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Dry machinable aluminum castings

Additions of small but effective amounts of one or more of bismuth, indium, lead and/or tin to an aluminum casting alloy markedly improved the dry machinability of a casting made from the modified alloy. The added elements, which are softer and lower melting than the matrix alloy form as small globules in the microstructure of the aluminum casting. These globules do not adversely affect the strength or hardness of the casting but enable surfaces of the casting to be machined without the use of a cooling and lubricating machining fluid.

Aluminum alloy castings are used in making many articles of manufacture. In the automobile industry, for example, many engine and transmission parts, chassis parts, body parts and interior parts are made of silicon-containing aluminum alloy castings. Many of these parts such as engine blocks, cylinder heads, crank cases, transmission cases and the like are initially formed as castings using sand molding, permanent mold, high pressure die casting and lost foam processes. These casting processes are capable of forming complex shapes to reasonably close tolerances. But after the castings have been trimmed, ground and cleaned by sand blasting (or various other blast-cleaning processes), many surfaces of the parts still have to be machined to specified dimensions within very close tolerances.

Engine and transmission castings, for example, may require precision machining processes such as milling, honing, and/or drilling and reaming. In these machining processes the casting is carefully positioned in a fixture and a cutting tool, carried and powered by an operator or computer controlled machine tool, cuts a cast surface to remove chips of cast metal to bring the surface to a specified finish and dimension. During the metal removal operation the machined surface is flooded with a machining fluid for the purposes of cooling and lubricating the region impacted by the cutting tool. The lubrication promotes cutting by minimizing adherence of tool and work. Ultimately, the machining fluid is drained from the machining area for recovery and re-use, or for disposal.

It is an object of this invention to provide a method for making aluminum alloy castings that can be machined without the use of a machining fluid. In accordance with this invention such a practice is termed ¡°dry machining.¡± It is a more specific object of this invention to provide an aluminum alloy casting that can be dry machined.

The relatively high silicon content of aluminum casting alloys increases the difficulty with which they are machined and has required the use of a machining fluid, typically a liquid based fluid. The purpose and goal of this invention is to accomplish dry machining of certain compositionally modified aluminum alloy castings without damage of the part and with tool life that is comparable to fluid lubricated and cooled machining.

In accordance with the invention suitable silicon-containing, aluminum casting alloys are modified to contain relatively small amounts of certain finely dispersed elements that are softer and lower melting than the aluminum casting alloy matrix material, and which significantly increase the machinability of surfaces of a casting into which they are incorporated. These elements include bismuth, indium, lead and tin and one or more of them may be added to the casting alloy. These lubricity-imparting additives are not very soluble in the solidified aluminum rich matrix phase of the castings although they may combine with alloying constituents such as magnesium. Thus, they are dispersed as very small, globular bodies in the cast metallurgical microstructure. And in this form, the dispersed phase of low melting elements surprisingly enables drilling and other metal removal machining of surfaces of the casting without the use of machining fluids. Sufficiently low amounts of one or more of soft elements are added to the casting alloy so that the dispersed, relatively low melting, soft phase (either as a pure additive phase or mixed with another constituent of the alloy in a low melting phase) is present in the solid casting more or less uniformly through the casting, and surfaces of choice can be machined regardless of the position of the machined surface.

Aluminum casting alloys typically contain a significant amount of silicon to increase the fluidity of the molten phase for castablity and mold filling. Silicon is also added to reduce the thermal expansion of the casting, as well as to increase its corrosion and wear resistance. The silicon content of aluminum alloys for casting may range from about four percent to about eighteen percent by weight of the cast alloy. Aluminum casting alloys for automotive applications also contain suitable amounts of one or more of copper, iron, manganese and/or magnesium for solid solution strengthening and for formation of strengthening phases. Other alloying constituents or impurities such as nickel, zinc, titanium, chromium and rare earth elements may also be present in the casting alloy to enhance the physical properties of a cast product.

But in accordance with this invention, small additions of one or more of bismuth, indium, lead and/or tin are made to these casting alloys for internal lubricity and dry machining of the castings. Typically a total of at least about 0.5% by weight of low melting elements, alone or in combination, is added to the melt before casting. Preferably the total addition of these soft, lubricity imparting elements does not exceed about two percent by weight of the casting so that the other properties of the casting are not significantly altered. Bismuth and/or tin are preferred additives.

These and other objects and advantages of the invention will become more apparent from a detailed description of preferred embodiments which follows.