Figure 5 shows the cross-section and optical micrographs of laser welded SiCp/6061Al joint. A few pores existed in heat affected zone and some irregular shaped shrinkage voids appeared in weld metal, which would weaken the tensile strength of the welded joint. Meanwhile, a large number of brittle needle-like Al₄C₃ phases appeared in the center of the weld seam, as shown in Figure 5C. The length of these needlelike precipitates was in the range of 40∼70 μm. Large Si crystals were also evident in the modified composite structure. According to the chemical formula of interface reaction, the growth of Al₄C₃ was usually accompanied by the precipitation of Si element.

The XRD results in Figure 6A demonstrated that the weld metal was mainly composed of Al matrix, Si and Al₄C₃. Only a small amount of SiC still existed in weld metal. Combined with the studies of other scholars (Yue et al., 1997; Bassani et al., 2007), the brittle needle-like phases were Al₄C₃, which was an important reason for the degradation of the tensile strength of the joint. With the increasing temperatures, the following reaction (1) occurred at the interface between Al matrix and SiC particles, which led to the formation of Al₄C₃ and Si. 4 A l + 3 S i C = A l 4 C 3 + 3 S i (1) The reaction (1) not only consumed the reinforced phase particles (SiC) in the molten pool, but also generated brittle needle-like phase Al₄C₃ which greatly reduced the mechanical properties of the welded joints. This was also consistent with the previous researches (Huang et al., 2001; Chao et al., 2013), which needed to be solved from the perspective of new process and composition regulation.

It is generally accepted that the interface reaction (1) depended on temperature and the exposure time. At high temperatures, SiC would gradually dissolve, C elements were released from the broken Si-C covalent bonds and diffused into liquid Al, and reacted with the Al elements to form Al₄C₃ precipitates. This could be verified by only few SiC particles existed in the central part of weld metal.

Figures 6B,C show the SEM micrograph of the region between weld metal and heat affected zone. EDX analyses were also carried out for the identification of the marked phases in Figure 6C. Referring to the XRD, phase A with a chemical composition of Al-2.6, C-42.1 and Si-55.3 by at% was identified as SiC. It was obvious that the SiC particles retained their original clear cut edges without any evidence of interfacial reaction phases. The chemical compositions (at%) of B and C phases were: Al-65.3, C-31.6, Si 3.1, and Al-78.1, C-7.1, Si-14.8, respectively. Therefore, these phases were recognized as Al₄C₃ and A1 matrix, respectively. The characterized results also indicated that Al₄C₃ phases were nucleated at the surface of SiC. In addition, needle-like Al₄C₃ precipitates in this region also became finer than that in the middle of weld metal where higher heat input was suffered. As clearly visualized in high magnification SEM micrographs of Figure 6C, the composite structure in this region was subjected to a clear variation after laser welding.

Figure 7 shows cross-section and optical micrographs of SiC_p/6061Al laser welded joint with Ti interlayer. As can be seen in Figures 7A,B, porosity and voids defects in the welded joints almost disappeared. By comparing Figures 5C, 7C, the microstructures of the weld metal were significantly changed after the addition of Ti. The distribution of acicular phases was more disordered. The length of acicular Al₄C₃ phases in the weld seam was significantly reduced to about 20–30 μm, which was very important for improving the tensile strength of the welded joint. Si crystals with diameters of about 5 μm, were also evident in the modified composite structure, which was similar to that without Ti addition.

SEM micrographs of SiC_p/6061Al laser welded joint with Ti interlayer at various regions with different magnifications are displayed in Figure 8. Figure 8A presents the whole welded joint with obvious interface between base metal and weld metal. The microstructure consisted of grey matrix, white particles and dark grey needlelike precipitates, as shown in Figure 8B. According to research by Guo (2010), the grey matrix, white particles and dark needlelike precipitates in Figure 8C were probably Al-Si eutectic structures, newly formed reinforcement particle TiC and Al₄C₃ phases, respectively.

In order to determine their element compositions, the element distribution in the typical area of weld metal was given and the measured results are as shown in Figure 9. It was inferred from Figure 9A–C that the white particles phases were mainly composed of Ti and C elements, which assumed to be the formed TiC phases. XRD results in Figure 10 also proved this assumption. Al and Si occupied the majority of the grey matrix, as depicted in Figures 9D,E. It was obvious that an enrich-Si irregular phases with light grey color presented in the Al matrix. According to the research by Chen et al. (2000) and Guo et al. (2012), the addition of Ti should also cause the presence of Ti-Al and Ti-Si compounds in weld metal. However, no Ti-Al or Ti-Si compounds were detected in the present study. This was mainly due to an insufficient Ti used to react with Al and Si.

When compared Figures 8B,C, few needle-like Al₄C₃ phases were observed in the central part of welded joints while it was easily formed near the fusion line due to only a small amount of Ti diffusing into this region. This made weld metal adjacent to fusion line the weakest position in the whole welded joint.

As reported by Chen et al. (2002), the affinity between Ti and C in welding process was greater than that of Al and C, which meant that Ti was easier to react with C under high temperature. Therefore, the chemical reaction (2) would take precedence over the reaction (1). Then, the formation of the Al₄C₃ was suppressed. Instead, the newly generated white phase TiC was distributed in the weld seam. However, at a higher magnification, it was found that the TiC particles still had local segregation, and the TiC particles were not compact in combination with the Al matrix, as shown in Figure 8D. Furthermore, the Si formed during the reaction was distributed in Al matrix, as arrowed in Figure 7C. T i + S i C = T i C + S i (2)

Based on the above analysis, laser welding process with Ti interlayer possessed several advantages: the addition of Ti could obviously improve the fluidity of molten pool, promote the formation of intact microstructure and reduce the number of defects such as pores and porosity. More importantly, the presence of Ti could effectively restrict the formation of Al₄C₃ brittle phase, and generate in-situ TiC particles in the weld metal. Under this condition, the weld metal was transformed into Al matrix reinforced by TiC particles, granular Si crystals and some finer Al₄C₃ phases. Therefore, the weldability of the material and the joint quality were improved.

Cross-section and micrographs of SiC_p/6061Al obtained by ULBW-Ti process are illustrated in Figure 11. It can be seen that ULAW-Ti process could also achieve welded joints without pores and crack, compared with the former two welding process. More importantly, ultrasonic vibration would promote the wetting between melt pool and base metal, which could be verified by the smooth transition of welded joints.

In addition, Figure 11B shows that a certain number of Al₄C₃ phases with a length of 5–20 μm still retained in the weld seam. This indicated that ultrasonic vibration did not eliminate the formation of Al₄C₃ phases, but reduced the size of this needle-like phase, compared to LBW-Ti process.

Combined with the SEM micrograph in Figure 11C, another benefit of ultrasonic vibration on weld metal was to homogenize TiC particles. The weld metal made by LAW-Ti process contained many TiC particle clusters. More white TiC particles existed in the center of weld metal and less Ti diffused to the weld metal near the fusion line, which could not effectively limit the growth of Al₄C₃. Microstructural heterogeneity was induced by laser welding process with Ti interlayer.

Ultrasonic vibration could stir the molten pool, thus promoting the diffusion of active Ti element in the weld metal to the periphery of the weld seam. Absence of voids and appearance of homogeneous distribution of TiC particles were more likely to achieve under the ultrasonic vibration.

The ultimate tensile strengths (UTS) of welded joints under three different welding process are shown in Figure 12. For laser welded joints without interlayer and ultrasonic, a joint efficiency of 49.4% was obtained. Two factors could be used to explain this low value of the tensile properties. The first was pore and voids defects presented in welded joints. The other was due to the large number of needle-like phases Al₄C₃ formed in the joint during laser welding. According to the fracture path in Figures 13A,D, the welded joints obtained by LBW process initially fractured along the densely pores in the heat-affected zone under tensile stress. The relatively straight fracture path was almost perpendicular to the tensile direction.

For laser welding process with Ti interlayer, active element Ti improved the flowability of weld pool, which was favorable for the flotation and overflow of bubbles. The wettability between the reinforcing phase and Al matrix phase was also enhanced. This could be proved by the disappearance of pores and voids in the weld seam. In addition, heating and melting of Ti took away a part of the laser power and reduced the welding heat input on the composite. Huang et al. (2001) suggested that the size of Al₄C₃ decreased with the decrease of the heat input. On the other hand, element C in the molten pool reacted preferably with Ti to generate TiC particles, as described in reaction (2). The length of Al₄C₃ was significantly reduced to 10–20 μm. The tensile strength of the joint was improved about 15% compared to LBW process. However, the tensile strength of the joints produced by LBW-Ti process was still at a low level. This was elucidated as follows, laser welding with fast cooling rate weakened the convection stirring in the molten pool. Consequently, the diffusion ability of Ti element became poor. The newly formed TiC particles also displayed particle accumulated phenomenon to some extent. Therefore, many needle-like Al₄C₃ phases formed in the weld seam near the fusion line, which produced inferior joints. Figures 13B,E also confirmed that the fracture occurred along the weld metal near the fusion line under tensile stress. Compared to the fracture path of LBW process, the fracture path of laser welded joint with Ti interlayer was more curved.

For ultrasonic-assisted laser welding process with Ti interlayer, the tensile strength of the joint reached up to 275 MPa, which was 77.2% that of the base metal. This suggested that the ultrasonic vibration had positive effects on microstructural evolution. According to results in Figure 13, needle-like Al₄C₃ in the welds was further refined with the assistance of ultrasonic. Ultrasonic vibration also promoted the diffusion and migration of Ti elements in the molten pool. The formed weld metal transformed into Al matrix reinforced by dispersed TiC particle. Also, the Al matrix microstructure should be refined when ultrasonic wave was applied on the workpiece.

As depicted in Figures 13C,F, the fracture occurred at the center of the weld seam. The fracture path was most curved and the final fracture was at approximately 10° to the tensile stress, indicating that its tensile strength had been significantly improved.

Figure 14 shows micrographs of fractured tensile specimens at different conditions. It could be found from Figure 14A that some pores were presented in the upper-left part of fracture surface, which reduced the load bearing capacity. Selvam et al. (2021) also stated that these pores would cause spots of accumulated stress. The result was in accordance with microstructural analysis in Part 3.1. In addition, relatively flat fracture surface obtained by LBW-N was obvious, indicating a brittle fracture. For fracture obtained by LBW-Ti process, many large cleavage planes were found in the fracture surface, as demonstrated in Figure 14C.

Compared with the first two processes, the ultrasonic-assisted laser welded joint with Ti interlayer possessed a smaller scale of cleavage planes because of reduced the size and quantity of the needle-like phases. Some dimples on the surface of the fracture were found, indicating that the joint had a certain degree of plasticity, as shown in Figure 14D.