Abstract:
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.

Description:
TECHNICAL FIELD  
       [0001]     This invention pertains to the machining of aluminum alloy castings. More specifically, this invention pertains to the machining of aluminum castings without the use of a metalworking fluid for lubrication and/or cooling.  
       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 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.  
         [0003]     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.  
         [0004]     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.  
       SUMMARY OF THE INVENTION  
       [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.  
         [0006]     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.  
         [0007]     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.  
         [0008]     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.  
         [0009]     These and other objects and advantages of the invention will become more apparent from a detailed description of preferred embodiments which follows. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is an oblique view of a cast cylinder block for a V8 internal combustion engine for an automobile, and  
         [0011]      FIG. 2  is a photomicrograph (at 1000×) of the microstructure of Aluminum Alloy B319 casting alloy showing globules of bismuth as the soft, low melting additive for dry machining.  
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0012]     This invention is applicable, for example, in making cast parts in large volume for automotive applications. Vehicle engine and transmission parts are examples of such parts. Most automotive castings require some machining to produce surfaces to a shape and/or dimensional specification. The machining requires the uses of high quality and expensive cutting tools such as drills, reamers and milling and honing tools. Heretofore the machining has also required the use of machining fluids for part and tool protection and for machine chip removal. The machining practices have required close management to produce high quality cast parts with good tool life and related management of machining costs.  
         [0013]     This invention is applicable to the making of cast aluminum parts and enables dry machining of surfaces of the casting without uneconomical reduction of cutting tool life. Cast aluminum parts are made from many known casting alloys. Among those commonly used for automobile parts are, for example, Aluminum Alloys 319.0, B319.0, 333.0, 336.0, A356.0, 356.0, A360.0, A380.0, 381.0, 383.0, and 390.0. The principal alloying components of these commercial alloys in nominal parts by weight are as follows: 319-Si6Cu3, B319-Si6Cu4Mg, A356-Si7Mg, 333-Si9Cu3, 336-Si12Cu, 356-Si7Mg(Fe), A356-Si7Mg, A360-Si10Mg, A380-Si8Cu3Fe, 381-Si10Cu4Fe, 383-Si10Cu2Fe1, and 390-Si17Cu4Fe1. These alloys also contain other elements as impurities or as additives, each of which may affect the physical, chemical or mechanical properties of the cast product. In accordance with this invention, however, small additions of one or more of bismuth, indium, lead, and or tin are made to aluminum alloys such as these alloys for dry machinability. The addition of one, or a combination, of these lubricity-imparting elements is in the range of about 0.5% to about 2% by weight of the casting.  
         [0014]     FIGS.  1  is an oblique, outline view of a cast aluminum engine cylinder block  10  for a V8 engine. Such an engine component is often cast from an aluminum casting alloy such as a 319 alloy, a 356 alloy, or a 390 alloy. Such castings, especially if they are of a complex part such as cylinder block  10 , require a substantial amount of machining in their manufacture to finished parts. For example, each of the eight cylinder bores  12  (four are visible in  FIG. 1 ) is honed to a close dimensional tolerance and degree of roundness. At the top of cylinder bores  12 , cylinder block casting  10  has a deck portion  14  that is machined very flat to seal with a cylinder head casting, not shown. Several bolt holes  16  are bored or drilled from deck surface  14  for secure attachment of a cylinder head on each V-portion of cylinder block  10 . As is known and illustrated in  FIG. 1 , an engine block casting has many bolt holes, coolant passages, oil passages and the like that are drilled or drilled and reamed or otherwise machined in the manufacture of such a cast product. And there is a long succession of such castings in an engine production line so that machining operations and the cost of machining tools is very important in such a manufacturing operation. It is now found that in many applications of the machining of aluminum castings, the addition of a suitable quantity of soft, low melting point element permits the dry machining of the thus self-lubricated cast alloy surface.  
         [0015]     Aluminum alloy B319 is a casting alloy used in cylinder block, cylinder head and inlet manifold applications. The specified composition of B319 is, by weight, 5.0% to 7.5% silicon, 3.0% to 5.0% copper, 1.0% max iron, 0.1% to 0.6% manganese, 0.1% to 0.5% magnesium. 0.3% max nickel, 2.0% max zinc, 0.3% max lead, 0.1% max tin, 0.15% max titanium, a total of 0.15% other elements and the balance aluminum. A specific B319 alloy that was free of lead and tin was used as a starting material in the following examples and tests.  
         [0016]     Drilling tests without any machining fluid were conducted on a cast plate of B319 alloy to obtain baseline dry machining data. The macro-hardness of the surface of the plate was determined to be 74 to 80 Brinell and its microhardness was 90 Knoop units. In the machining tests, commercial one-quarter inch diameter, tungsten carbide drills were used to drill closed end holes to a depth of three-quarters of an inch. Only twelve holes could be drilled in the unlubricated B319 plate before the drill had to be discarded. The drilling of the twelve holes required an average power of 2.8 Kw and torque values reaching 2.6 Nm.  
         [0000]     The Modified B319 Alloys  
         [0017]     Samples of the B319 aluminum alloy were then modified by the addition of bismuth. The bismuth-containing B319 material was prepared as follows.  
         [0018]     Bismuth needles (½-in length by ⅛-in wide at mid-section) were added in the desired amount (0.2%, 0.5% and 1% by weight in these examples) to melted aluminum B319 alloy at 1360° F. using a perforated spoon/ladle. The needles were gently stirred and dispersed into the melt with the spoon moving the melt in a circular pattern with the needles held at a level of about two inches below the melt surface. This was continued for about two minutes and then the melt was held at temperature for 30 minutes. The alloy melt was then stirred for one minute and degassed with nitrogen gas using a rotary degasser at 650-700 rpm for about 15 minutes (for a normal melt of 30 lbs). The alloy melt was then gently skimmed and the temperature stabilized at 1310° F. for about 5 minutes before the crucible was pulled out of the furnace. The alloy, having cooled to 1260° F., was poured into Zircon sand molds. Following shakeout and cleaning, the cast plates were heat treated using a conventional T-5 aluminum alloy heat treatment schedule to minimize bismuth segregation.  
         [0019]     B319 aluminum casting alloys were prepared respectively containing, by weight, 0.2% bismuth, 0.5% bismuth and 1.0% bismuth.  FIG. 2  is a photomicrograph at 1000-fold magnification showing a portion of a bismuth-containing B319 casting. The photomicrograph shows a matrix material of Al—Si eutectic material  200  and globular bismuth  202  adhered to needles of AlFeSi intermetallic phases  204 . While the eutectic acicular silicon needles make a casting more difficult to machine, the small amount of soft bismuth globules markedly increase its machinability.  
         [0020]     Tin, indium and/or lead-containing aluminum containing alloys can be prepared in similar manner with care taken to disperse the lower melting additive into the melt of aluminum casting alloy.  
         [0021]     Hardness Testing of the Bi Modified B319 Material:  
                                                                     Microhardness   Macrohardness           (Knoop)   (Brinell)                                        Conventional B 319   90   74 to 80           B 319 + 0.5% bismuth   90   74                      
 
         [0022]     It is seen that the addition of 0.5% by weight of bismuth did not appreciably reduce the surface hardness of the cast plates. But, as will be seen, the bismuth additions did change the machinability of the plates. Microstructure analysis of conventional B 319 and Bi containing B 319 showed no difference among the two alloys except that small globules of elemental bismuth are clearly visible in the B319 alloy with 0.5% bismuth added as seen in  FIG. 2 . These small globules are believed to be responsible for the lubricious characteristics of the alloys containing bismuth and/or tin, indium and lead.  
         [0023]     Dry machining tests where rows of one-quarter inch closed holes were drilled to a depth of three-quarters inch using the separate self-lubricating bismuth-containing B319 alloy plates showed lower horsepower and torque, good hole integrity, small chip size, and at least two orders of magnitude improvement in tool life compared to dry machining of the conventional B319 alloy plate.  
                                                                             Tool Life   Power   Torque           (no. of holes)   (Kw)   (Nm)                                        Conventional Al B319   12   2.8   2.6           Al B319 + 0.2% Bi   667   2.8   2.0           Al B319 + 0.5% Bi   &gt;5000   1.9   1.5           Al B319 + 1.0% Bi   &gt;5000   1.8   1.2                      
 
         [0024]     The benefits to dry machining of Bi containing B319 aluminum alloy are thus demonstrated. The tool life and power consumption values are comparable to those obtained when machining bismuth-free B319 alloy castings using machining fluids. And the costs of handling and disposing of the fluids is avoided. Benefits of dry machining are still appreciable even if it is not practical to use dry machining in all operations on a casting or family of castings.  
         [0025]     Similar improvements in the machinability of the family of aluminum casting alloys are obtained by additions of about one-half percent to about two percent by weight of one or more of bismuth, indium, lead and/or tin. As stated, these elements can be used alone or in combination within the specified limits of 0.5 to 2%. Bismuth and/or tin are preferred because of a combination of relatively low cost and ease of handling.  
         [0026]     The practice of the invention has been illustrated by additions of single elements to a specific aluminum casting alloy in a series of drilling tests. However, these lubricity adding elements may be beneficially used either individually or in combination in other casting alloys and in other machining operations. The scope of the invention is limited only by the following claims.