In the process of aluminum alloy welding, due to the different types, properties and welding structures of materials, various cracks can appear in the welded joints, and the shape and distribution characteristics of the cracks are very complex. According to their generated parts, they can be divided into the following two types of cracks form:
(1) Cracks in weld metal: longitudinal cracks, transverse cracks, crater cracks, hair or arc cracks, root cracks and microcracks (especially in multi-layer welding).
(2) Cracks in the heat-affected zone: weld toe cracks, laminar cracks and microscopic thermal cracks near the fusion line. According to the temperature range of crack generation, it is divided into hot crack and cold crack. Hot crack is generated at high temperature during welding, which is mainly caused by the segregation of alloy elements on the grain boundary or the existence of low melting point substances.
Depending on the material of the metal to be welded, the shape, temperature range and main reasons for the occurrence of hot cracks are also different. Hot cracks can be divided into three categories: crystallization cracks, liquefaction cracks and polygonal cracks. Crystallization cracks are mainly produced in hot cracks. During the crystallization process of the weld, near the solidus line, due to the shrinkage of the solidified metal, the residual liquid metal cannot be filled in time.
Intergranular cracking occurs under the action of solidification shrinkage stress or external force, which mainly occurs in carbon steel, low alloy steel welds and some aluminum alloys with more impurities. Liquefaction cracks are generated under the action of shrinkage stress during solidification of grain boundaries heated to high temperatures in the heat-affected zone.
During the test, it was found that when the surface of the filler material was not sufficiently cleaned, there were still many inclusions and a small amount of pores in the weld after welding. In the three sets of tests, since the welding filler material is a casting structure, which contains high melting point substances, it will still exist in the weld after welding, and the casting structure is relatively sparse and has many holes, which is easy to absorb the components and oil containing crystal water. quality, they will be a factor in the generation of porosity during the welding process. When the weld is under tensile stress, these inclusions and pores often become the key sites for inducing microcracks.
Further observation by microscopy revealed that there was a clear tendency for these inclusions and pore-induced microcracks to intersect with each other. However, it is still difficult to judge whether the harmful effect of the inclusions is mainly manifested as a stress concentration source to induce cracks, or it is mainly manifested as a brittle phase to induce cracks.
In addition, it is generally believed that pores in aluminum-magnesium alloy welds do not have a significant impact on the tensile strength of the weld metal. phenomenon of cracks.
Whether the phenomenon of porosity-induced microcracks is only a secondary phenomenon or one of the main factors causing a substantial decrease in the tensile strength of welds remains to be further studied.
