The present invention relates generally to improved high strength aluminum base alloys having improved hot tear resistance when solidified into cast products.
It is well-known that alloying ingredients improve the properties of an aluminum-based material. High-strength aluminum alloys (which typically contain less than ninety five percent by weight aluminum, and possess a heat-treated yield strength in excess of approximately thirty thousand pounds per square inch) can be made compatible with low-cost fabrication techniques such as casting. For example, the addition of silicon (Si) to aluminum results in an improved ability of the molten metal to flow, thereby improving castability. Other alloying agents, such as copper, can be used to improve the strength characteristics of aluminum-based alloys, although traditionally at the expense of castability. Others may include both.
One such alloy, created by the Assignee of the present invention, is known as GM220, an alloy that can be easily cast into engine blocks, heads and other components for high-strength, weight-sensitive applications. Nevertheless, many aluminum alloys such as GM220 are not able to fully realize the efficient, low cost manufacturing that casting has to offer because of the tendency to form a defect known as hot tearing. In hot tearing, the strength of the solidifying melt is insufficient to withstand the stress related to thermal contraction of the metal due to solidification shrinkage and thermal expansion. Such phenomenon is exacerbated when the mold is more rigid, such as in metal molds.
Modified fabrication techniques, such as mold thermal control and higher pouring temperatures, can be used to reduce this problem; however, such approaches are often limited by the geometry of the molded part, as well as the complexity and concomitant expense. Likewise, casting simple billets with extensive post-cast machining significantly increase the cost, and therefore defeat the purpose of low-cost casting operations.
Grain refining is a preferred way to reduce the likelihood of hot tearing in castable aluminum alloys. In grain refining, a master alloy (for example, a titanium (Ti)-containing or related agent) is added to the molten metal just prior to pouring. In one well-known form, such master alloy may be a metal carbides, nitride and boride, such as titanium diboride (TiB2). It is believed that these master alloys introduce microscopic particles as nucleating agents into the melt.
Despite the addition of such master alloys, it is still desirable that an improved method of grain refinement of high strength, aluminum-based casting alloys be developed. It is further desirable that such grain refinement be useful in the production of an aluminum-based alloy without the attendant problem of hot tearing.