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Aluminum Alloy Forging Coarse Grain Problem
Jun 22, 2018

Aluminum alloy forging coarse grain problem.

Various types of deformed aluminum alloys have a tendency to form coarse-grained structures with different degrees. The coarse-grained structure is less noticeable by forgings and die forgings, whereas 2A1 (LD10), 2A70 (LD7), 2A02 (LY2) alloys, especially 6A02 (LD2), are often found to have coarse-grained structures.


For example, 7A04 (LC4) alloys on forgings from 2A50 (LD5), 2A11 (LY11), and 2A16 (LY16) alloys


The coarse grains of the forgings are mostly caused by coarse crystals or overheated structures of raw materials. The reason for local coarse grain is complicated.


The surface coarse crystals of the forgings are mostly the result of the remaining of coarse-grained rings on the surface of the forgings. The coarse-grained ring on the surface of the aluminum alloy bar deteriorates the plasticity, and cracks are easily generated during forging. When the hammer is forged, it should start tapping, breaking the surface coarse-grained ring, increasing the plasticity of the metal, and then gradually increasing the blow to make the metal full of mold.


In localized parts where the degree of deformation of the forgings is small, coarse crystals are often caused by the degree of critical deformation. In order to avoid the formation of coarse crystals during recrystallization, the deformation at the final forging temperature is to avoid the critical deformation (12% to 15%). In regions where the degree of deformation is large and the relative flow of metals is intense, the orientation of the crystal grains tends to be uniform, and the recrystallization energy is high, and aggregation recrystallization may occur to form coarse crystals upon subsequent heat treatment.


In addition, excessive deformation, excessive number of forgings, rough mold surface, low final forging temperature, high quenching temperature, and long time will lead to coarse grains. For example, coarse crystals are particularly likely to occur in the vicinity of the forging flash area.


The coarse-grained structure will significantly reduce the tensile strength, yield strength, and elongation of the forgings, and have an adverse effect on fatigue strength and seismic performance. Proper technological measures should be taken to prevent coarse-grain defects. Generally speaking, it is necessary to consider: selecting reasonable blank shape and size, reasonable blank making and pre-forging steps, reasonable deformation-temperature conditions, improved tools, smoothness of mold surface, uniform lubrication, and the like.