Keywords

1 Introduction

Wire + Arc Additive Manufacturing (WAAM) is a novel process where a wire is fed through an electric or plasma arc at a constant rate to build a near net shape part. WAAM titanium alloy Ti-6Al-4V has found its applications in the aerospace industry to produce large parts at high deposition rate, affordable cost, and much reduced lead time comparing to the traditional manufacturing and the powder bed fusion AM processes (Williams et al. 2016). WAAM Ti-6Al-4V parts are practically defect-free, i.e. material density is 99.99%. However, it is recognised that feedstock contamination may occur during the wire production and/or part building process that can lead to process induced porosity defects (Wang et al. 2013). In addition, complex thermal cycles and directional solidification during the WAAM process result in non-conventional microstructure and texture.

High cycle fatigue properties of WAAM Ti-6Al-4V produced by single bead deposition method showed a 10% longer fatigue life compared to the mill annealed Ti-6Al-4V (Wang et al. 2013). Vertical build samples (loading axis parallel to the AM build direction) showed better fatigue properties (Wang et al. 2013). The lower fatigue strength of the horizontal samples (loading axis perpendicular to the build direction can be explained by the orientation of the prior β grains that are perpendicular to the loading axis in case of horizontal specimens, which makes it easier for the micro-cracks to grow rapidly resulting in fatigue life reduction. Presence of defects and non-conventional microstructure will also influence the fatigue performance and may limit the industrial adoption process of WAAM processed materials. So far, few studies have been done on the effects of porosity and non-conventional microstructure on the fatigue properties of WAAM Ti-6Al-4V. Published studies on WAAM Ti-6Al-4V have shown that defects are the preferred fatigue crack initiation sites that can lead to fatigue strength reduction (Wang et al. 2013; Biswal et al. 2019). Biswal et al. investigated the effect of designed porosity on fatigue performance of WAAM Ti-6Al-4V built by the oscillation strategy. Fatigue strength of porosity specimens was reduced by a factor of 1.5 compared to specimens without porosity (Biswal et al. 2019).

Fatigue crack propagation rate is mainly affected by the microstructure features and process induced residual stresses; defects and surface roughness have no or negligible effect. Our previous study on single bead build WAAM Ti-6Al-4V (Zhang et al. 2017) found that crack growth rate was lower when crack propagated across the build layers comparing to crack growing parallel to layers, albeit the small difference. In contrast, Xie et al. (2018) showed crack growth rate being 5% lower when crack grew in parallel with the build layers, and related this to the continuous interaction of the crack tip with the columnar β grains, which resulted in greater resistance to crack growth rate repeatedly. On the other hand, Lorant (2010) did not find any anisotropy in the crack growth rate property in WAAM Ti-6Al-4V. Studies on this front so far (Zhang et al. 2017; Xie et al. 2018; Lorant 2010) are inconclusive in terms of crack growth rate anisotropy in WAAM Ti-6Al-4V. The difference in these studies is likely caused by the competing mechanisms from microstructure and residual stress. Furthermore, crack growth rate properties in Zhang et al. (2017) were only investigated with the material by single bead deposition method, which is limited to the maximum build thickness of 7 mm. For thicker structures, parallel and oscillation build methods are currently available. Hence, it is important to study the effect of different deposition methods on fatigue crack growth rate and the property anisotropy.

This paper is focused on the material properties crucial to structural integrity and durability of WAAM Ti-6Al-4V produced by different deposition methods, which are the influence of defects on fatigue crack initiation and the effect of microstructure on fatigue crack growth rate. The key objectives are: (a) to develop predictive methods for porosity defect and its effect on fatigue life taking into account of the defect size, location, distribution and current NDT capabilities; (b) to study fatigue crack propagation behaviour in two build methods and two crack orientations, in order to provide crack growth rate properties for damage tolerance design for this alloy.

2 Methods

2.1 Experimental Design

WAAM Ti-6Al-4V material was deposited using a grade-5 Ti-6Al-4V wire with a wire diameter 1.2 mm, using a HiVE machine that had a Fronius Plasma 10 module attached to a rotator, wire feeder and the wire spool around the plasma torch. The base plate was a hot-rolled Ti-6Al-4V of 12 mm thickness and was clamped to a rigid steel backing block during deposition. Heat from the substrate was extracted by a water-cooled backing plate. Argon gas of 99.99% purity was used as shielding gas (placed ahead of the torch and as well as the trailing end of the torch), directed precisely at the melt pool to avoid oxidation.

The test program to study the effect of porosity on high cycle fatigue consisted of two batches of WAAM Ti-6Al-4V specimens, (1) material was deposited using the standard processing route as shown in Fig. 1(a), referred to as reference group, (2) using contaminated wires to deposit the specimen gauge section as shown in Fig. 1(b), referred to as porosity group. The wire was contaminated using water displacement 40th formula (WD-40®), details can be found in (Biswal et al. 2019). The chemical composition (% by weight) of all the influential elements in both the batches was within the permissible limits laid out by ASTM F3302-18.

Fig. 1.
figure 1

Specimen extraction: (a) reference specimens, (b) porosity specimens.

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The test programme to study fatigue crack growth behavior was performed using two different material deposition methods, i.e. parallel path and oscillation builds. Schematic of the two deposition methods is shown in Fig. 2(a). Two WAAM Ti-6Al-4V walls were deposited, one by the parallel and one by oscillation method, both are of 450 mm length, 150 mm height and 22 mm thick as shown in Fig. 2(b). After the deposition, the walls were separated from the substrate plate and compact tension samples were extracted. Figure 2(b) shows the schematic of the two sample orientations with respect to the build layers.

Fig. 2.
figure 2

(a) Two different deposition methods for building two WAAM Ti-6Al-4V walls, (b) schematic of compact tension sample extraction and crack orientations

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2.2 Specimen Design and Testing Method

2.2.1 Tensile and Fatigue Testing

Flat tensile specimens were machined according to ASTM E8 as shown in Fig. 3(a). Load controlled fatigue test specimens were extracted from the porosity wall in vertical orientation with respect to the build direction and machined according to ASTM E466 standard as shown in Fig. 3(b). Three specimens were subjected to interrupted fatigue-tomography testing which have been referred to as Fatigue-Tomography (FT) specimens FT-1, FT-2 and FT-3 while the remaining specimens were used to develop the S-N curves with and without porosity defects. Specimen FT-1 was subjected to interrupted fatigue test with six X-ray computed tomography (CT) measurements taken at the test start and at suitable intervals during the fatigue test. The remaining two specimens, i.e. FT-2 and FT-3, were scanned before the start of the test and fatigue tested till failure.

Fig. 3.
figure 3

Specimens for (a) tensile test, (b) load-controlled fatigue test; (c) X-ray computer tomography set-up for porosity mapping (test facility at TU Dortmund, Germany); (d) compact tension specimen and dimensions.

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The interrupted fatigue-tomography test set-up consisted of two units: a 50 kN load controlled fatigue test unit (Shimadzu) and an X-ray CT scanning unit (X TH 160 system, Nikon Metrology). The entire gauge section (5 mm diameter and 10 mm length) of the specimen was scanned by the X-ray CT to map the initial porosity distribution. Specimen FT-1 was then fatigue tested at an applied stress amplitude of 315 MPa, stress ratio of 0.1 and frequency of 20 Hz for 2 × 104 cycles which is approximately 65% of the expected fatigue life of the specimen based on published test results (Biswal et al. 2019). At this point the fatigue test was interrupted to perform X-ray CT scanning, following which the fatigue test was resumed. Subsequent fatigue tests were interrupted at every few thousand cycles to perform X-ray CT scans. All the X-ray CT scans were conducted at an accelerating voltage of 120 kV, current of 58 µA and voxel size of 20 µm (effective pixel size of 10 µm). Exposure time was set at 500 ms while capturing 1583 projections (rotation step size 0.22°), which resulted in each interrupted scan time of approximately 50 min. The scanned files were analysed using Volume graphics® software (VG Studio Max 2.2).

2.2.2 Fatigue Crack Growth Testing

For each deposition strategy, five compact tension samples were extracted, two for crack parallel to the layers and three for crack perpendicular to the layers. The compact tension samples were extracted from the mid thickness of the wall. Figure 3(d) shows the geometry and dimension of the compact tension sample. Crack growth testing was performed according to the ASTM E647 standard, at a maximum load of 3 kN, load ratio 0.1 and 10 Hz frequency.

3 Results and Discussion

3.1 Effect of Porosity Defects on High Cycle Fatigue

3.1.1 Porosity Analysis

The size distribution of gas pores was studied using the X-ray CT scans and it was evident that the reference group specimens can consist of micron sized pores (62 ± 23 µm). In contrast, the size of gas pores in the porosity group specimens (206 ± 80 µm) were found to be much larger. Further, the X-ray CT scans showed that the gas pore morphology in both the groups was near spherical and the spatial distribution was isolated from each other, such that material density was found to be 99.99% and 99.96% in the reference and porosity groups respectively.

3.1.2 Tensile and Fatigue Tests

Tensile test results (Fig. 4a) showed that the tensile strength and yield strength of both the groups were comparable (tensile strength 859 ± 4 MPa and yield strength 802 ± 7 MPa). However, the uniform elongation measured in the two groups was widely separated (porosity group 4% and reference group 10%). Similarly, the S-N test data (Fig. 4b) showed that the fatigue limit i.e. applied stress range for a life of 107 cycles, was 540 MPa for the reference group and 360 MPa for the porosity group at an applied stress ratio of 0.1. This indicates that WAAM Ti-6Al-4V has a notch fatigue factor of 1.5 and notch sensitivity of 0.5 for spherical gas pores. It is worth mentioning that the fatigue performance of as-built (i.e. non-heat treated, polished) WAAM Ti-6Al-4V specimens was close to the wrought and HIPed AM Ti-6Al-4V alloy.

Fig. 4.
figure 4

Reference data and sources: wrought (MMPDS 2008); powder fusion AM - hot isostatically pressed (HIP) AM: selective laser melting (Bagehorn et al. 2017; Kahlin et al. 2017); electron beam melting (Svensson and Ackelid 2009; Brandl et al. 2011; Hrabe et al. 2015; Kahlin et al. 2017; Shui et al. 2017).

Results of (a) static tensile test, (b) fatigue test at applied stress ratio 0.1;

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3.1.3 Interrupted Fatigue-Tomography Test

Interrupted fatigue-tomography test (Fig. 5) showed tortuous cracks originated preferentially at sub-surface pores that grew towards the nearest free surface. This can be explained by the higher values of the stress concentration factor (SCF) for sub-surface pores, which is at least 25% higher than that of an internal pore and depends on the distance of the pore to the specimen free surface. The location of the cracks were found to be at the mid-riff section of the pores as shown in Fig. 5(c), so the critical sub-surface pores were expanding under cyclic loads.

Fig. 5.
figure 5

Images of interrupted fatigue-tomography test showing the overall cross-sectional (or top view) of the X-ray CT scan taken at: (a) zero cycle, just before the fatigue test, (b) 3.2 × 104 cycles (specimen failed at 32380 cycles), (c) three dimensional view of the scanned section at 3.2 × 104 cycles.

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3.1.4 Stress Intensity Factor Range Applied to Gas Pore Defects

Since the applied stress range and the porosity defect size are both play an important role in determining the fatigue performance, the stress intensity factor (SIF) range was used to correlate the fatigue test results. According to Murakami’s model (Murakami and Endo 1986), a spherical gas pore can be treated as a planar crack of size equal to the square root of the projected area of the pore. Stress intensity factor range was calculated by Murakami’s equation as shown here by Eq. (1).

$$ \Delta K = C \times \Delta \sigma \sqrt {\pi \sqrt {area} } $$
(1)

where ∆K is the stress intensity factor range, ∆σ the applied stress range, √area the square root of the projected area of the pore, and parameter C is 0.5 for internal defects and 0.65 for surface defects.

Fatigue test result presented in Fig. 4(b) have been re-plotted as a relation of ΔK vs. N in Fig. 6 showing a much better correlation between the fatigue life and the fracture mechanics parameter.

Fig. 6.
figure 6

Correlation between fatigue life and stress intensity factor range (SIF range) for porosity defects tested in this study (SIF range is calculated using the Murakami’s equation, Eq. 1). Note: encircled data points denote crack initiation at internal pores; the majority failed by cracks initiating from sub-surface pores.

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3.2 Fatigue Crack Growth Behaviour

Measured fatigue crack growth rates (FCGR) for the parallel path and oscillation wave builds are shown in Fig. 7, for two different crack orientations, and compared with traditional method processed Ti-6Al-4V, i.e. cast, wrought (mill annealed) and wrought (beta annealed). Numbers in the figure legends represent repeated fatigue tests for each case.

Fig. 7.
figure 7

Fatigue crack growth rates and comparison of two different build methods: (a) crack propagating parallel to layers, (b) crack across the layers

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3.2.1 Crack Orientation

When crack propagated in parallel to the WAAM build layers, the influence of different deposition method on FCGR is negligible, as shown in Fig. 7a. In this crack orientation, crack growth rate was lower than that of the wrought material (mill annealed), but greater than that of casting and beta annealed before ΔK reached 18 MPa√m; beyond this value, crack growth rate in WAAM and beta annealed conditions are similar.

When crack growth was across the WAAM layers, Fig. 7b, WAAM Ti-6Al-4V showed lower crack growth rate compared to the wrought and similar crack growth rates as the cast and the beta annealed. The oscillation build had much lower crack growth rate in two tests, but another test had much higher growth rate, which is similar to that from the parallel build. Therefore, it is not yet conclusive for the oscillation wave build; further research is required on the competing mechanisms of microstructure and residual stress; the latter depends on specimen extraction location. If we analyse the crack propagation with respect to the columnar prior-β grain orientation, the crack in this orientation propagated in parallel with the columnar prior-β grain orientation.

The role of the macro- and micro-structure and morphology of the microstructural constituents can play an important role on the fatigue crack behaviour of a material. In Ti-6Al-4V, size and morphology of α lath are the primary microstructural parameters that control the plastic zone ahead of the crack tip, therefore the fatigue crack growth rate (Han et al. 2016; Tan et al. 2018). If we analyse the crack growth rate with respect to the microstructure, the low crack growth rate in the cast and beta annealed materials is owing to the presence of fully lamellar and lamellar plus basket weave microstructure respectively. On the other hand, the mill annealed wrought material consists of duplex microstructure with near equiaxed grains, and consequently leads to faster crack growth rate.

3.2.2 Build Methods and Effect on Property Anisotropy

Degrees of anisotropy in fatigue crack growth rate for the parallel path and oscillation wave built samples are presented in Fig. 8. The parallel build samples did not show considerable anisotropy in crack growth rate, with only a small difference between the two crack orientations at ΔK values of 12-15 MPa√m, Fig. 8a. The oscillation build samples showed significant anisotropy in Fig. 8b, where samples with crack crossing the layers showed much lower crack growth rate, except the Test No. 2.

Fig. 8.
figure 8

Crack growth rates in both crack orientations in (a) parallel and (b) oscillation builds.

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The lower crack growth rate in the across layer samples is thought to be associated with the crack propagating through the layer banding along the build direction. Ho et al. (2019) presented detailed microstructure analysis of single path build WAAM Ti-6Al-4V showing that the banding appearance regions are either heat affected zones (HAZ) or segregation areas that are formed due to repeated thermal cycling during the deposition process. The study also showed that the top of each HAZ zone is associated with very fine α packets with very fine spacing along the build direction, resulting in more resistance to crack propagation across the layers comparing to crack growth in parallel to the layers. For the crack across the layer specimens, the crack tip passes through each layer band, so it continuously encounters fine α packets in the HAZ and segregation zones that will slow down the crack growth rate. Detailed microstructure analysis of layer banding can be found in (Ho et al. 2019). It remains to be studied whether the parallel and oscillation deposition methods have also influenced the HAZ and segregation zones. The absence of anisotropic crack growth rate in the parallel path build samples might be associated with the very low or absence of HAZ and segregation zones. The anisotropic crack growth rate observed in the oscillation build samples might be due to the presence of pronounced HAZ and segregation zones and the crack propagating through these regions in crack across the layer samples. Hence, a lower crack growth rate is observed.

4 Conclusions

  • Porosity defects were embedded to specimen gauge section by using contaminated wires during the additive manufacturing. X-ray computed tomography showed spherical pores distributed scarcely (i.e. isolated from each other). Interrupted fatigue-tomography tests showed preferential crack initiation from the near surface pores; the subsequent crack growth was directed towards the nearest free surface.

  • Porosity defects did not affect the tensile strengths, however the ductility was reduced from 10% to 4% and fatigue strength (in terms of cyclic stress range at stress ratio 0.1) was reduced from 540 MPa (reference, defect-free) to 360 MPa.

  • Good correlation was found between the fatigue life and the stress intensity factor (SIF) range of the crack initiating defect, where the SIF for pore geometry was calculated using Murakami’s equation.

  • Fatigue crack growth rate in WAAM Ti-6Al-4V is lower than the mill annealed wrought but higher than the β annealed wrought and casting materials.

  • The parallel path deposition method showed higher crack growth rate than the oscillation build when crack is across the layers. However, there is virtually no difference between the two build methods when crack is parallel to the layers.

  • Anisotropic fatigue crack growth rate was observed only for the oscillation build, whereas the parallel path build had almost isotropic crack growth rates between the two major crack orientations.