Dry machining of soft metal-modified aluminum castings with carbon-coated tools

Abstract

Additions of small but effective amounts of one or more of bismuth, indium, lead and / or tin to a silicon-containing aluminum casting alloy markedly improves the dry machinability of a casting made from the modified alloy. But dry machining of such castings is synergistically improved by the use of cutting tools with cutting surfaces coated with microcrystalline diamond or diamond-like carbon coatings. The use of such carbon-coated tools enables remarkable improvement in dry machinability even at reduced levels of the soft metal in the alloy.

Description

TECHNICAL FIELD [0001] This invention pertains to the machining of silicon-containing aluminum castings without the use of a metalworking fluid for lubrication and / or cooling (dry machining). More specifically this invention pertains to the markedly improved dry machining of castings of silicon-aluminum alloys containing microstructurally-dispersed soft metal additives while using cutting tools with carbon-based coatings. BACKGROUND OF THE INVENTION [0002] 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...

AI Technical Summary

Benefits of technology

[0005] 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. This invention provides a synergistic improvement in dry machining of such castings by using a combination of a relatively small amount of soft-metal additive in the cast alloy and a carbon-coated cutting tool. Such combinations have provided benefits in dry machining tool life that were unforeseen from optimized use of either practice alone.

[0006] The invention uses a cutting tool, such as a drill, that has a durable carbon-based coating on the cutting or working surfaces. The combination of the aluminum alloy composition and the coated cutting tool permits practical dry machining of aluminum castings with relatively small amounts of dry machining additive and long tool life. Indeed, preferred combinations of soft metal additive-containing aluminum alloys and tungsten carbide cutting tools with carbon-based coatings have demonstrated surprisingly successful dry machining benefits.

[0007] 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.

[0008] 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 and other applications such as aerospace 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.

[0009] 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 to enable dry machining of the castings. Typically a total of at least about 0.03% by weight of low melting elements, alone or in combination, is added to the melt before casting. The minimum amount of the additive depends on the casting composition, its microstructure, and on the selected additive(s). Preferably the total addition of one or more 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.

[0010] In general it is preferred to use tungsten carbide cutting tools in dry machining operations of this invention for a suitable combination of durability and cost. The cutting surfaces of the cutting tools are coated with a carbon-based material in dry machining of aluminum castings. For example, it is preferred to perform the machining using tungsten carbide drills that are coated with a diamond-like carbon coating (DLC, a combination of particles of sp2 and sp3 molecularly bonded carbon atoms) or with microcrystalline diamond particles (sp3 bonded). The use of such coated carbide cutting tools often permits dry machining of aluminum castings with a smaller amount of the soft metal additive(s) in the cast alloy.

Problems solved by technology

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.

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.

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