From a practical
Also, the crack growth rate is propor- tional to the area of the crack. Imagine you have two cracks in the
material, and the larger crack is twice as large as the smaller one. In this case, the larger crack will grow four times as fast as the smaller crack, and as it becomes pro- gressively larger with each fatigue cycle, it continues to grow faster. Terefore, fatigue life is controlled by the largest “crack,” or void, in the material. In most commercial castings, this is the largest pore, which acts as a built-in crack. In addition to tensile properties,
solidification rate affects pore size and fatigue strength. According to indus- try data, pore diameter is reduced by rapid solidification and grain refine- ment. Tese considerations have been confirmed experimentally. Unfortunately, determining the size of the largest pore by standard metal- lographic examination is not straight- forward. Te distribution of porosity in commercial castings is non-uniform, so extreme value statistics must be used to overcome this concern. From a practical point of view,
control of porosity is the single most important factor in obtaining good fatigue life in net-shaped castings. Compared to porosity, the strength level of the alloy (as determined by heat treatment or alloy composition) is less important. Tis means that the best results are obtained by: • good degassing and melt treatment; • effective grain refinement ; • proper modification practice;
point of view, control of porosity is the most important f
actor in obtaining good fatigue life in net-shaped castings.
• rapid solidification. In addition, casting processes that
apply pressure to solidifying castings reduce the amount of porosity and size of the resulting pores. It is also possible to reduce porosity in castings by the use of hot isostatic pressure treatment. Te amount of porosity in a cast- ing depends on several factors. Listed roughly in order of importance, they are: • solidification rate; • gas content; • metal cleanliness; • pressure in the casting; • modification; • grain refinement. Oxide films also are an important
source of casting defects. A thin oxide film forms on the surface as soon as liquid aluminum comes in contact with air. If the liquid is quiescent, the oxide stays on the surface and does not affect metal quality. However, if there is any turbulence or splashing, the oxide film is mixed or folded into the melt, and the quality of the casting suffers.
One important practical implica-
tion on metal quality is that oxide films assist in porosity formation. Hydrogen has low solubility in solid metal—only 6% of the gas is soluble in the liquid, so it tends to form dispersed micro-pores during solidification. Because of the high surface tension of liquid metal, the pores cannot nucleate homogenously, so gas pores form on oxide films in the melt. If the metal is filtered carefully, no porosity will form, even if significant amounts of gas dissolve in the metal. Te following operation is seen in
many metalcasting facilities. Metal is pumped into a crucible, which is then transferred by forklift or crane before it is poured into a holding furnace. Ten, a ladle dips out of the holding furnace to pour metal into the mold. Each transfer of liquid metal produces an aluminum waterfall. Te resulting splashing gener- ates oxide films, which are folded into the liquid and carried into the casting. In contrast, a level-pour transfer has been employed for many years by primary aluminum suppliers. A launder or trough carries liquid metal from the melting furnace to the casting pit. Once the launder is filled, the metal flows in the lower part of the channel, and the surface is quiescent. Because the transfer is level, there are no waterfalls or oxide films folded into the melt. Normally, the metal also passes through an in-line degasser and a filter box. With these practices and treatment, castings that are low in gas can be produced nearly free from oxide films and other inclusions. Unfortunately, one last hurdle must
be cleared for improving the quality of aluminum castings. Metal must enter and fill the mold in a way that does not produce additional oxides or other defects. Runner design’s effect on tensile and fatigue properties in aluminum castings represents the last frontier for metalcasters. Designing a runner to eliminate turbulence in mold filling would allow metalcast- ers to produce oxide-free, net-shaped castings having the same reliability as forgings and machined wrought alloy components.
Casting facilities can transfer molten aluminum from the furnace to the mold in a trough through an automated pouring system, which can lead to reduced gas and inclusion-free castings.
34 | MODERN CASTING June 2011
Geoffrey Sigworth is a technical specialist in aluminum melting for Foseco, Cleveland. Tis paper was adapted from a paper originally presented at the 115th Metalcasting Congress (Paper # 11-013).
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