A simple test for assessing solidification cracking susceptibility and checking validity of susceptibility prediction

Abstract

Cracking during solidification is a serious defect in welding and casting. The crack susceptibility of one Al alloy relative to that of another has been predicted recently using the maximum steepness |dT/d(fS)1/2| as the crack susceptibility index, where T is temperature and fS the fraction solid. To check the validity of predictions based on the index, a simple test for the solidification cracking susceptibility was developed. In this test a stationary sheet was lap welded to a sheet moving at speed V in a straight line normal to the welding direction. V is the transverse tensile deformation rate of the mushy zone. As V was increased from zero, the transition from no crack propagation to full crack propagation occurred over a range of V, called the transition range. The crack susceptibility is lower if the transition range is located at a higher V level because the fact that a faster deformation rate V is required to cause crack propagation means less crack susceptibility. It was shown that the V level of the transition range increased, i.e., the crack susceptibility decreased, in the order of 6061 Al, 7075 Al, 2024 Al and 2219 Al. This is consistent with the predicted one. Thus, the present test can be used to check the validity of predicting the crack susceptibility based on the crack susceptibility index. The present test addressed some of the issues in the Varestraint test, in which the workpiece is bent suddenly during welding to induce cracking, thus inducing an unrealistically high deformation rate and unable to determine the deformation rate of the mushy zone that causes cracking. Other advantages of the present test were also discussed. Since the Varestraint test has long been the most widely used test for studying the important subject of solidification cracking, the present test is particularly significant.