal alloy may not be the optimal for the MMCs with such an alloy as the matrix material. Meanwhile, a number of studies have also confirmed that the addition of reinforcement particles into metal matrix during laser-assisted AM process will result in higher porosity and reduce material ductility [14, 42, 53]. This certainly makes the optimization of AM process parameters more challenging. As such, it is believed that more research efforts are expected to achieve low material porosity of the laser-assisted AM-produced MMCs. Also, other common defects including deformation, detrimental residual stress, and surface roughness have been extensively investigated and discussed for the AM-produced metal alloys, but the effort has still been lacking for AM-produced MMCs. For those defects, the formation mechanism and mitigation strategies should be developed for AM-produced MMCs. In this regard, research is also called for to compare different laser-assisted AM processes in terms of defect mitigation and performance improvement. Industry has been particularly interested in learning the differences among the AM processes in producing MMCs, and thus, proper technical and business decisions can be made.

MMC reinforcements for laser-assisted AM

Future research should also investigate the suitability of other advanced reinforcement materials in laser-assisted AM of MMCs. Most studies reviewed in this paper adopt particulate reinforcements. However, other types of reinforcement, such as platelets, whiskers, or fibers, have different reinforcing mechanisms when introduced into metal matrix material, and they could also significantly enhance material performance under specific loading circumstances. Those reinforcements are frequently adopted in conventional manufacturing of MMCs, but they have been hardly investigated in laser-assisted AM processes. Meanwhile, the difficulty of achieving good wettability between the reinforcing particles and matrix is also regarded as a major obstacle [39, 41, 42]. Therefore, the development of new reinforcement material, as well as infiltrant materials with higher permeability and bondability with matrix materials, is urgently called for. This is particularly true for the laser-assisted AM processes where the extremely short process cycle is generally not favorable for diffusion. Also, the interaction between the reinforcements and the laser during laser-assisted AM of MMCs deserves significant research attention. There are many unknown phenomena in the process such as decomposition and vaporization of reinforcement particle, and understanding the phenomena will greatly help the microstructure and mechanical properties of the AM-produced MMCs. In addition, the geometry and size effects of the raw matrix and reinforcement particles are rarely studied in laser-assisted AM of MMCs, while these parameters are confirmed critical to product porosity and microstructure in conventional powder metallurgy [113,114,115].

Dispersion of reinforcements

The processing techniques of making MMCs also possess a major opportunity for improvement. One technical challenge of manufacturing MMC via laser-assisted AM techniques may be the difficulty of achieving a uniform distribution of particles, especially when nano-sized reinforcing particles are adopted. Agglomeration of reinforcing particles has been reported in many studies, and this limits the performance of final MMC products [34, 52, 74]. As investigated, mechanical mixing (e.g., ball milling, liquid state stirring) is the most adopted method to prepare raw powder mixture for making MMCs via laser-assisted AM. These methods may have a detrimental effect on particle morphology, introduce impurities, and lead to limited dispersing effect. Meanwhile, various powder mixing methods are proposed in conventional powder metallurgy, such as ultrasonic processing, particle surface modification, addition of dispersing agent, and applying external magnetic field. Therefore, more efficient particles dispersion techniques are expected to reduce the agglomeration of nano-sized reinforcement particles in laser-assisted AM processes.

There is another level of complexity in achieving uniform dispersion of reinforcing particles in laser-assisted AM processes. In PBF processes, the spreading of a fresh layer of powder by a recoating mechanism may alter the dispersion of reinforcement in the matrix powders, and in turn affect the final dispersion in the MMC. In DED processes, powder mixture is transported from a powder feeder, going through the deposition head, to exist the nozzle. The powder transport will certainly change the dispersion of reinforcement in the matrix. In this regard, both issues represent challenges, and they will need to be investigated to improve the uniform dispersion of reinforcement. Besides, the possible redistribution of reinforcement due to laser material interaction and raid solidification during laser-assisted AM processes should also be studied [116].

Post-processing

Moreover, due to the nature of laser-assisted AM processes, the as-built components have been plagued by challenging issues such as high porosity, high gradient residual stress, geometry distortion, and low surface quality. Post-treatment is reasonably expected to mitigate these negative effects and improve components performance. The research on post-treating AM-produced regular metal alloys is abundant—various methods of heat treatment, hot isostatic pressing, ultrasonic peening, laser shock peening, mechanical rolling, and machining have been attempted. However, the research on post-treating AM-produced MMCs has been lagging behind. As such, the adoption of post-treatments, for homogenizing microstructure, improving matrix/reinforcement interface, and reducing the porosity of laser-assisted AM-produced MMCs, should be explored. Finally, from a systematic point of view, laser-assisted AM process has proven to be economically inefficient requiring costly raw powder preparation and expensive post-processing. In order to effectively reduce the cost and realize the industrial applications of laser-assisted AM techniques, further investigation is required to lower the cost of raw powder preparation and post-processing.

Modeling and simulation

In order to understand the material melting/solidification, matrix/reinforcement interaction, as well as residual stress and defects formation during laser-assisted AM processes, modeling and simulation is regarded as another indispensable research area. For laser-assisted AM processes, the rapid thermal cycle and localized melting/solidification create major challenges to any in situ observation approaches. Modeling efforts at atomistic, micro-, meso-, and macro-levels are all appreciated. With reliable predictive models, the fundamental mechanisms can be understood, and thus, the laser-assisted AM process for making MMCs can be optimized. As a result, the costly trial-and-error experiments can be minimized. In this regard, serious challenges still exist in developing the predictive thermo-mechanical models for the various sub-processes during the laser-assisted AM processes, including temporal material property change, microstructure evolution, matrix/reinforcement interaction, and precipitate formation. Moreover, the current design of MMC systems is primarily based on the trial-and-error mechanism, which is time-consuming and costly. Thus, advanced predictive methods such as Calphad (calculation of phase diagram) are expected t be more actively adopted in the future design of new MMC material systems.

Conclusions

Development of MMCs by laser-assisted AM techniques is a promising area for both academia and industry. In recent years, significant efforts have been made to obtain various MMCs, primarily by powder bed fusion (PBF) processes and directed energy deposition (DED) processes. This paper surveys those developments. To the best of our knowledge, this is the first comprehensive review on this topic. The reviewed research works are categorizes based on the types of metal matrix materials, namely, aluminum/aluminum alloys, copper/copper alloys, iron/iron alloys, nickel/nickel alloys, titanium/titanium alloys, and other types of metals. The microstructure and resultant properties especially mechanical properties are summarized, and the findings of these studies are thoroughly summarized and compared. It is indicated that although strengthening AM-produced metal parts using second phase materials is promising, there still exist significant challenges and limitations for MMCs processed via laser-assisted AM methods. By providing such a critical analysis on the existing literature, the future research opportunities are pointed out. It is hoped that this comprehensive review will stimulate more exciting research to address the fundamental scientific problems and develop understanding for industry to adopt laser-assisted AM to produce MMCs with confidence.