Aluminum alloy die forgings are widely used in parts with medium and high strength because of their small specific gravity, good thermal conductivity, high conductivity and corrosion resistance. The technological characteristics of heating die forging of aluminium alloy are poor fluidity, low plasticity, good thermal conductivity, narrow forging temperature range, strict initial and final forging temperature requirements. The performance characteristics of heating die forging are that aluminium alloy does not produce isomeric transformation, mainly depends on the correct control of forging mechanical parameters to improve the metal structure, so that metal streamlines are evenly and continuously distributed along the shape of forgings to improve mechanical properties. The quality characteristic of heating die forging is that aluminum alloy is easy to produce two major quality defects, folding and cracking.
Among them, folding can reduce the bearing area of parts, and it is easy to produce stress concentration in service and become the source of fatigue failure, which has great harm. The folding waste accounts for 70%-80% of the total forging waste, and the cracking waste accounts for 5%-10% of the total forging waste.
The causes of folding are large cross-section size of billet, unreasonable shape of billet, partial pressing forming, small radius of fillet at the transition point of die chamber, and too much reduction at one time during operation.
Cracks are mainly caused by the obvious anisotropy of extruded aluminium bars, in which the longitudinal mechanical properties are significantly higher than the transverse mechanical properties (the longitudinal toughness is the largest, while the transverse toughness is the smallest). At the beginning of final forging, under the condition of unequal three-dimensional stress or non-three-dimensional compressive stress, there exists maximum shear stress, which can easily lead to the failure of grain-to-grain relationship, which is not conducive to the development of slip deformation, and oblique cracks are caused by poor deformation ability.
At the same time, the additional stress caused by uneven deformation and the thermal stress caused by uneven temperature are larger, and the part with large deformation interacts with the part with small deformation. When the tensile stress exceeds the strength of this part, the cracking will occur.