When applied to castings, the term “quality” refers to both degrees of soundness (freedom from porosity, cracking and surface imperfections) and levels of mechanical properties (strength and ductility). The microstructural features that most strongly affect mechanical properties are
- (a) grain size and shape
- (b) dendrite-arm spacing
- (c) size and distribution of second phase particles and inclusions
Castings with fine, equiaxed grains have the best combinations of strength and ductility. To some extent, the size and shape of grains can be controlled by the addition of grain refiners, but use of low pouring temperatures and high cooling rates are the preferred methods.
All commercial aluminum alloys contain multiple phases, as well as oxide and gas inclusions. During freezing, inclusions and second-phase particles segregate to the spaces between dendrite arms. The farther apart the dendrite arms are, the coarser the distribution of micro constituents and the more pronounced their adverse effects on properties. Thus, small interdendritic spacing is necessary for high casting quality. Although several factors affect spacing to some extent, the only efficient way of ensuring fine spacing is the use of rapid cooling.
The finer the dispersion of inclusions and second-phase particles, the better the properties of the castings. Fine dispersion requires that particles be small; large masses of oxides or intermetallic compounds produce excessive brittleness. Controlling the size and shape of micro constituents can be done to some extent by controlling composition, but is accomplished more efficiently by minimizing the period of time during which micro constituents can grow. Like minimizing grain size and interdendritic spacing, minimizing the time for the growth of micro constituents calls for rapid cooling. Thus, it is evident that a high cooling rate is of paramount importance in obtaining good casting quality.
However, it should be kept in mind that in die casting, although cooling rates are very high, air tends to be trapped in the casting, which gives rise to appreciable amounts of porosity at the center. Extensive research has been conducted to find ways of reducing such porosity; however, it is impossible to eliminate completely and die castings often are lower in strength than low-pressure or gravity-fed mold castings, which are more sound in spite of slower cooling.
The production of “premium-quality” castings is an example of understanding and using the solidification process to a good advantage. In the production of premium-quality castings, composite molds combining several mold materials are used to take advantage of the special properties of each casting process. High mechanical properties in designated areas are obtained by the use of special chills, Plaster sections and risers may be used to extend the feeding range of the casting.
Premium-quality castings are made to the tight radiographic and mechanical specifications required for aerospace and other critical applications. Basic linear tolerances of + 15 mils/in. are possible with aluminum alloy castings, depending on mold material, equipment, and available fixtures, and a minimum wall thickness of 3.8 mm (0.15 in.) is typical. Aluminum alloys commonly poured as premium-quality castings include C355, A356, and A357.
- Federal QQ
- B26/B26M – 14
- SAE AMS-STD-2175
- Conflict of Materials
- Test Bars
- Material Certification