Selective laser melting (SLM) can be used to prepare functionally gradient materials (FGMs) for local customization of performance. In this study, CuSn10/AlSi10Mg functional gradient materials were prepared by SLM, and the effect of the material composition ratio on the microstructure of the CuSn10/AlSi10Mg transition layer was investigated. The phase and quantity of the transition layer were calculated using CALPHAD, the microstructural evolution of the interface region of the gradient materials was discussed based on electron backscattering diffraction (EBSD) results, and the formation mechanism of cracks in the interface region was revealed. The results show that the microstructure of the CuSn10/AlSi10Mg transition layer consists of a matrix of Al₄Cu₉ and Al₂Cu with columnar and fine equiaxed grains. In the transition layer zone (from the copper alloy side to the aluminum alloy side), with an increase in the AlSi10Mg content, the matrix content does not change significantly, whereas the content of Al/Cu intermetallic compounds changes sharply. The Al₄Cu₉ phase first precipitates and its content gradually decreases, whereas the Al₂Cu phase precipitates later and its content gradually increases, and a large amount of Al/Cu intermetallic compounds are generated around the cracks. The main reason for the formation of severe cracks in the transition zone is that the directly generated Al₄Cu₉ phase is prone to large volume changes (4.4%), leading to stress concentration and initial microcracks. The large volume change (4.3%) caused by the transformation of the Al₂Cu phase and Cu enriched in the matrix into the Al₄Cu₉ phase (indirectly generated) further exacerbates the stress concentration and ultimately leads to macrocracking. Avoiding the direct and indirect generation of Al₄Cu₉ is the primary means of solving the problem of cracking. The microhardness of the transition layer is higher than that of the matrix on both sides. The highest hardness is observed at the crack (804 HV), similar to that of Al₄Cu₉.

In this study, CuSn10/AlSi10Mg gradient functional materials are prepared by SLM through two gradient paths (19 and 16 layers of different compositional gradients are designed for samples 1 and 2, respectively). The microstructures of the different transition regions are observed by optical microscopy (OM) and scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). To reveal the microstructural evolution, the phase compositions of the transition regions are measured using X-ray diffraction (XRD) and EBSD. Finally, the microhardness is measured using a microhardness tester to understand the changes in mechanical properties.

Sample 1 (19-layer transition composition) prepared using SLM forms more cracks, generates transverse cracks, and causes macroscopic cracking throughout the sample. Sample 2 (16-layer transition composition) forms slight cracks, and the transverse cracks disappears. Although both transition compositions have cracks, the 19-layer transition is significantly more severe than the 16-layer transition. More importantly, nearly all the cracks are generated in the Al-rich transition region (Fig. 2). During the printing process, the distribution of Al along the deposition direction gradually increases from zero at the beginning to a uniform distribution at the end, which is consistent with the spot scanning results. However, both Cu and Sn are uniformly distributed throughout the transition region, which further confirms that the CuSn10 alloy is continuously remelted and is then diffused to the upper layer during the printing process, resulting in the enrichment of Cu in the region of the Al alloy (Fig. 4). In the transition region, the phases mainly consist of the matrix phase α-Cu/α-Al and Al/Cu intermetallic compounds, and the intermetallic compounds are mainly Al₄Cu₉ and Al₂Cu. From the Cu alloy side to the Al alloy side, the content of the matrix does not change significantly with the addition of the Al alloy. In addition, Al₄Cu₉ first precipitates and then gradually decreases, and it is dominant at 40% AlSi10Mg. With a continuous increase in the Al alloy content, the Al₂Cu phase precipitates later and gradually increases, exceeding the Al₄Cu₉ phase at 50% AlSi10Mg content (Figs. 5?7).

The microstructure of the SLMed CuSn10/AlSi10Mg gradient material is composed of columnar and fine equiaxed grains that grow in the direction of the center of the molten pool, and the equiaxed grains close to the boundary of the molten pool have a random grain orientation. In the transition region, the phases mainly consist of α?Cu/α?Al matrix and Al/Cu intermetallic compounds, and the intermetallic compounds are mainly Al₄Cu₉ and Al₂Cu. From the Cu10Sn side to the AlSi10Mg side, with the addition of the Al alloy, the content of the matrix does not change significantly, but Al₄Cu₉ first precipitates and gradually decreases, and it dominates at 40% AlSi10Mg. With a continuous increase in the aluminum alloy, the Al₂Cu phase precipitates later and gradually increases, exceeding the content of the Al₄Cu₉ phase at 50% AlSi10Mg. A large amount of the Al₄Cu₉ phase is generated around the microcracks in the transition region. However, a large amount of the Al₂Cu phase is generated around the macrocracks, and nearly all cracks mainly occur in the Al-rich transition region. The volume change of the generated Al₄Cu₉ is the highest (4.4%), and the reaction between Al₂Cu and the Cu matrix forming the Al₄Cu₉ phase exhibits the second-highest volume change (4.3%), whereas the volume change forming the Al₂Cu phase is only 0.3%. The Al₄Cu₉ phase nucleates in both Al- and Cu-rich solid solutions, whereas the Al₂Cu phase can only nucleate in the Al-rich region. Therefore, the reason for crack formation is that the direct generation of the Al₄Cu₉ phase in the transition region is prone to forming a stress concentration that generates the initial microcracking. The indirectly formed Al₄Cu₉ (the reaction between Al₂Cu and excess Cu in the matrix) causes a large volume change and further aggravates the stress concentration, resulting in severe macrocracks. Avoiding the generation of the Al₄Cu₉ phase (including direct and indirect formations) is the primary means of solving the cracking problems. The microhardness of the transition layer region is affected by the intermetallic compound content. From the Cu alloy side to the Al alloy side, the microhardness first increases and then decreases, and it is higher in the transition region than that of the substrate. This trend is consistent with the number of intermetallic compounds. The highest microhardness (804 HV) is observed at the cracks, which is very close to that of Al₄Cu₉. This further verifies that enriched intermetallic compounds are the main reason for crack formation.