Cracks were found on aluminum cast components made of the material EN AC-43000 (AlSi10Mg(a)). These cracks were corrected with a repair weld. To avoid stress peaks caused by welding, the component was then solution annealed again and aged hot. As part of a material analysis quality control in the laboratory of TAZ GmbH, it should be validated whether the repair process is acceptable for the customer. For this purpose, the corresponding areas should be prepared metallographically and documented with the light microscope or the laser scanning microscope. The metallographic section sample was contrasted for a better representation of the weld seam and the microscopic structure. The common pickling solution according to Keller was slightly modified and optimized by our metallographer. The weld seam was first shown macroscopically. It can be seen that welding was done from both sides. Since several weld nuggets can be detected, a multi-layer weld seam can be assumed. At low magnification, significantly finer dendrites can be seen in the weld compared to the base material.
Anomalies were then documented in detail. Above all, small gas pores can be detected in the welding zone.
After the macroscopic examination, the microstructure was documented in the laser scanning microscope. The welding filler material used shows a near-eutectic, homogeneous structure with a dendritic alpha phase and a moulded, refined eutectic. The refinement can be recognized by the formed, spheroidal particles in the eutectic, which would appear elongated and needle-like without heat treatment.
The base material shows a similar picture. Here, too, a dendritic alpha phase and a moulded-in, refined eutectic can be seen, but there are also elongated AlFeSi phases. This suggests a slightly different chemical composition between the base material and the filler material.
The micrographs were also checked with a hardness test. For this purpose, a linear hardness curve was drawn up from the base material via the center of the weld seam to the opposite base material and the hardness profile was shown graphically. It can be seen that the filler metal is about 50 HV softer than the base material. The conspicuously low hardness value at 15 mm is probably due to a gas pore that happened to be hit exactly.
Conclusion: The microstructure and the hardness values indicate that the solution annealing was carried out correctly after the repair welding. The increased hardness of the base material also indicates (hot) aging. Because the hardness is significantly lower and no elongated AlFeSi phase can be found, it can be assumed that a different material was selected as the welding filler material.