Unfortunately, this plot has limited predictive value, since the area fraction of defects on the fracture surface is only a post mortem value. However, it shows foundrymen the dan- gers of even small amounts of porosity; and it informs us as to the metal cleanliness needed to produce the highest qual- ity castings. It also shows clearly that elongation to fracture is the most sensitive indicator for the presence of porosity or other defects in castings.


Fatigue Failure


Aluminum castings are often used in structural components subject to cycles of applied stress. Over their commercial lifetime millions of stress cycles may occur. In these appli- cations it is important to characterize their fatigue life. This is especially true for safety critical applications, such as au- tomotive suspension components.


The commercial importance of fatigue has provided the mo- tivation for extensive studies by automotive and university researchers. What follows is, by necessity, a simplified ap- proach to the problem of fatigue failure. The intention is to offer practical guidelines for design engineers, so they know what to look and ask for in a casting.


We consider a simple case, illustrated in Figure 10. This shows a piece of material subjected to a tensile stress in the vertical direction (arrows). In the center is a circular-shaped crack. Because the crack provides no mechanical strength, stress accumulates there. The stress intensity parameter is:


Equation 8


where a is the crack radius and σ is the applied stress. When the load is applied 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 usually represented by this equation:


Equation 9


where N represents the number of times the stress has been applied and C is an empirical constant. Equation [9] applies to the intermediate region (stage II) of crack growth, and is called the Paris Equation. For aluminum alloys the exponent (n) is very nearly equal to four, which means that:


Equation 10


In other words, the crack growth rate is proportional to the square of the radius, or to the area of the crack.


Now suppose we have two cracks present in the material. The larger crack is twice as large as the smaller one. From equa- tion [10] we find that the larger crack will grow four times as fast as the smaller crack and as it becomes larger with each fatigue cycle, it continues to grow faster and faster. The prac- tical implication is obvious: Fatigue life is controlled by the largest ‘crack’. In most commercial castings, this is the largest pore. For practical purposes, large pores are built-in cracks.


An example of how a pore may nucleate a fatigue failure is shown in Figure 11. (This fatigue test specimen was taken from an A356-T6 alloy casting.)16


The effect of pore size on fatigue has been shown clearly in several recent studies. Fatigue tests were made with A356- T6 alloy at a stress ratio, R = -1. Several types of castings were studied: two gravity poured castings, a squeeze cast- ing and a semi-solid metal (SSM) casting. These castings exhibited a range of defect sizes; the high pressure die cast


Figure 10. Crack in fatigue failure. 14


Figure 11. Fracture surface of fatigue specimen showing incipient pore.16


International Journal of Metalcasting/Winter 11


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