Crack Tendency and Stress Generation of Aluminum Alloys

In the process of solidification and shrinkage, the alloy is prone to crack because of excessive external resistance. There is a general tendency of cracking in aluminium die casting, but the tendency of cracking in Al-Si and Al-Mg alloys is greater.

Cracks are different from hot cracks and cold cracks. Observation of cracks shows that the shape of cracks is tortuous and irregular, the surface is strongly oxidized, the glossy cracks are hot cracks, and the head is cold cracks.

Hot cracking occurs during solidification of alloys, i.e. most alloys have formed dendritic skeleton and begin to solidify, but there are still a few liquids in dendritic parts. At this time, the linear shrinkage of alloys is the largest and the thermal strength of alloys is very low. For example, the shrinkage pressure is produced when the strain of alloys is larger than the allowable strain of alloy strength at this temperature.

Cold cracking is caused by severe obstruction or external force during solid shrinkage, such as premature sand drop or mechanical damage during cleaning. Cold cracking rarely occurs.

When the solidification and cooling process of the casting is blocked, the stress will be deformed. The residual stress will exist in the casting within the elastic strength range, which will reduce the design strength of the casting. The deformation of the casting will occur during the use and processing, which will affect the accuracy and assembly of the casting.

Casting stress consists of thermal stress, phase transformation stress and shrinkage stress. Casting stress of die castings can be eliminated by natural failure of annealed cargo.