Abstract
The solubility of hydrogen in liquid and solid aluminum is reviewed. Based on classical nucleation theory, it is shown that pores cannot nucleate either homogeneously or heterogeneously in liquid aluminum. Results of in situ studies on pore formation show that pores appear at low hydrogen supersaturation levels, bypassing nucleation completely. The results are explained based on the bifilm theory introduced by Prof. John Campbell, as this theory is currently the most appropriate, and most likely, the only mechanism for pores to form. Examples for the effect of hydrogen on pore formation are given by using extreme data from the literature. It is concluded that a fundamental change in how hydrogen is viewed is needed in aluminum casting industry.
Highlights
Pores have been observed [25] in situ to initiate hot tears, which are common in high strength cast aluminum alloys, such as the Al-Cu system
Understanding pore formation is paramount to lowering production costs, increasing their quality and performance, and their wider use
Classical nucleation theory is applied to formation of hydrogen pores in aluminum castings
Summary
There are many studies in the literature in which pores have been shown to degrade mechanical properties, such as tensile strength [1,2,3,4,5,6,7,8,9,10,11,12,13], elongation [6,7,8,11,14,15,16], and fatigue life [17,18,19,20,21,22,23,24] of aluminum alloy castings. Understanding pore formation is paramount to lowering production costs, increasing their quality and performance, and their wider use. It is understandable why prevention or at least minimization of the number and size of pores has been the main emphasis of foundry engineering research for several decades. Casting processes such as squeeze casting and post-casting processes such as hot isostatic pressing have been used to prevent and heal the pores in castings, respectively. A proof-by-contradiction method is used to compare assumptions, observations, and the principles of physics, and to determine the most appropriate mechanism of hydrogen pore formation
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