Patent Publication Number: US-2002003012-A1

Title: Forged scroll parts and production process thereof

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
     [0001] This application is an application filed under 35 U.S.C. §111(a) claiming benefit pursuant to 35 U.S.C. §119(e)(1) of the filing date of Provisional Application 60/230,807 filed Sep. 7, 2000 pursuant to 35 U.S.C. §111(b). 
    
    
     
       FIELD OF THE INVENTION  
       [0002] The present invention relates to an aluminum alloy-made forged part for an orbiting scroll and/or a fixed scroll, which is assembled into a scroll compressor employed mainly in an air conditioner; and to a process for producing the forged part.  
       BACKGROUND OF THE INVENTION  
       [0003] In recent years, scroll compressors have become of interest as air conditioner compressors. One reason is that such a scroll compressor contains a small number of parts and is driven silently. The scroll compressor includes a fixed scroll having a spiral wrap portion  11 , as shown in FIG. 1, and an orbiting scroll having a spiral wrap portion whose shape is similar to that of the portion  11 . The spiral wrap portion of the orbiting scroll is driven for orbital movement so that these spiral wrap portions face each other.  
       [0004] In many cases a fixed or orbiting scroll (hereinafter simply referred to as a “scroll”), which serves as a main part of a scroll compressor, is produced from aluminum alloy in order to reduce the weight of the compressor. The scroll is produced by, for example, casting or forging. In order to provide a scroll with strength and reliability, forging is advantageously carried out for producing the scroll. Since the scroll has a complicated shape, it must be produced through hot forging.  
       [0005]FIG. 2 shows a conventional production process for an aluminum alloy scroll part by forging.  
       [0006] Usually, a round bar material obtained through extrusion is employed as a stock material for forging. Firstly, an aluminum alloy prepared by mixing alloy components and melting is cast through continuous casting into a billet (BL) for extrusion having a large diameter of 200 mmφ or more (“φ” used herein represents “diameter”.). After the inside of the BL is homogenized through heat treatment, the BL is cut into pieces such that they have identical volumes to provide round bars, each having predetermined length and diameter, and each piece is subjected to extrusion to form a round bar.  
       [0007] Usually, the diameter of the extruded round bar is almost equal to the outer diameter of a forged part. The round bar is cut into pieces, and the pieces are employed as a stock material for forging. As described below, if necessary, the cut piece may be previously shaped by forging or machining into a piece having a shape similar to that of the scroll part in order to facilitate production of a scroll part, before forging of the stock material to employ the shaped piece as a stock material for forging.  
       [0008] The stock material is forged into a scroll part usually through hot forging. In order to provide the forged part with strength, the forged part is usually subjected to solution (quenching) and aging heat treatment after forging.  
       [0009] Thereafter, if necessary, a portion of the surface of the part is subjected to machining in order to enhance precision in the size of the forged part.  
       [0010]FIG. 4 is a schematic cross-sectional view showing a conventional forging process for a scroll. A workpiece  4  placed in a die  2  is pressed downward with a punch  1  to form the wrap portion  11 . Usually, the distance that the punch  1  moves is determined to be consistent in order to make the thickness of a flange portion  12  of the scroll consistent.  
       [0011] In order to precisely forge a workpiece into a scroll wrap, Japanese Patent Application Laid-Open (kokai) Nos. 54-159712, 59-61542, and 62-89545 disclose a process for forging an aluminum alloy-made scroll, in which the workpiece is subjected to forging or machining in advance to provide the piece with a previous shape, and then the workpiece is forged into the scroll wrap. The reason why such a process is carried out is as follows. The wrap portion  11  has a spiral shape, the height of the portion is large, and the wrap portion is connected to the flange portion  12 . Therefore, when a workpiece is forged into a scroll as shown in FIG. 4, forming a wrap portion having a uniform height is difficult, and thus a workpiece having an intermediate shape is formed in advance. When the process is carried out, the produced scroll is provided with a shape with some degree of precision. However, the process requires designing an intermediate shape which matches the final shape of the scroll, and preparation of a forging die employed for intermediate processing. Consequently, the process includes complicated steps and involves high costs, presenting difficulty in practice.  
       [0012] Japanese Patent Application Laid-Open (kokai) Nos. 60-102243 and 06-23474, among other publications, disclose a back pressure forging process in which a workpiece prepared only by cutting a round bar is employed without subjecting the workpiece to pre-processing before forging, and, during forging of the workpiece, a load is applied to the end portion of a scroll wrap  11  in a direction opposite a forging direction in order to control material flow to realize a uniform flow into a wrap-shaped mold and to reduce variation in the height of the scroll wrap  11 . According to the process, by using a workpiece prepared only by cutting a round bar, a scroll in which there is a reduction in variation in the height of a wrap portion  11  can be produced at low cost and high productivity.  
       [0013] The process will be described in more detail. In the back pressure forging process for a scroll, as shown in cross-sectional views of FIGS. 5 and 6, a workpiece  4  is pressed with a punch  1 , and the workpiece is forged into a die space for wrap formation  2   a  of a die  2  to form a wrap. During forging, a back pressure lower than a punch pressure is applied through knock pins  7  and knockouts  6  to the end of the wrap in a direction opposite that of forging thereby making the height L 2  of the wrap uniform, as shown in a cross sectional view of a forged part (FIG. 7).  
       [0014] The back pressure forging process regulates, to some extent, the effect for making the height of a spiral wrap of a forged scroll part uniform.  
       [0015] However, although variation in the height of a wrap of a scroll is regulated to some extent through the back pressure forging process, wrap height varies between individual scrolls unless the thickness of individual cut materials; i.e., the weight of individual workpieces, is strictly controlled. Therefore, a margin for machining of the end of a wrap must be controlled in every forged part during a post-processing step. Alternatively, in consideration of different wrap heights among scroll products, slightly-large-sized scrolls must be forged to provide scrolls with a large margin for machining during a post-processing step, resulting in low yield.  
       [0016] In the forging process, when a workpiece is forged into a scroll, the thickness (L 1 ) of a flange portion is controlled by a stroke of the punch  1 , and the remaining portion of the workpiece is forged into a wrap portion. Therefore, the difference in the volume of the workpiece before forging is reflected in the difference in the height (L 2 ) of the wrap portion.  
       [0017] Conventionally, in order to smoothly carry out forging of a workpiece without production loss, the workpiece is prepared by cutting a round bar material having a diameter nearly equal to the maximum outer diameter of a forged scroll (i.e., the outer diameter of a flange portion). Therefore, variation in the thickness of the cut material is reflected in variation in volume of the workpiece; i.e., variation in the height of a wrap portion of the scroll.  
       [0018] The area of a horizontal cross section of a wrap portion is about ⅓ to ⅕ that of a horizontal cross section of a workpiece. Accordingly, the variation in the cut length of the workpiece is multiplied by a factor of 3 to 5 in height of the wrap portion. Therefore, since a margin of the end of the wrap for machining in a post-processing step cannot be reduced, the amount of time for machining of scrolls cannot be reduced, and material-based yield cannot be enhanced.  
       [0019] In consideration of conditions under which scrolls are used, an aluminum alloy material containing a large amount of silicon is employed for producing a scroll to enhance strength and wear resistance of the scroll. The material is hard, and thus a blade for cutting the material is easily worn. Therefore, compared with a conventionally-used alloy, variation in the thickness of the aluminum alloy material increases during cutting, greatly affecting variation in wrap height between individual forged scrolls.  
       [0020] As described above, an aluminum alloy material is employed for producing a scroll in order to reduce the weight of the scroll. In consideration of the balance between strength, wear resistance, and processability, Al-Si alloy materials have mainly been developed among a variety of aluminum alloy materials. When characteristics of the material are regulated to impart wear resistance to the material, fine Si particles are uniformly dispersed in an aluminum base. Development of alloy materials other than Al-Si alloy materials has encountered difficulty at present. Therefore, such an alloy material is not employed in practice, and basically modifications of Al-Si alloy materials has been carried out.  
       [0021] In such an Al-Si alloy material, crystallization of Si particles is necessary for enhancing wear resistance of the material. However, crystallization of coarse primary Si crystals having a size of tens of μm or more causes wear of a blade during machining, resulting in a product to having a rough machined surface. In addition, when such coarse primary Si crystals segregate at a portion of a scroll subjected to high stress, fatigue breakage initiates at the portion where the scroll is employed, greatly impairing reliability of the scroll. Furthermore, as described above, when such an Al-Si alloy material is cut, wear of a blade is accelerated. Thus, variation in the thickness of the material increases during cutting. Therefore, such an Al-Si alloy material preferably has a structure in which crystallization of coarse primary Si crystals is suppressed and fine eutectic Si particles having a size of several μm are uniformly dispersed.  
       [0022] As described above, in a conventional production process, such an aluminum alloy material is formed through cutting into an extrusion round bar material. In order to form the round bar material, the alloy material is usually cast through continuous casting into a billet (BL) having a relatively large diameter (200 mmφ or more). Therefore, the billet is solidified slowly during casting, and thus crystallization of coarse primary Si crystals having a size of 100 μm or more tends to occur, and control of the distribution of Si particles in the billet is difficult. Furthermore, as described above, variation in the thickness of the billet may occur during cutting. In addition, primary Si crystals remain in a forged scroll product as a large, hard impurity, and the crystals may cause problems in machining of the forged scroll and reduction in strength thereof.  
       SUMMARY OF THE INVENTION  
       [0023] The present invention provides a forged scroll part employed in a scroll compressor and a production process for the scroll part, which is produced from an aluminum alloy material which enables reduction in variation of wrap height within a forged part and between forged parts, reduction in a margin for machining in post-processing, and suppression of occurrence of coarse primary Si crystals that would cause wear of a blade during machining and reduction in strength of the forged scroll part.  
       [0024] In order to solve the aforementioned problems, the present invention provides:  
       [0025] (1) a forged scroll part produced from an aluminum alloy material comprising:  
       [0026] Si: 8.0-12.5 mass %;  
       [0027] Cu: 1.0-5.0 mass %; and  
       [0028] Mg: 0.2-1.3 mass %,  
       [0029] wherein the scroll part contains substantially no Si particles having a size of 15 μm or more, and the mean Si particle size is 3 μm or less.  
       [0030] If necessary, the forged part may further comprise:  
       [0031] Ni: 2.0 mass % or less; and/or  
       [0032] one or more species selected from among Sr, Ca, Na, and Sb: total 0.5 mass % or less.  
       [0033] The strength of the forged part is enhanced through solution heat treatment, quenching, and aging, and, if necessary, the forged part is subjected to machining and is imparted with characteristics satisfactory for a practically-used scroll part.  
       [0034] The present invention also provides:  
       [0035] (2) a process for producing the forged part, which comprises a step for casting an aluminum alloy material into a round bar having a diameter of 130 mmφ or less through continuous casting of an aluminum alloy material comprising Si: 8.0-12.5 mass %, Cu: 1.0-5.0 mass %, and Mg: 0.2-1.3 mass %; a step for cutting the aluminum alloy round bar into a stock material for forging; a step for subjecting the stock material to upset at an upsetting ratio of 20-70% to form a pre-shaped product (hereinafter “a workpiece”); and a forging step for applying pressure onto the workpiece with a punch at a temperature of 300-450° C. to form a scroll wrap in a direction of punch pressing, wherein the forging step includes a single step in which a forged scroll part is press-formed while back pressure is applied to the end of the wrap of scroll part in a direction opposite that of a punch pressure.  
       [0036] The present invention also provides:  
       [0037] (3) a process for producing the forged part, which comprises a step for casting an aluminum alloy material into a rod having a diameter of 85 mmφ or less through continuous casting of an aluminum alloy material comprising Si: 8.0-12.5 mass %, Cu: 1.0-5.0 mass %, and Mg: 0.2-1.3 mass %; a step for cutting the aluminum alloy round bar into a stock material for forging; a step for subjecting the stock material to upset at an upsetting ratio of 20-70% to form a workpiece; and a forging step for applying pressure onto the workpiece by use of a punch at a temperature of 300-450° C. to form a scroll wrap in a direction of punch pressing, wherein the forging step includes a single step in which a forged scroll part is press-formed while back pressure is applied to the end of the wrap of scroll part in a direction opposite that of a punch pressure.  
       [0038] In the aforementioned production process, the alloy material may further comprise Ni: 2.0 mass % or less, and/or one or more species selected from among Sr, Ca, Na, and Sb: total 0.5 mass % or less. Preferably, after completion of forging, the forged part is subjected to homogenization heat treatment at 480-520° C. for 30 minutes to four hours, and/or to surface peeling.  
       [0039] Preferably, a work lubrication process in which the workpiece is coated with graphite film in advance is carried out in combination with a die lubrication process in which a graphite-containing oily lubricant is applied to a die as a lubrication process during forging. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0040]FIG. 1 is a schematic representation of a forged scroll part.  
     [0041]FIG. 2 is a flow chart of a process in which a conventional material for extrusion is employed as a stock material for forging.  
     [0042]FIG. 3 is a flow chart of the process of the present invention.  
     [0043]FIG. 4 is a schematic cross-sectional view showing a conventional forging process for a scroll.  
     [0044]FIG. 5 is a cross-sectional view of a material, a punch, and a die before the forging step of the present invention.  
     [0045]FIG. 6 is a cross-sectional view of a material, a punch, and a die during the forging step of the present invention.  
     [0046]FIG. 7 is a schematic representation showing a cross-sectional view of a forged scroll part. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0047] The present invention will be described in more detail.  
     [0048] The aluminum alloy scroll is usually produced from an Si-containing aluminum alloy in order to impart wear resistance to the scroll. Crystallization of fine particles of added Si enhances wear resistance of the scroll against another scroll.  
     [0049] When the content of Si contained in the alloy is about 11 mass % or less, fine eutectic Si particles having a size of several μm dispersedly crystallize in an Al base in proportion to the content of Si, and the Si particles enhance wear resistance of the alloy scroll. Therefore, the content of Si is preferably high. When the content of Si contained in the alloy is less than 8.0 mass %, a sliding part, such as a scroll formed from the alloy, exhibits unsatisfactory wear resistance.  
     [0050] In contrast, when the content of Si contained in the alloy is in excess of 12.5 mass %, crystallization of primary Si crystals occurs. The primary Si crystals tend to become large to have a size as large as tens of μm. The large Si crystals cause wear of a blade during cutting, and cause loss of the edge of a cutting tool during machining in post-processing, resulting in a problem in finishing. In addition, when the crystals segregate at a portion in the vicinity of the outer surface of a forged part, which is susceptible to stress concentration, breakage of the forged part initiates at the portion, resulting in lowering of mechanical strength. Therefore, the upper limit of Si content is 12.5 mass %.  
     [0051] When Cu is added to the aluminum alloy in an amount of several %, strength of the Al base is enhanced through post heat treatment. Addition of Cu also contributes to enhancement of wear resistance of the alloy. However, when the content of Cu contained in the alloy is less than 1.0 mass %, Cu does not contribute to enhancement of strength of the alloy. When the content of Cu is in excess of 5.0 mass %, Cu does not contribute to enhancement of strength of the alloy commensurate with the content of Cu. Therefore, the content of Cu is preferably 1.0-5.0 mass %.  
     [0052] Mg combines with Si, precipitating in the form of Mg 2 Si in the alloy after heat treatment, and this precipitation contributes to hardening of the alloy. Mg also forms MgSiCu through precipitation after heat treatment, and the compound contributes to hardening of the alloy. Such Mg compounds enhance the strength of the alloy. When the content of Mg is less than 0.2 mass %, Mg fails to exert such a effect. When the content of Mg is in excess of 1.3 mass %, the effect of Mg does not increase commensurate with the content of Mg. In addition, an oxide generates and invades the alloy during casting, resulting in defects of the alloy. Therefore, the content of Mg is preferably 0.2-1.3 mass %.  
     [0053] If necessary, the alloy of the present invention may further contain Ni in an amount of 2.0 mass % or less. Addition of a small amount of Ni exerts an effect of enhancing heat resistance of the alloy. When the content of Ni is 0.1 mass % or less, Ni fails to exert the above effect. When the content of Ni is in excess of 2.0 mass %, large crystals generate, resulting in lowering of toughness of the alloy. Therefore, the content of Ni is preferably 0.1-2.0 mass %.  
     [0054] In the alloy of the present invention, eutectic Si particles contribute to enhancement of wear resistance of the alloy. In order to uniformly disperse the Si particles in the alloy and to suppress generation of coarse primary Si crystals, the alloy may contain one or more species selected from among Sr, Ca, Na, and Sb in a total amount of 0.5 mass % or less. Preferably, Sb is contained in an amount of 0.05-0.5 mass %, and Sr is contained in an amount of 0.005-0.05 mass %. Sr is particularly preferable, since addition of a trace amount of Sr exerts the above effect, and weight loss of the alloy during melting is small.  
     [0055] A characteristic feature of the present invention resides in the alloy composition or in the process including a melting step of the alloy and a forging step, particularly in the steps of the process. With reference to FIG. 2, there will be described a conventional production process for a forged part by use of a material for extrusion as a stock material for forging. In the conventional process, an aluminum alloy round bar for extrusion is cut into pieces and the piece is employed as a stock material for forging, since a flange portion of a scroll has a round shape and has an outer diameter of about 80-130 mmφ. Firstly, the aluminum alloy is melted, and cast through continuous casting into billets for extrusion. Each billet is subjected to homogenization heat treatment, and cut into pieces having a length of tens of cm, and each piece is extruded through an extrusion machine into a round bar having a diameter nearly equal to that of a scroll. Subsequently, the round bar is cut in a direction perpendicular to the longitudinal direction of the bar to obtain a stock material for forging. The material is heated, a lubricant is applied to the material, and then the material is subjected to hot forging.  
     [0056] When the alloy is cast into a usual billet for extrusion, the billet usually has a diameter as large as 200 mmφ or more. Therefore, since the billet is cooled slowly and solidified gradually, crystallization of coarse primary Si crystals having a size of about 100 μm easily occurs when the content of Si in the alloy is in excess of 10%. Consequently, the crystals may remain in the extruded round bar of small diameter. The primary Si crystals tend to segregate at the center portion of the billet, at which the cooling rate of the billet is particularly low. When the content of Si is nearly equal to 12%, the primary Si crystals generate randomly in the entirety of the cross section of the billet.  
     [0057]FIG. 3 shows an example of the process of the present invention. In the present invention, in order to avoid generation of coarse primary Si crystals, an aluminum alloy is cast through continuous casting into a round bar material having a diameter of 130 mmφ or less. In contrast to a conventional billet for extrusion, a round bar material having diameter of 130 mmφ or less, which is obtained through continuous casting, is cooled very rapidly and thus solidified rapidly. Therefore, eutectic Si particles become fine in the round bar material, and even when the content of Si is in excess of 10 mass %, coarse primary Si crystals, which generate in the conventional billet, do not generate in the round bar material. Particularly, in the case in which the aforementioned modification elements, such as Sr, Ca, Na, and Sb, are added to the alloy in an amount up to the Si content of 12.5 mass %, generation of primary Si crystals is substantially not observed in the round bar material; i.e., the aforementioned problem is avoided. The round bar material substantially comprises no Si particles when Si particles having a size of 15 μm or more are substantially not observed in the following process.  
     [0058] According to the process using the alloy composition, eutectic Si particles having a size of 15 μm or more are substantially not observed and the particle size is usually about 10 μm at most. The mean particle size is 3 μm or less. As used herein, the phrase “substantially not observed” refers to “the percentage of non-observation in a field of view under a microscope is 99% or more.” In the present invention, the round bar material substantially comprises no Si particles having a size of 15 μm or more since Si particles are not substantially observed in the foregoing process.  
     [0059] The particle size of Si may be directly determined from a photomicrograph of the round bar material. Preferably, the particle size is obtained through image processing by use of a microscope image analyzer (e.g., Luzex), since a correct value is obtained through this technique. As used herein, the term “particle size” refers to the diameter of a circle having the same area as that of the particle.  
     [0060] The diameter of a cast round bar material is preferably small, since the material having a small diameter is solidified rapidly. When the diameter of the material is small, eutectic Si particles in the material easily become fine, and generation of primary Si crystals is greatly suppressed. Therefore, a round bar material having a diameter of 85 mmφ or less is more preferable as a cast material from the viewpoint that such a material exhibits excellent upsetting effect as described below.  
     [0061] The material of the present invention may be cast to have a diameter that matches the outer diameter of a scroll product, and cut into a stock material for forging. A characteristic feature of the present invention is that the cast material has a diameter smaller than that of the outer diameter of a scroll product; the cast material is cut to have a length corresponding to the weight of a forged scroll part; and the cut material is subjected to upsetting to attain a desired diameter. The diameter of the material after upsetting is determined to match the outer diameter of a flange portion of the scroll product. Through cutting of the continuously cast bar material having a small diameter and upsetting of the cut material, the material exhibits improved ductility and fatigue characteristics, due to uniform dispersion of Si particles.  
     [0062] Upsetting of the cut round bar material may be carried out through free-forging; i.e., through application, under two punches, of pressure onto the material in a vertical direction to sufficiently enlarge the diameter of the material. However, upsetting of the material is preferably carried out through die-forging, in which the outer diameter of the material is determined by a die, in order to enhance precision in the diameter and the thickness of the material and to carry out scroll forging, which is the next step, at high productivity.  
     [0063] During upsetting, the upsetting ratio of the material is appropriately 20-70%.  
     [0064] The upsetting ratio is obtained by the following formula:  
     [0065] Upsetting ratio (%)  
     [0066] =100×(cross-sectional area of material after processing—cross-sectional area of material before processing)/cross-sectional area of material after processing  
     [0067] =100×(height of material before processing—height of material after processing)/height of material before processing  
     [0068] Usually, upsetting may be carried out at room temperature when the upsetting ratio is low. Preferably, upsetting is carried out after the material is heated, since the upsetting ratio can be increased. However, even in the case in which upsetting is carried out at elevated temperature, when the upsetting ratio is very high, cracking occurs on the circumferential surface of the material beyond material ductility. In addition, since the ratio of the height of the material to the outer diameter thereof becomes high, buckling of the material occurs during upsetting, and thus a high-quality upset material cannot be obtained. Therefore, in the case of the material of the present invention, the upsetting ratio is appropriately 70% or less, preferably 60% or less. When the upsetting ratio is less than 20%, the material may fail to exhibit improved ductility and fatigue characteristics. In addition, variation in characteristics of the below-described material for forging is not reduced satisfactorily.  
     [0069] When upsetting is carried out, the material is usually heated. The material may be heated before upsetting, and then subjected to upsetting. However, in order to improve the surface condition of the material during peeling and facing as described below and to enhance the shapability of the material during upsetting, the material is preferably subjected to homogenization heat treatment before upsetting. The homogenization heat treatment is appropriately carried out at 480-520° C. for 30 minutes to four hours. When the temperature is lower than 480° C., the material is not satisfactorily homogenized. When the temperature is higher than 520° C., eutectic fusion occurs at boundaries between crystal particles. The temperature is preferably 495-510° C. When the treatment time is less than 30 minutes, the material is not satisfactorily homogenized. When the time is in excess of four hours, eutectic Si particles tend to become large.  
     [0070] If necessary, the surface of the material may be subjected to peeling and facing in advance. Through peeling and facing, precision in the diameter of the material is enhanced, and the condition of the circumferential surface of a workpiece after upsetting is improved.  
     [0071] The process for casting an aluminum alloy into a round bar material having a small diameter, which includes cutting the cast material into a stock material for forging; and subjecting the stock material to upsetting to form a workpiece, has the following three advantages.  
     [0072] A first advantage is that generation of primary Si crystals is suppressed and eutectic Si particles become fine in the cast material, since the material is cooled rapidly as described above. When the cast material is subjected to plastic working to some extent, the material exhibits improved ductility and fatigue characteristics.  
     [0073] A second advantage will be described in relation to the following reason. Variation in the length of the cut cast round bar material leads to variation in the volume (weight) of the stock material for forging, which results in variation in the height of a wrap portion of a forged scroll part. The cast round bar material is usually cut by use of a round sawing machine. When the cast round bar has a small diameter, the material is accurately fed into the sawing machine to determine the length of the material, and thus variation in the length (thickness) of the cut material tends to be low. In addition, when the diameter of the cast round bar material is small, the area of the cross section of the material is small. Therefore, even if variation in the length (thickness) of the material occurs, the variation in the volume (weight) of the material is low compared with that in a material having a larger diameter. More specifically, when the diameter of the cast round bar material is small, variation in the volume (weight) of the stock material for forging becomes low, resulting in low variation in the height of a wrap portion of a forged scroll part.  
     [0074] A third advantage is enhancement of material-based yield. When the round bar material is cut into the stock material for forging having a predetermined length, unwanted pieces are obtained from both ends of the bar material, and powdery chips are generated. The amount of loss of the material attributed to the powdery chips is determined by the thickness of a cutting blade and the diameter of the round bar material. Specifically, when different stock materials having the same volume are cut from round bar materials having different diameters, the amount of powdery chips which are formed when a stock material is cut from a round bar material having a large diameter is larger than that of powdery chips which are formed when a stock material is cut from a round bar material having a small diameter. Therefore, when a stock material for forging is cut from a round bar material having a small diameter, loss of the material is reduced. As a result, the stock material for forging can be obtained at high yield, which leads to an economical benefit.  
     [0075] In view of the foregoing, when the upsetting ratio is low during upsetting of the stock material for forging, the aforementioned advantages are obtained to an unsatisfactory degree. Therefore, the upsetting ratio is 20% or more, preferably 40% or more.  
     [0076] A pre-shaped material which has undergone the aforementioned upsetting, i.e., a workpiece, is subjected to hot forging. The diameter of the workpiece is determined to match the outer diameter of a flange portion of a scroll part.  
     [0077] An aluminum alloy material is subjected to hot forging at 300-450° C., preferably at 350-450° C. When the hot forging temperature is very low, the material fails to be formed into a predetermined shape or cracking occurs in the material, whereas when the temperature is very high, swelling or buckling of the material may occur.  
     [0078] When a workpiece is subjected to hot forging, in order to prevent seizing of a workpiece into a forging die, a lubricant is usually applied to the workpiece and the die. In general, when an aluminum alloy material is subjected to hot forging, a liquid lubricant containing a mixture of graphite and water or mineral oil is widely employed. Usually, when a workpiece is forged into a product having a simple shape, satisfactory lubrication and release effects are obtained through mere spraying of a lubricant directly onto a forging die. However, in the case in which a workpiece is forged into a product having a complicated shape, when lubrication is not carried out thoroughly, a lubricant becomes short, and thus the workpiece is forged into a poorly-shaped product, or the workpiece is seized into a die and cannot be forged into the product. In order to solve such problems, a workpiece is immersed into a liquid lubricant in advance to coat the piece with a lubrication film. When a workpiece is forged into a scroll having a complicated shape, the workpiece is forged in a die having a wrap-shaped deep groove to form a wrap portion having a large height. Therefore, since a lubricant fails to cover the entirety of the wrap-contoured inner walls of the die when only spraying is carried out, shaping and release of the workpiece is not satisfactorily carried out; i.e., forging of the workpiece is difficult. In order to solve such a problem, preliminary immersion of the workpiece into the lubricant is carried out in combination with spraying of the lubricant onto the die. As a result, improved lubrication and release effects are obtained, and forging at high productivity is realized.  
     [0079] In order to coat a workpiece having lubrication film thereon, a solution prepared by mixing a solvent with a graphite lubricant is applied to the workpiece. In order to increase productivity, a lubricant prepared by diluting the solution with a rapid-drying solvent is applied or sprayed to the workpiece.  
     [0080] In a most economical process, a lubricant is prepared by mixing and dispersing graphite powder into water serving as a solvent, a workpiece is heated and then immersed into the lubricant, and the resultant workpiece is dried. In this case, the workpiece must be heated at a temperature at which water serving as a solvent is evaporated or dried within a very short time. When the heating temperature of the workpiece is lower than the boiling point of water, the lubricant fails to dry and remains on the workpiece after immersion of the workpiece; i.e., the lubricant is not rapidly dried. Therefore, the heating temperature of the workpiece must be 100° C. or higher. In consideration of productivity, the heating temperature is preferably 130° C. or higher. The upper limit of the heating temperature may be a temperature at which deterioration of the workpiece, such as melting, does not occur. Briefly, the heating temperature is 500° C. or lower, preferably 450° C. or lower. The workpiece is usually heated in a heating furnace. Alternatively, the residual heat of the workpiece after hot upsetting may be utilized; i.e., the workpiece is immersed into a lubricant immediately after upsetting. In this case, the lubricant is baked onto the workpiece which has undergone upsetting, and the workpiece is removed from the lubricant and then dried.  
     [0081] Through this procedure, cutting, heating, upsetting, lubrication, and forging may be carried out successively, resulting in high productivity.  
     [0082] Upsetting and forging may be carried out simultaneously in a single pressing apparatus. In this case, continuous production of a scroll part is possible through carrying out cutting, heating, lubrication, upsetting, and forging successively.  
     [0083] A workpiece that has undergone upsetting and lubrication is forged into a scroll as follows. If necessary, the workpiece is additionally heated, and the workpiece is pressed downward with a punch  1  into a die space  2   a  to form a wrap portion downward in the die space  2   a.  Before the workpiece is pressed with the punch  1 , knockouts  6  connected through knock pins  7  to a back pressure apparatus are inserted, in advance, in the die space  2   a  for forming a wrap, such that the knockouts  6  reach the vicinity of the upper end of the die space  2   a.  When the workpiece is pressed into the die space  2   a  to form a wrap, pressure is applied to the end of the wrap in a direction opposite the pressing direction from the back pressure apparatus through a back pressure plate  3 , the knock pins  7 , and the knockouts  6  to form the wrap having a uniform height.  
     [0084] When the back pressure is not applied to the workpiece during forging, the amount of the metal that flows into the wrap-forming portions in the die can become nonuniform. Therefore, the object of applying the back pressure is to obtain a uniform amount of metal flow into the wrap-forming portions in the die. The amount of the back pressure can regulate the uniformity of the amount of the metal flow into the wrap-forming portions in the die. Accordingly, by applying the appropriate back pressure to the workpiece, the amount of metal flow into the wrap-forming portions in the die can be uniform, and thus the height of the wrap portions of the product can be uniform. When the back pressure is very high, buckling of the wrap occurs during wrap formation, and a good product is not obtained. Therefore, when a forged part such as a scroll, in which the ratio of the area of a horizontal cross-section of a wrap portion to that of a horizontal cross-section of a flange portion is about ⅓ to ⅕, and the height of the wrap portion is 4 to 10 times the thickness of the wrap portion, is formed at the aforementioned heating temperature, the surface pressure applied to the end of the wrap is appropriately 40-120 N/mm 2 , preferably 60-100 N/mm 2 .  
     [0085] In order to impart strength and wear resistance to the forged scroll part, the scroll part must be subjected to solution (quenching) and aging treatment. The solution temperature is preferably 490-500° C. After the scroll part is subjected to quenching in water, the scroll is subjected to aging for hardening under appropriate conditions; i.e., temperature: 160-210° C., time: 1-8 hours. Through this procedure, the scroll part is imparted with a satisfactory hardness of about HRB 70-85.  
     [0086] If necessary, the heat-treated forged scroll part is further subjected to machining to precisely regulate the height and the shape of the wrap portion. The thus-produced scroll part can be provided in a compressor or the like.  
     EXAMPLES  
     [0087] The present invention will next be described by way of Examples. Unless indicated otherwise herein, all parts, percents, ratios and the like are by weight.  
     [0088] Production of a workpiece for forging of the present invention  
     [0089] Alloy materials A through F shown in Table 1 were employed in Examples 1 through 8, and alloy materials G and H shown in Table 1 were employed in Comparative Examples 5 and 6. Each alloy material was cast into a bar having a diameter of 82 mmφ and a length of 5,000 mm through continuous casting at a casting rate of about 300 mm/minute. The bar was subjected to homogenization heat treatment at 500° C. for one hour, and then subjected to facing by use of a peeling machine to attain a diameter of 78 mmφ.  
     [0090] Subsequently, the bar was cut into workpieces having a thickness of 65 mm by use of a round saw having a thickness of 2.5 mm.  
     [0091] Each workpiece was heated at about 400° C. in a heating furnace, and the disk-shaped workpiece was pressed with a punch into a die by use of a 630-ton press machine to attain a diameter of 114 mmφ to upset the workpiece through die-forging. The upsetting ratio is obtained from the following calculation:  
     [0092] upsetting ratio={1-(78/114) 2 }×100=53%.  
     [0093] When the bar was cut into workpieces, chips (45 g per workpiece) were formed.  
     [0094] Production of a workpiece for forging through a conventional process  
     [0095] Alloy materials B and C shown in Table 1; i.e., alloy materials of Examples 4 and 5, were employed in Comparative Examples 3 and 4. Each alloy material was cast into a billet for extrusion having a diameter of 200 mmφ through continuous casting at a casting rate of about 150 mm/minute. The billet was subjected to homogenization heat treatment at 500° C. for one hour, and then extruded into a stock material having an outer diameter of 114 mmφ, which is equal to that of the above upset workpiece. The stock material was cut into workpieces having a thickness of 30.4 mm, so that that the volume of each workpiece was the same as that of the above upset workpiece, by use of a round saw having a thickness of 2.5 mm.  
     [0096] When the stock material was cut into workpieces, chips (80 g per work piece) were formed. The amount of loss of the material was about twice the loss of the material in the cases of Examples 1 through 8 in which the round bar obtained through continuous casting was cut into workpieces.  
     [0097] Observation of internal metallographical structure of a workpiece for forging  
     [0098] Subsequently, in order to observe the internal metallographical structure of each of the above-prepared workpieces, or to measure the size and the weight of the workpiece, 10 upset workpieces or 10 cut workpieces were collected as samples.  
     [0099] After the sizes and the weights of these 10 workpieces were measured, a 20 mm-square sample was cut out of the center portion of each workpiece having a diameter of 114 mmφ, and the internal microstructure of the sample was observed.  
     [0100] Through this observation, the existence of primary Si crystals, the size of the crystals, the number of the crystals, and the size of eutectic Si particles were measured. The weight of each sample was measured by use of an even balance. The thickness of each sample was measured at two points per sample by using of a micrometer. The results are shown in Table 2. The weight and the thickness are shown by a range of 10 samples.  
     [0101] The results reveal that, when a workpiece is subjected to upsetting, coarse primary Si crystals are not formed in the workpiece, variation in the size and the weight of the workpiece is reduced, and production yield is improved; i.e., a highly-reliable workpiece exhibiting high precision in size can be produced economically.  
     [0102] Scroll forging  
     [0103] Subsequently, the above upset workpiece and the above extruded-and-cut workpiece were heated at 200° C. in a heating furnace, and then each workpiece was immersed into a water-containing graphite lubricant for several seconds, then removed therefrom to coat the workpiece with a lubrication film.  
     [0104] While the workpiece was heated at 400° C., the workpiece was subjected to forging at a punch pressure of 450 tons and at a back surface pressure of 40-120 N/mm 2  to produce a scroll having a flange diameter of about 115 mmφ, a flange thickness of about 23.0 mm, a wrap height of 39.6 mm, and a wrap thickness of 5.7 mm. The ratio of the area of a horizontal cross section of the flange to that of a horizontal cross section of the wrap was about 4.0.  
     [0105] In Comparative Examples 1 and 2, upset workpieces obtained from alloy material A were subjected to forging at back pressures of 30 and 130 N/mm 2 , respectively.  
     [0106] Under the aforementioned conditions, 50 workpieces of each Example or each Comparative Example were successively subjected to forging to produce 50 scroll parts. Difference in the height (the maximum height—the minimum height) of the scroll wrap of each forged part was measured to obtain variation in wrap height difference between the 50 forged parts. In addition, the mean height of the wrap of each forged part (the mean value of the heights of the wrap measured at three points  11   a,    11   b,  and  11   c  shown in FIG. 1, wherein  11   a  represents a spiral initiation point;  11   c  represents a spiral termination point; and  11   b  represents a point on a line joining the points  11   a  and  11   c,  the point  11   b  being adjacent to the point  11   c ) was measured to obtain variation in mean wrap height between the 50 forged parts. Furthermore, the shape of the wrap of each forged port was observed.  
     [0107] The results are shown in Table 3. The results reveal that, when the back pressure is 30 N/mm 2 , difference in wrap height of one forged part is in excess of 1 mm; i.e., the height of the wrap becomes non-uniform when the back pressure is low. In contrast, when the back pressure is 130 N/mm 2 , buckling of the wrap occurs, and a good forged part is not produced.  
     [0108] The results reveal that variation in mean wrap height between forged parts produced from workpieces obtained through the conventional process including extrusion and cutting is 1.0 mm or more. That is, as shown in Table 2, variation in volume between the workpieces causes variation in wrap height between the forged parts.  
     [0109] According to the present invention, variation in height of the wrap of one forged part falls within 0.5 mm, and variation in mean wrap height between forged parts also falls within 0.5 mm. That is, a forged part having a good shape can be produced.  
     [0110] Subsequently, 10 forged parts of each of Examples 4 and 5 and Comparative Examples 3 through 6 were heated at 500° C., and then subjected to quenching in water. Subsequently, the parts were subjected to aging treatment at 180° C. for six hours. Thereafter, a tensile test piece was obtained from each forged part, and tensile characteristics of the forged part were evaluated. In addition, the side wall of the wrap of each forged part was machined about 0.5 mm by use of an end mill, and then the machined surface was observed. Furthermore, the workpiece for forging was subjected to heat treatment in a manner similar to that of the above procedure, and a fatigue test piece was obtained from the workpiece. The fatigue test piece was subjected to a test by use of an Ono-type rotating bending fatigue test apparatus, and fatigue characteristics of the workpiece were evaluated on the basis of breakage stress at 10 7  cycles. The results are shown in Table 4.  
     [0111] The results reveal that, when an upset stock material is employed as a workpiece, the fracture elongation of the workpiece was improved, and thus a forged part exhibiting high fatigue strength and having an excellent machined surface was produced. That is, when formation of coarse primary Si crystals was suppressed, the above effects are obtained.  
     [0112] In order to observe the internal metallographical structure of the forged part, a test piece was cut out from the central portion of the forged part of each of Examples 1 through 8 after aging treatment, and the test piece was subjected to observation of microstructure. Consequently, primary Si crystals were not observed in each test piece, and change in the size of eutectic Si particles attributed to forging and heat treatment was not confirmed.  
     [0113] In Comparative Examples 5 and 6, in which the Si content of the alloy material falls outside the range of the present invention, scratches were formed on the machined surface of a forged part, the scratches were attributed to formation of primary Si crystals, and the strength of the forged part was lowered. Such a forged part is not suitable for a scroll.  
               TABLE 1                          Alloy material (unit: wt %) subjected to test and production process for forged part                             Stock material   Forging           for forging   back                                     Chemical analysis value (mass %)   Diameter       pressure                                                             Alloy   Test No.   Si   Cu   Mg   Ni   Sb   Sr   Others   (mmφ)   Working   N/mm 2                                                                       A   Example. 1   10.2   2.9   0.5   —   —   —   Bal.   82   Upsetting   80           Example. 2   10.2   2.9   0.5   —   —   —   Bal.   82   Upsetting   40           Example. 3   10.2   2.9   0.5   —   —   —   Bal.   82   Upsetting   120           Comp. Ex. 1   10.2   2.9   0.5   —   —   —   Bal.   82   Upsetting   30           Comp. Ex. 2   10.2   2.9   0.5   —   —   —   Bal   82   Upsetting   130       B   Example. 4   11.5   4.5   0.6   —   —   —   Bal   82   Upsetting   80           Comp. Ex. 3   11.5   4.5   0.6   —   —   —   —   200   Extrusion   80       C   Example. 5   10.4   2.6   0.3   —   —   —   Bal   82   Upsetting   80           Comp. Ex. 4   10.4   2.6   0.3   —   —   —   Bal.   200   Extrusion   80       D   Example. 6   8.9   2.1   0.4   —   0.22   —   Bal   82   Upsetting   80       E   Example. 7   12.0   1.2   1.1   1.2   0.25   —   Bal   82   Upsetting   80       F   Example. 8   11.2   4.6   0.7   —   —   0.01   Bal   82   Upsetting   80       H   Comp. Ex. 5   13.1   4.8   0.5   —   —   —   Bal.   82   Upsetting   80       G   Comp. Ex. 6   7.0   0.3   0.2   —   —   —   Bal.   82   Upsetting   80                  
 
     [0114]               TABLE 2                          Metallographical observation and size measurement of workpiece for forging                             Internal microstructure                                     Primary Si crystal   Eutectic Si particle                                             Maximum   Mean   Maximum   Size   Note                                                                     size   size   size   Diameter   Thickness   Weight           Alloy   Test No.   Number   (μm)   (μm)   (μm)   (mmφ)   (mm)   (g)                                                                 Examples   A   Ex. 1   None   —   2.0   4.8   114.0   30.40-30.49   841-843           B   Ex. 4   None   —   2.1   6.7   114.0   30.35-30.51   845-848           C   Ex. 5   None   —   2.0   4.4   114.0   30.38-30.52   840-842           D   Ex. 6   None   —   1.9   4.4   114.0   30.37-30.50   839-842           E   Ex. 7   None   —   2.1   7.2   114.0   30.42-30.52   841-843           F   Ex. 8   None   —   2.1   5.3   114.0   30.44-30.51   845-847       Comparative   B   Comp. Ex. 3   5   100   2.5   10.3   114.0   30.20-30.58   844-850       Examples   C   Comp. Ex. 4   2    52   3.0   15.5   114.0   30.33-30.63   840-845           H   Comp. Ex. 5   5   110   2.0   8.4   114.0   30.37-30.46   845-848           G   Comp. Ex. 6   None   —   1.8   4.8   114.0   30.41-30.49   840-842                    
     [0115]               TABLE 3                          Size measurement and observation of forged part in each test                                         Difference in                       wrap height   50 Forged parts           Back   in one forged   Mean wrap           pressure   part/mm   height/mm                                                     Alloy   Test   Work piece   N/mm 2     (Max.-Min.)   Minimum   Maximum   Note                                                             Example   A   Ex. 1   Upset piece   80   0.3 to 0.4   39.4   39.7                   Ex. 2   Upset piece   40   0.3 to 0.5   39.0   39.4               Ex. 3   Upset piece   120   0.2 to 0.4   39.2   39.5           B   Ex. 4   Upset piece   80   0.3 to 0.4   39.2   39.6           C   Ex. 5   Upset piece   80   0.3 to 0.4   39.4   39.7           D   Ex. 6   Upset piece   80   0.3 to 0.4   39.2   39.7       Comparative   A   Comp.   Upset piece   30   1.3 to 2.0   —   —   Variation in       Example       Ex. 1                       wrap height               Comp.   Upset piece   130   0.2 to 0.4   39.0   39.3   Buckling of               Ex. 2                       wrap           B   Comp.   Extruded piece   80   0.3 to 0.5   38.2   39.8               Ex. 3           C   Comp.   Upset piece   80   0.3 to 0.5   38.4   39.7               Ex. 4                    
     [0116]               TABLE 4                          Mechanical characteristics and machining test of forged part                                     Fatigue               Tensile characteristics   characteristics                                                         0.2% proof   Tensile   Fracture   (room temperature)   Observation of                   stress   strength   elongation   10 7  cycle   machined           Alloy   Test   (MPa)   (MPa)   (%)   (MPa)   surface                                                         Example   B   Ex. 4   401   456   6.3   210   No tool scratch           C   Ex. 5   322   403   13.8   190   No tool scratch       Comparative   B   Comp.   408   448   3.2   180   Tool scratch       Example       Ex. 3           C   Comp.   330   415   10.8   165   Tool scratch               Ex. 4           H   Comp.   410   458   3.8   170   Tool scratch               Ex. 5           G   Comp.   200   301   15.1   130   No tool scratch               Ex. 6                    
     [0117] According to the alloy material and the forging process of the present invention, mass-production of an aluminum alloy-made forged scroll, in which formation of primary Si crystals which cause lowering of strength of the scroll and adversely affecting machining of the scroll, is suppressed can be achieved. According to the present invention, variation in wrap height of one forged scroll can be reduced and variation in mean wrap height between forged scrolls can also be reduced.  
     [0118] While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.