developed empirically to represent the resulting quality index:
Q=UTS + 150 log E
where Q=Quality Index (MPa), UTS=Ultimate Tensile Strength (MPa) and E=Elongation to Fracture (%) The index has the following qualities:
• It describes the relative tradeoff between strength and elongation. If you need increased strength or elon- gation in a casting, the heat treatment can be changed accordingly. The same effect may also be obtained by increasing or decreasing magnesium content in Al-Si-Mg alloys.
• It allows buyers of castings to compare two different castings that may have received different heat treatments, or whose chemical com- position is different.
• It gives an indication of improvements that might be made in any particular casting.
• Because the iso-quality and iso-yield strength contours appear as straight lines in the quality plots, the relationship indicates all important tensile properties. Experimental measurements and theoretical considerations have shown
that the maximum value of the quality index possible in Al-Si-Mg (A356/357) alloy castings is 470-500 Mpa. This value is obtained in castings that so- lidify in about 30 seconds or less.
Exploring Fatigue Failure Aluminum castings are often used in
structural components subject to cycles of applied stress. Over their commer- cial lifetime, millions of stress cycles may occur. In these applications, it is important to characterize the compo- nents’ fatigue life. This is especially true for safety critical applications, such as automotive suspension components. The commercial importance of fa-
tigue has provided the motivation for extensive studies by automotive and university researchers. A simplified ap- proach to the problem of fatigue failure can offer practical guidelines for design engineers, so they know what to look and ask for in a casting. Consider a piece of material subjected to tensile stress in the vertical direction. In the center is a circular-shaped crack. Because the crack provides no mechanical strength, stress
accumulates there. When the load is ap- plied cyclically to the material, the crack grows a little bit each time the load is applied. We want to know how fast the crack grows, and how long it will take for failure to occur. The growth of the crack with each cycle is related to the number of times the stress has been applied. Also, the crack growth rate is proportional 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 progressively larger with each fatigue cycle, it continues to grow fast- er. Therefore, 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 industry data, pore diameter is reduced by rapid solidification and grain refinement. These considerations have been con- firmed experimentally. Unfortunately, determining the size
of the largest pore by standard metal- lographic examination is not straight- forward. The 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. This means that best results are obtained by: • good degassing and melt treatment; • effective grain refinement ; • proper modification practice; • rapid solidification. Also, some new casting processes
apply pressure to solidifying castings to 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. Numer- ous studies have shown that fatigue life can be improved in this way.
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40 Metal Casting Design anD PurChasing MarCh/aPril 2011
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