Critical assessment of reduced pressure test. Part 1: Porosity phenomena

Abstract During the production of aluminium ingots and castings, the surface oxide on the liquid may be folded into the bulk liquid to produce crack-like defects (bifilms) that are extremely thin, but can be extensive, and so constitute seriously detrimental defects. In this work, it has been found that bifilms are the initiator and hydrogen is only a contributor to 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 discriminating use of the RPT clearly reveals the existence of bifilms, and the effect of hydrogen on porosity formation. However, it seems that the RPT is of little use to evaluate the hydrogen content of the alloy. To investigate these effects, two alloys were studied in laboratory experiments LM0 (99.5%Al), LM4 (Al– 5Si–3Cu) and one in an industrial environment LM24 (Al–8Si–3Cu–Fe).

[1]  Julie Huang,et al.  Simulation of microporosity formation in modified and unmodified A356 alloy castings , 1998 .

[2]  J. Conley,et al.  Modeling the effects of cooling rate, hydrogen content, grain refiner and modifier on microporosity formation in Al A356 alloys , 2000 .

[3]  E. Chang,et al.  An analysis on porosity formation in A356 aluminium alloy castings , 2002 .

[4]  J. Campbell,et al.  Liquid metal quality , 2002 .

[5]  V. Voller,et al.  A model of inverse segregation: the role of microporosity , 1995 .

[6]  H. Nagaumi.  Prediction of porosity contents and examination of porosity formation in Al–4.4%Mg DC slab , 2001 .

[7]  M. Rappaz,et al.  Modeling of inverse segregation and porosity formation in directionally solidified aluminum alloys , 1995 .

[8]  Y. W. Lee,et al.  Modeling of feeding behavior of solidifying Al-7Si-0.3Mg alloy plate casting , 1990 .

[9]  A. L. Maples,et al.  A thermodynamic prediction for microporosity formation in aluminum-rich Al-Cu alloys , 1987 .

[10]  G. Sigworth,et al.  Mechanisms of porosity formation during solidification: A theoretical analysis , 1993 .

[11]  Kambiz Vafai,et al.  Development and investigation of three-phase model of the mushy zone for analysis of porosity formation in solidifying castings , 1995 .

[12]  J. C. Fisher.  The Fracture of Liquids , 1948 .

[13]  Michel Rappaz,et al.  Modeling of casting, welding and advanced solidification processes-V : proceedings of the fifth International Conference on Modeling of Casting and Welding Processes, held in Davos Switzerland, September 16-21, 1990 , 1991 .

[14]  R. D. Pehlke,et al.  Mathematical modeling of porosity formation in solidification , 1985 .

[15]  G. Sigworth,et al.  Evolution of porosity in long freezing range alloys , 1993 .