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 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 a dry machining method that uses a combination of a machinable aluminum alloy casting composition and a cutting tool with a carbon-based coating on the working surfaces of the tool.  
       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. 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 sp 2  and sp 3  molecularly bonded carbon atoms) or with microcrystalline diamond particles (sp 3  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.  
         [0011]     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  
       [0012]      FIG. 1  is an oblique view of a cast cylinder block for a V8 internal combustion engine for an automobile, and  
         [0013]      FIG. 2  is a bar graph summarizing number of holes drilled (a measure of cutting tool life) for castings of a representative B319 aluminum alloy composition, with and without lead additions, using tungsten carbide drills with and without a diamond-like carbon coating on the cutting surfaces of the drills.  
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0014]     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 use 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. The machining practices have required close management to produce high quality cast parts with good tool life and related management of machining costs.  
         [0015]     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, 390.0, and 396.0. 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, 390-Si17Cu4Fe1, and 396-Si11Cu2.25Fe0.45. Among those used for aerospace parts are, for example, Aluminum Alloys 4215F—Si5Cu 1.2Mg0.5, 4218G-Si7Mg0.5, 4219G-Si7Mg0.5, and 4241-Si7Mg0.58. 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.  
         [0016]     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. For many dry machining applications the addition of one, or a combination, of these lubricity-imparting elements is suitably in the range of about 0.03% to about 2% by weight of the casting. The additive-containing aluminum alloy casting is machined with a cutting tool having a carbon-based coating on the cutting surfaces. In general, it is preferred to use tungsten carbide cutting tools in the practice of this invention where the cutting tools are provided with a DLC coated or microcrystalline diamond coated cutting surfaces. The use of a cutting tool with a suitable carbon-based coating in the dry machining of a soft metal additive-containing aluminum alloy casting can produce dramatic improvements in tool life.  
         [0017]      FIG. 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, a 390 alloy, or a 396 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  18  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, a combination of a carbon coated tungsten carbide cutting tool with a soft, low melting point element-containing aluminum alloy casting permits practical, high throughput dry machining of the self-lubricated cast alloy surface.  
         [0018]     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 containing &lt;0.02% lead was used as a starting material in some of the following examples and tests.  
         [0019]     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, uncoated cemented tungsten carbide drills were used to drill closed end holes to a depth of three-quarters of an inch. The drilling of such closed end holes is considered a particularly challenging operation for successful dry machining. Only eleven holes could be drilled in the unlubricated B319 plate before the drill had to be discarded. The drilling of the eleven holes required an average power of 3.8 Kw and torque values reaching 2.0 Nm.  
         [0000]     B319 Alloys Modified with Lead  
         [0020]     Samples of the B319 aluminum alloy (&lt;0.02% by weight lead) were then modified by the addition of lead. The lead-containing B319 material was prepared as follows.  
         [0021]     Lead particles were added to attain the desired amount (0.03%, 0.05%, 0.08%, and 0.15% by weight in these examples) to melted aluminum B319 alloy at 1360° F. using a perforated spoon/ladle. The particles were gently stirred and dispersed into the melt with the spoon moving the melt in a circular pattern with the particles 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 lead segregation.  
         [0022]     B319 aluminum casting alloys were prepared respectively containing, by weight, 0.03%, 0.05%, 0.08%, and 0.15% lead. Examination of the cast materials confirmed that the lead was distributed as fine globules throughout the microstructure of the casting. The casting also contained eutectic acicular silicon needles. While the silicon needles make a casting more difficult to machine, the small amount of soft lead globules were used to increase its machinability.  
                                                 Hardness Testing of the Pb-modified B319 Material:                    Microhardness   Macrohardness           Substrate   (Knoop)   (Brinell)                       Conventional B319   90   74 to 80           B319 + 0.15% lead   90   74                      
 
         [0023]     It is seen that the addition of 0.15% by weight of lead did not appreciably reduce the surface hardness of the cast plates. But, as will be seen, the lead additions did change the machinability of the plates especially when tungsten carbide cutting tools with diamond-like carbon coatings and microcrystalline diamond coatings were used.  
         [0000]     Tool Life Tests using Lead-Containing B319 Castings and Tungsten Carbide Drills Coated with Carbon-Based Materials  
         [0024]     In the following tests lead-containing B319 aluminum alloy cast plates were used. Casting alloys and cast plates were prepared respectively containing, by weight, 0.03% lead, 0.05%, lead, 0.08% lead, and 0.15% lead. Holes were drilled in the cast material with uncoated tungsten carbide drills, and carbide drills having a diamond-like carbon coating (DLC) or a microcrystalline diamond coating on their cutting surfaces. Tungsten carbide drills are, of course, commercially available as are tungsten carbide cutting tools that have DLC coatings or microcrystalline diamond coatings on their cutting surfaces. DLC coatings comprise a bonded mixture of graphite (sp 2  carbon) particles and diamond (sp 3 ) particles. Cutting tools are also available with microcrystalline diamond coatings but they are more expensive than DLC coated tools. These carbon-based cutting tool coatings lower the coefficient of friction between tool and substrate.  
         [0025]     Dry machining tests were conducted on the lead-containing cast plate substrates with one-quarter inch DLC coated tungsten carbide drills, one-quarter inch microcrystalline diamond coated drills, and, for comparison, with uncoated tungsten carbide drills. The uncoated drills and microcrystalline diamond coated drills were rotated at a speed of 61 m/min and fed into the cast substrates at a rate of 0.18 m/rev. The DLC coated tungsten carbide drills were rotated at a speed of 213 m/min with a feed rate of 0.18 m/rev. Rows of ¼ inch closed holes were drilled to a depth of ¾ inch using the respective coated and uncoated tungsten carbide drills.  
         [0026]     Comparisons in drill life during dry machining were first made with uncoated tungsten carbide drills and tungsten carbide drills coated with DLC. Results of these tests are presented graphically in  FIG. 2 . As a baseline test, an uncoated tungsten carbide drill was used to drill one-quarter inch holes in a plate of commercial B319 alloy with &lt;0.02% by weight lead. Only 11 holes were drilled in the unlubricated plate before the drill had to be discarded. This value of 11 holes drilled is the left-most entry bar entry in  FIG. 2 . In a second baseline test, a plate of B319 alloy containing 0.08% by weight lead was used as the casting specimen and one-quarter inch holes were drilled with an uncoated tungsten carbide drill. The increased soft metal content permitted the drilling of 58 holes before the drill became unusable. This data is illustrated graphically in the second-from-left bar entry in  FIG. 2 . In a third dry machining test, a tungsten carbide drill with a DLC coating was used to drill one-quarter inch holes in a plate of commercial B319 alloy with &lt;0.02% by weight lead. Ninety-eight, one-quarter inch holes were drilled by dry machining in the unlubricated plate before the drill became unusable. The gradual increase in drill life is illustrated in  FIG. 2 .  
         [0027]     A dramatic increase in drill life was obtained when a tungsten carbide drill with a DLC coating on its cutting surface was used to drill holes in a plate of B319 alloy containing 0.08% by weight lead. More than 4000, one-quarter inch, closed end holes were drilled before the drill was unable to form additional good holes. This result demonstrates a synergistic effect for dry machining in the combination of the use of a carbon-coated tungsten carbide drill and a lead-containing B319 aluminum alloy.  
         [0028]     A series of comparative dry machining tests was also conducted using uncoated tungsten carbide drills and tungsten carbide drills coated with bonded microcrystalline diamond particles. The following table summarizes tool life and power and torque requirements on the various substrates and using either coated or uncoated tungsten carbide drills. In each test the drilled substrate was a base B319 aluminum alloy or the base alloy modified with an indicated addition of lead.  
                                                                     Lead   Drill   Tool   Power   Torque       (wt. %)   Coating   Life   (kW)   (Nm)                                0.00   None   11   3.8   2.0       0.03   None   13   3.2   2.4       0.05   None   22   2.8   2.0       0.03   Diamond   760   0.6   1.6       0.05   Diamond   &gt;3500   0.5   1.3       0.08   Diamond   &gt;3500   0.4   1.1       0.15   Diamond   &gt;10,000   0.4   1.2                  
 
         [0029]     The benefits to dry machining of Pb-containing B319 aluminum alloy using carbon-coated tungsten carbide drills are thus demonstrated. The tool life and power consumption values are comparable to those obtained when machining additive-free B319 alloy castings using machining fluids.  
         [0030]     The practice of this invention has been illustrated by the presence of single lubricious elements in 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.