Determination of metal quality of aluminium and its alloys
Aluminium alloy castings are being used increasingly in safety-critical applications in the automotive and aerospace industries. To produce castings of sufficient quality, it is, therefore, important to understand the mechanisms of the formation of defects in aluminium melts, and important to have a reliable and simple means of detection. During the production of aluminium ingots and castings, the surface oxide on the liquid is folded in to produce crack-like defects (bifilms) that are extremely thin, but can be extremely extensive, and so constitute seriously detrimental defects. However, the presence of bifilms has not been widely accepted, because there has been no single metal quality test that has been able to resolve features that are only nanometres, or sometimes micrometres, in thickness. In the past, porosity has usually been held solely responsible for most failures in aluminium alloys, and hydrogen has been blamed as the actual cause. In this work it has been found that bifilms are the initiator and hydrogen is only a contributor in the porosity formation process. For the first time, evidence is presented for the contribution of air (or perhaps more strictly, residual nitrogen from air) as an additional gas, adding to hydrogen in pores in cast Al alloys. The Reduced Pressure Test (RPT) is used which is a simple and widely known test, is cost effective and involves no complicated equipment or consumables, thus recommending it for implementation on the foundry floor. On this basis, several Al-Si based alloys were studied. A quality index -Bifilm Index- is introduced to quantify the results of the reduced pressure test. In addition, mechanical tests were carried out to correlate bifilm index with mechanical properties.