Introduction

The piston is one of the most important parts of an automobile engine. Pistons have traditionally been fabricated using Fe-based and Al-based materials. A Fe-based piston has a large inertia that is due to its higher density, and has a high endurance of heat and mechanical loads during operation. On the other hand, the Al-based material is used for the reduction of weight of an engine in recent years. In particular, Al-Si12CuNi cast Al alloy, which is standardized as JIS AC8A (referred to as AC8A), has high fatigue strength and low expansion in high temperature condition (Ref 1). However, when cast Al alloy is used as the material of the piston head part, some problems such as cracks are generated with a full-load durability test and/or a thermal-shock test (Ref 2, 3).

In a previous report (Ref 4), some of the authors clarified the fatigue analysis result of the engine piston by the finite element method. In this report, the head of the piston with AC8A had the number of cycles to failure of 107 or less. That is, the head with AC8A had poor durability during engine operation, and fatigue crack will be generated from this portion. Some researchers have reported that the improvement of durability for fatigue cracking of a cast Al alloy could be successfully achieved (Ref 5-8). However, those methods at the present moment are too expensive to be in common use for an engine piston (Ref 9). On the other hand, the head of the piston with AA6061 wrought alloy, which is standardized as JIS A6061 in 6000 series of Al alloy (referred to as A6061), had the number of cycles to failure of 107 or more (Ref 4). It is conceivable that the piston durability during engine operation is remarkably improved if A6061 is used as the main composition of a piston. Nevertheless, A6061 is difficult to use for a piston since it has large expansion at a high temperature (Ref 1). Hence, it is suggested that combining A6061 as the piston head and AC8A as the piston skirt that improved durability of the piston during engine operating can be achieved (Fig. 1). However, the ability to join this combination is difficult, because one of the materials is a cast alloy. Therefore, an appropriate joining method between a wrought material such as A6061 and a cast material such as AC8A needs investigation for automotive industry.

Fig. 1
figure 1

Joining between A6061 solid bar and AC8A pipe for engine piston

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Friction welding is well known as one of the solid state joining methods. This method is very useful for the joining of dissimilar materials, and the welding process can be easily automated. Also, this welding method has several advantages over fusion welding, e.g., high energy efficiency, narrower heat-affected zone (HAZ), and low welding cost (Ref 10-12). In particular, friction welding is able to produce joints with high reliability easily; it is widely used in the automotive industry and applied to fabricate important parts such as drive shafts and engine valves. Many researchers have reported that the mechanical and metallurgical properties of the friction welded joints of similar Al-based material combinations show desirable characteristics. For example, the friction welding condition for the joint which had good tensile strength of same Al-based material combinations was individually demonstrated as follows: JIS 1000 (Ref 13), 2000 (Ref 14, 15), 5000 (Ref 16-19), 6000 (Ref 20, 21), and 7000 series alloys (Ref 14, 22-24). In those ways, some researchers including the authors have reported that the joining of similar Al-based material combinations could be successfully achieved with a relatively good joint, although the friction welding conditions differed in those reports. On the other hand, research on the friction welding of cast alloy has been few in comparison with that of similar Al-based material combinations, because the weldability of cast materials differed with expanded wrought materials. In this case, the tensile strength of Al-Si11Cu3(Fe) alloy (JIS ADC12) joint (Ref 25) and AlSi7 Mg alloy (JIS AC4CH) joint (Ref 26) were investigated. Furthermore, the result between expanded wrought materials and casting materials have scarcely been investigated because a cast alloy has poor weldability although the friction welding condition for the joint which had the tensile strength of following combinations were investigated: the joint between 5052 alloy (JIS A5052) and AlSi7Mg0.3 alloy (JIS AC4C) by Kato and Tokisue (Ref 27), and the joint between Al-Cu alloy matrix composites and Al-Si5Cu1 Mg alloy (JIS AC4D) by Kuroki et al. (Ref 28). That is, the friction welding conditions of dissimilar Al-based material combinations with good joint properties has not been fully clarified, so that the friction welding conditions for material combinations are determined by trial and error. In particular, clarifications of the joining mechanism are strongly required concerning the weldability of the combination between an expanded wrought material and a cast material because the joint made by those combinations will be applied to an engine piston.

This study presents the characteristics of friction welding between a solid bar of A6061 and a cast pipe of AC8A, and discusses improvements of a joining method to obtain high joint properties. In this report, the authors present the mechanical properties of welded joints, under various friction welding conditions. The authors also show the mechanical properties of the joint made with the conventional friction welding method, in contrast to those made with the friction welding technique developed by some of the authors. Furthermore, the authors propose that this friction welding technique is suitable for welding of using a pipe shape. Based on those results, the authors describe the possibility with the friction welded joint between a solid bar of A6061 expanded wrought Al alloy and AC8A cast pipe for obtaining good joint strength.

Experimental Procedure

The materials used were 6061-T6 (JIS A6061-T6, referred to as A6061) expanded wrought rod with a diameter of 32 mm and Al-Si12CuNi-T6 (JIS AC8A-T6, referred to as AC8A) high pressure casting block. The chemical compositions and mechanical properties such as the ultimate tensile strength (UTS), 0.2% yield strength (0.2%YS), and elongation for materials used are shown in Table 1. In this connection, the 0.2% yield strength for AC8A was not investigated because this material was a cast Al alloy. A6061 rod was machined to a 30-mm diameter for the weld faying (contacting) surface, as shown in Fig. 2a. On the other hand, AC8A block was cut in a rectangular shape, and then the rectangular block was machined to the specimen with hollow cylinder shape boring an outer diameter of 30 mm and an inner diameter of 24 mm for the weld faying surface, as shown in Fig. 2b. Those shapes were designed to imitate an engine piston. All weld faying surfaces of friction welding specimens were polished with a surface grinding machine before joining in order to eliminate the effect of surface roughness on mechanical properties of dissimilar material joints (Ref 29-31).

Table 1 Chemical compositions and mechanical properties of materials used
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Fig. 2
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Shapes and dimensions of friction welding specimens

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A continuous (direct) drive friction welding machine was used for the joining. During the friction welding operations, the friction welding condition was set to the following combinations: a friction speed of 27.5 s−1 (1650 rpm), friction pressures of 25 or 100 MPa, a range of friction times from 0.3 to 1.1 s, a range of forge pressures from 25 to 150 MPa, and a forge time of 6.0 s. The friction torque was measured with a load-cell, and recorded using a personal computer through an A/D converter with sampling times of 0.001 s. All joint tensile test specimens had only the outer flash (burr or collar) removed, i.e., the inner flash was not removed, because the target of the joint is an engine piston. By the way, A6061 and AC8A are heat treatable Al alloy and their mechanical properties are improved by heat treatment, such as solution treatment plus artificial aging (T6 treatment). Hence, all joint tensile test specimens were kept at room temperature for 5 weeks because the post weld aging of Al alloy with T6 treatment is carried out at room temperature, i.e., natural aging treatment (Ref 23, 32, 33). The joint tensile tests were carried out at room temperature.

Results of the Conventional Method

Joining Behavior and Friction Torque Curve

To clarify the joining behavior in details of the joint characteristics using the conventional friction welding method (conventional method), the joining behavior and friction torque during the welding process of the joint was investigated. Figure 3 shows an example of the relationship between the joining behavior and the friction torque during the friction process of the joint. In this case, the joint was made with a friction pressure of 25 MPa, a friction time of 1.6 s, and a forge pressure of 150 MPa. Photos 1-4 in Fig. 3a correspond to the friction torque of (1) to (4) in Fig. 3b, respectively. Photo 1 shows the state at the weld faying surfaces as they contacted each other, then the friction torque was rapidly increased. When the friction torque reached the initial peak of (2), the AC8A side was slightly upset (deformed) and exhausted the outer flash from the weld interface, as shown in Photo 2. Thereafter, the friction welding was continued as shown in Photo 3, and it was maintained until a friction time of 1.6 s, which was set before this experiment. Then, the friction torque was increased by braking for the rotation stop. Immediately after that behavior, the AC8A side was crushed by added forge pressure as shown in Photo 4. In this connection, the joint was able to successfully make with a forge pressure of 25 MPa, although those data will be demonstrated later. Therefore, the forge pressure should be set to below 150 MPa, because the AC8A side could not keep the pipe shape with increasing of friction torque by braking (see Fig. 3b). On the other hand, the friction welding for the solid bar shape of the same material combinations in the preliminary experiment (Ref 4) was able to successfully make with high friction pressure such as 100 MPa. However, the joint in this study with a high friction pressure such as 100 MPa could not successfully make as shown in Fig. 3a-4, since the AC8A side was crushed by the generating of friction torque during the friction process (the data are not shown here). Hence, the friction and forge pressures should be set to suitable conditions for prevention of breaking of AC8A side, because the pipe shape of AC8A side was not kept for those setting pressures.

Fig. 3
figure 3

Example of joining behavior and friction torque curve by conventional method: friction pressure of 25 MPa, friction time of 1.6 s, and forge pressure of 150 MPa

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Fig. 4
figure 4

Cross-sectional appearances of weld interface regions of joint by conventional method at various forge pressures: friction pressure of 25 MPa and friction time of 1.1 s

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Observation of Cross Section of Joint

Figure 4 shows the cross section of the joint at various forge pressures, wherein was made by the conventional method. In this case, the joint was made with a friction pressure of 25 MPa and a friction time of 1.1 s. When the joint was made with a forge pressure of 25 MPa as shown in Fig. 4a, the AC8A side had heavy deformation, and the A6061 side mostly retained the solid bar shape. In particular, the weld interface was not kept to the first contacted position (initial contact position) of both weld faying surfaces, and that traveled to the longitudinal direction of AC8A side as shown in Fig. 5. That is, the traveling phenomena of the weld interface were observed in the joint in this friction time (1.1 s). The traveling phenomena of the weld interface in the joint will be described later. In addition, this joint had the gap between the transferred AC8A on the A6061 and AC8A sides, i.e., the non-joined (not-joined) region at the traveled weld interface was observed. The outer and inner flashes were mainly expelled between the transferred AC8A on the A6061 and AC8A sides, and it was increased with increasing forge pressure (Fig. 4b through e). The quantities of the transferred AC8A on the A6061 side and the non-joined region at the traveled weld interface decreased with increased forge pressure because that was expelled as the flash. However, the joint without traveled weld interface was not obtained.

Fig. 5
figure 5

Example of traveling phenomenon of weld interface of joint

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Joint Tensile Strength

Figure 6 shows the relationship between the forge pressure and the joint efficiency (joining efficiency) of the joints. The joint efficiency was defined as the ratio of joint tensile strength to the ultimate tensile strength of the AC8A base metal. Figure 7 shows examples of the appearances of the joint tensile tested specimens and those fractured surfaces. When the joint was made with a forge pressure of 25 MPa as shown in Fig. 6, it had approximately 8% joint efficiency. The joint efficiency increased with increasing forge pressure, and it had approximately 34% efficiency at a forge pressure of 75 MPa. Thereafter, the joint efficiency was maintained with increasing forge pressure, although with some scatter in the data at 100 and 125 MPa. That is, the joint with 100% joint efficiency was not obtained. All joints fractured between the traveled weld interface and the AC8A side (referred to as the mixed mode fracture), as shown in Fig. 7. In particular, the fractured surface showed two types of the dark and bright contrast such as areas A and B. Figure 8 shows the optical overviews at the fractured surface of AC8A base metal, and those of areas A and B on the A6061 side in Fig. 7. The area A showed a flat surface which was flatter than area B, although both had a brittle surface. In particular, a brittle surface of area A as shown in Fig. 8-ii resembled the fractured surface of the AC8A base metal as shown in Fig. 8-i. Further investigation will be needed to elucidate the detailed joint characteristics, e.g., by SEM observation via EDS analysis because it was considered that area B may be transformed during oxidization. Moreover, area B resembled the worn state of other Al alloy friction welded joints (Ref 18, 34). Hence, it was considered that area A was the part of the fracture at the AC8A side by tensile test and area B was the part of the shear fracture at the weld interface during the friction process. Furthermore, the dark contrast region of area A on the fractured surface of the joint with a forge pressure of 75 MPa as shown in Fig. 7b covered most of the fractured surface. The joint efficiency increased with increasing forge pressure from 25 to 75 MPa, which corresponded to increasing dark region of area A (see Fig. 6). Hence, to obtain the joint that has higher joint efficiency a forge pressure needs to be added higher than the friction pressure. However, all joints had the mixed mode fracture and the flatter surface such as area B, although that joint was estimated to have good strength in the micro-level. To use the joint as an engine piston, it is required that the joint does not fracture at the weld interface. That is, the joint must be fractured from AC8A side. On the other hand, the results for the fracture from the base metal side, which had the traveled weld interface, was not also demonstrated by the experiment of Kato and Tokisue (Ref 35). Hence, the fracture from the base metal side seems to be not easily obtained, although the details of which were not clarified in the Kato and Tokisue’s paper (Ref 35). Therefore, one of the main factors for the mixed mode fracture of the joint for this combination seems to be because of the traveling phenomena of the weld interface. That is, it is desirable to set short friction time to prevent the occurrence of the traveled weld interface from the joint.

Fig. 6
figure 6

Relationship between forge pressure and joint efficiency of joint by conventional method; friction pressure of 25 MPa and friction time of 1.1 s

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Fig. 7
figure 7

Examples of joint tensile test specimens after tensile testing and those fractured surfaces: forge pressure of (a) 25 MPa and (b) 75 MPa

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Fig. 8
figure 8

Optical overviews at fractured surface of AC8A base metal, and those of areas A and B on A6061 side in Fig. 7

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In an attempt to reduce the traveled distance of the weld interface, the joint was made with short friction time. Figure 9 shows an example of the friction torque curve and the appearance of the joint at a friction time of 0.3 s with a forge pressure of 125 MPa by the conventional method. The friction welding was maintained until a setting friction time of 0.3 s, since the friction torque was measured until this friction time as shown in the friction torque curve of Fig. 9. However, the AC8A side was crushed by adding forge pressure. That is, the joint was not able to successfully make, as shown in the photo of Fig. 9. On the other hand, the joint with a long friction time such as 1.1 s was able to successfully make as shown in Fig. 4e. Hence, the pipe shape of the AC8A side was not kept for this forge pressure, because it was able to estimate that this side did not have sufficient temperature rise at a friction time of 0.3 s. That is, the joint had insufficient friction heat for successful joining. Therefore, the crushing of the AC8A side seems to be caused by the increasing of the friction torque by braking, as shown in the friction torque curve of Fig. 9.

Fig. 9
figure 9

Example of friction torque curve and appearance of joint by conventional method; friction pressure of 25 MPa, friction time of 0.3 s, and forge pressure of 125 MPa

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Improving the Joining Method

Suggestions for the Friction Welding Technique

Based on the above results, it is necessary to reduce the increasing of the friction torque by braking during the rotation stop to obtain successful joining at short friction time. In previous reports, some of the authors suggested the new friction welding technique (Ref 23, 36). The steel or Al joint made by this technique did not have the increasing of the friction torque by braking. In addition, the joint had less axial shortening and less flash compared with the conventional method. Figure 10 shows the general view of a part of the friction welding machine for new friction welding technique. The fixed side chuck had an electromagnetic clutch, and the fixed side specimen was fixed with the chuck, wherein was connected with an electromagnetic clutch. When the clutch was released, the fixed side specimen was simultaneously rotated in conjunction with the rotating side specimen, i.e., the relative speed at the weld interface was instantly decreased to zero. Therefore, the increasing of the friction torque by braking was not generated. Furthermore, as the braking time was smaller than 0.04 s, i.e., one rotation of the specimen, its effect was negligible. That is, the braking time had a negligible effect on joining phenomena, and the braking deformation of the joint during the rotation stop was very small. The detailed characteristics of this welding technique have been described in previous reports (Ref 23, 36). In particular, this friction welding technique was suited to the joining for the thin-walled circular pipe (Ref 37). Hence, if the joint between A6061 solid bar and AC8A pipe with short friction time is made by this friction welding technique, it will have successful joining.

Fig. 10
figure 10

General view of a part of friction welding machine for new friction welding technique

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Figure 11 shows an example of the friction torque curve and the joint appearance for this friction welding technique. This joint was made at a friction time of 0.3 s and a forge pressure of 125 MPa. The friction torque simultaneously decreased to zero at a friction time of 0.3 s, i.e., when the friction process was finished, and the joining could be successfully achieved. That is, the friction torque did not have increased braking. Therefore, it is considered that this friction welding technique, in which the relative speed at the weld interface decreased to zero when the friction process was finished, would be suitable for the joining of the friction welding for material combination in this study.

Fig. 11
figure 11

Example of friction torque curve and appearance of joint by new friction welding technique; friction pressure of 25 MPa, friction time of 0.3 s, and forge pressure of 125 MPa

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Joint Tensile Strength

Figure 12 shows the relationship between the forge pressure and the joint efficiency of the joints at various friction times. In this case, a friction time was set to 0.3, 0.7, and 1.1 s, respectively. When the joint was made at a friction time of 0.3 s as shown in Fig. 12a, it had approximately 6% efficiency at a forge pressure of 25 MPa. Then, the joint efficiency increased with increasing forge pressure, and it was approximately 16% at a forge pressure of 75 MPa or higher. The joint efficiency also increased with increasing forge pressure at friction times of 0.7 and 1.1 s (see Fig. 12b, c), although that at a friction time of 1.1 s with a forge pressure of 125 MPa had scattering. In particular, the joint at a friction time of 0.7 s with a forge pressure of 125 MPa had approximately 54% joint efficiency as shown in Fig. 12b. This joint efficiency was higher than that of the joint, which was made by the conventional method (see Fig. 6). However, all joints had the mixed mode fracture as shown in Fig. 7a or b, and those fractured surfaces resembled that of the joint by the conventional method.

Fig. 12
figure 12

Relationship between forge pressure and joint efficiency of joint at various friction times; friction time of (a) 0.3 s, (b) 0.7 s, and (c) 1.1 s

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Figure 13 shows the cross section of the joints at various friction times. In this case, the joint was made with a forge pressure of 125 MPa. When the joint was made at a friction time of 0.3 s as shown in Fig. 13a, the inner weld interface had the non-joined region. Therefore, the joint efficiency was low as shown in Fig. 12a. The non-joined region was also observed in the joint at a friction time of 1.1 s as shown in Fig. 13c, although it decreased with increasing friction time. The weld interface of this joint traveled to the longitudinal direction of AC8A side from the initial position. That is, the traveling phenomena of the weld interface were observed in the joint. Hence, the joint was estimated to be difficult to obtain the fracture at the AC8A side, although the maximum joint efficiency was achieved at a friction time of 0.7 s with a forge pressure of 125 MPa. The generating of the traveling phenomena seems to be a cause on the difficulty of obtaining the fracture at the AC8A side of the joint.

Fig. 13
figure 13

Cross-sectional appearances of weld interface regions of joint at various friction times: forge pressure of 125 MPa

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Discussion

Based on the above results, it was considered that the joint was difficult to obtain the fracture at the AC8A side although the joint strength was increased with increasing forge pressure, because the traveling phenomena of the weld interface were generated during the friction process. To clarify the traveling phenomena in this study, the joint was made without the influence of the high forge pressure, which was an identical friction pressure. In this case, the joint was made with a forge pressure of 25 MPa by our friction welding technique. When the joint was made at a friction time of 0.3 s as shown in Fig. 14a, the weld interface slightly traveled to the longitudinal direction of AC8A side from the initial position. That is, the traveling phenomena of the weld interface were observed in this joint. Then, the quantities of transferred AC8A on the A6061 side increased with increasing friction time. Furthermore, when the joint was made with more short friction time such as 0.1 or 0.2 s, it was generated of the non-joined region at the weld interface although those data are not shown here, because the A6061 and AC8A sides were not unified in those friction times. Hence, it was able to be considered that the generating of the traveling phenomena was not completely removed from the joint, even though the joint will be made with another friction welding condition. The traveling phenomena of the weld interface were also observed in the combination of different diameters for same material combinations of friction welded joint (Ref 35, 38-41). Similarly, the traveling phenomena were also observed in friction stud welding, i.e., the combination of bar and plate (Ref 42-44). Therefore, the difference of the heat transfer at the weld interface in the combination of the different shapes seems to be a cause of the traveling phenomena of the weld interface. In case of the combination of different diameters, the large diameter side is difficult to heat in comparison with the small diameter side, and the generated friction heat is transferred to the regions of AC8A side that are not in contact. Thus, the temperature of the weld interface at the initial position is decreased and that is transferred to the high temperature of the small diameter side. The combination of friction welding in this study was carried out between solid bar and circular pipe, i.e., its result was able to regard as the combination of the different diameters. Hence, the traveling phenomena were also generated by the combination in this study. On the other hand, the traveling phenomena were estimated to be generating at a friction time of 0.3 s or less, because that was observed at this friction time (see Fig. 13a). However, the joint efficiency at a friction time of 0.3 s was low, even though the joint had high forge pressure (see Fig. 12a). That is, it was difficult to obtain the joint without the traveling phenomena of the weld interface, of which was made with the combination of the shape of the friction welding specimens between solid bar and pipe. Thus, it was clarified that the friction welding between a solid bar of A6061 and a cast pipe of AC8A was not desirable since the combination of the shape of the friction welding specimens was a cause of the traveling phenomena of the weld interface.

Fig. 14
figure 14

Cross-sectional appearances of weld interface regions of joint at various friction times: forge pressure of 25 MPa

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If the joint such as the combination with the same shape of the friction welding specimen will be successfully made without the traveling phenomena of the weld interface, the joint will be estimated to obtaining higher strength because the joint efficiency had approximately 54% as shown in Fig. 12b. In this connection, we were able to successfully obtain the joint without the traveling phenomena of the weld interface, which was made by the same combination of the pipe shape of the friction welding specimens (Ref 45). Those results will be described in a next report.

Conclusions

This paper described the characteristics of friction welding between a solid bar of 6061 (A6061) Al alloy and a pipe of Al-Si12CuNi (AC8A) Al cast alloy. The following conclusions are provided.

  1. 1.

    When the joint was made by a continuous drive friction welding machine (conventional method), the AC8A portion of the joint had heavy deformation and the A6061 side was hardly deformed. In particular, the joint could not be successfully made with the following friction welding conditions, because AC8A pipe side crushed due to insufficient friction heat or high pressure: a short friction time such as 0.3 s, high friction pressure such as 100 MPa, or high forge pressure such as 150 MPa. The deformation of AC8A side was caused by increasing friction torque during braking.

  2. 2.

    To prevent braking deformation until the rotation stops, a joint was made by a continuous drive friction welding machine that has an electromagnetic clutch. When the clutch was released, the relative speed between both specimens simultaneously decreased to zero. Joining at short friction time could be successfully achieved by this friction welding technique.

  3. 3.

    When the joint was made at a friction time of 0.3 s, it had approximately 6% efficiency at a forge pressure of 25 MPa. The joint tensile strength increased with increasing forge pressure, and it had approximately 16% efficiency at a forge pressure of 75 MPa or higher. The joint tensile strength also increased with increasing forge pressure at other friction times.

  4. 4.

    The maximum joint tensile strength had approximately 54% efficiency, when it was made with a friction pressure of 25 MPa, a friction time of 0.7 s, and a forge pressure of 125 MPa. However, all joints showed the fracture between the traveled weld interface and the AC8A side because the weld interface traveled to the longitudinal direction of AC8A side from the first contacted position of both weld faying surfaces.

  5. 5.

    It was clarified that the friction welding between a solid bar of A6061 and a cast pipe of AC8A was not desirable since the traveling phenomena of the weld interface were caused by the combination of the shape of the friction welding specimens.