Patent Publication Number: US-2019185967-A1

Title: Cast aluminum alloy for transmission clutch

Description:
FIELD 
     The present disclosure relates generally to aluminum alloys, and more particularly, to cast aluminum alloys that have improved casting quality and mechanical properties, as well as cast articles made therefrom, such as clutch housings made from permanent mold and high pressure die casting (HPDC). 
     INTRODUCTION 
     Cast aluminum alloys have been increasingly used in the automotive industry to replace cast iron or HSLA (High-Strength Low-Alloy) steels that are either Grob-cold formed or roller die formed, in applications such as engine blocks and transmission components to reduce mass. With the increasing demand for fuel economy, manufacturers in the car industry are now increasingly required to produce near-net-shape aluminum components with very thin walls and a combination of high mechanical properties such as tensile, ductility, and yield strength. HPDC affords the most economic production method for producing many of these components 
     In order to avoid discontinuities and particularly misruns in the very thin wall locations in the cast component, the molten alloy is injected into the die cavity rapidly enough that the entire cavity fills before any portion of the cavity begins to solidify. Hence, the injection is under high pressure and the molten metal is subject to turbulence as it is forced into a die and then rapidly solidifies. Since the air being replaced by the molten alloy has little time to escape, some of it is trapped and porosity results. Castings also contain pores resulting from gas vapor decomposition products of the organic die wall lubricants and porosity may also result from shrinkage during solidification. 
     A major drawback of the porosity resulting from the HPDC process is that aluminum alloy castings made from aluminums which ordinarily have the capacity to respond to age-hardening, cannot be artificially aged, that is, they cannot be treated at the high temperatures characteristic of artificial aging conditions. The internal pores containing gases or gas forming compounds in the high pressure die castings expand during conventional solution treatment at elevated temperatures, resulting in the formation of surface blisters on the castings. The presence of these blisters affects not only the appearance of castings but also dimensional stability and in some cases it can negatively impact particular mechanical properties of HPDC components. 
     Clearly a need exists in the art for an aluminum alloy suitable for HPDC, without compromising mechanical properties of the cast components such as transmission housings. 
     SUMMARY 
     This disclosure provides cast aluminum alloys that have improved casting quality and mechanical properties for manufacturing articles made therefrom, such as transmission clutch housings made from HPDC. 
     The alloy may contain at least one of the castability and strength-enhancement elements, such as silicon, copper, magnesium, chromium, zirconium, vanadium, cobalt, strontium, sodium, barium, titanium, iron, manganese, and/or zinc. The microstructure of the alloy may contain at least one insoluble solidified and/or precipitated particles with at least one alloying element. 
     In one exemplary embodiment, which may be combined with or separate from the other examples and features provided herein, an aluminum alloy suitable for high pressure die casting is provided. The aluminum alloy may contain: about 12 to about 16 weight percent silicon, about 0.0 to about 6.0 weight percent copper, about 0.2 to about 0.5 weight percent magnesium and about 0.1 to about 0.5 weight percent chromium; the aluminum alloy further including about 0 to about 1.0 weight percent each of iron, manganese, and zinc; the aluminum alloy further includes less than 0.1 weight percent each of strontium and phosphorous; and the aluminum alloy further including about 0.0 to about 0.25 weight percent other trace elements such as, but not limited to, Titanium, Zirconium, Vanadium, and Cobalt. Additional features may be provided, including but not limited to, the aluminum alloy further including about 74 to about 88 weight percent aluminum. 
     In yet another example, which may be combined with or separate from the other examples and features described herein, the aluminum alloy may consist essentially of about 13 weight percent silicon; about 0.0 to about 6.0 weight percent copper; about 0.35 weight percent magnesium; about 0.1 to about 0.5 weight percent chromium; about 0.4 weight percent each of iron; about 0.5 weight percent each of manganese; about 0.2 weight percent of zinc; less than 0.1 weight percent each of strontium and phosphorous; and about 0.0 to about 0.25 weight percent other trace elements such as, but not limited to, Titanium, Zirconium, Vanadium, and Cobalt. Additional features may be provided, including but not limited to, the aluminum alloy further including a balance of about 85 weight percent aluminum. 
     In yet another example, which may be combined with or separate from the other examples and features described herein, the aluminum alloy may include a solidus boundary at about 550° C. The aluminum alloy may include an as-cast yield strength of 185 MPa. The aluminum alloy may also include an as-cast ultimate yield strength of 310 MPa. The aluminum alloy may also include an as-cast elongation at room temperature of 2.8 percent. Further features of the aluminum alloy may include a T5 yield strength of 251 NiPa; a T5 ultimate tensile strength of 340 MPa; and a T5 elongation of 1.9 percent. 
     In still another example, which may be combined with or separate from the other examples and features described herein, the aluminum alloy may consist essentially of: about 13.0 to about 14.0 weight percent silicon; about 0.0 to about 2.0 weight percent copper; about 0.3 to about 0.4 weight percent magnesium; about 0.2 to about 0.3 weight percent chromium; about 0.3 to about 0.5 weight percent manganese; less than 0.5 weight percent each of iron and zinc; less than 0.05 weight percent each of strontium and phosphorous; about 0.0 to about 0.15 weight percent other trace elements such as, but not limited to, Titanium, Zirconium, Vanadium, and Cobalt; and the remaining balance being about 83 to about 86 weight percent aluminum. And a further additional feature such as the aluminum alloy includes a solidus boundary at about 500° C. 
     A casting article, such as a transmission clutch housing, is provided and cast from any of the versions of the aluminum alloy disclosed herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings are provided for illustration purposes only and are not intended to limit this disclosure or the claims appended hereto. 
         FIG. 1  is a top view of a transmission clutch housing casting in accordance with aspects of an exemplary embodiment; 
         FIG. 2  is a bottom view of a transmission clutch housing casting in accordance with aspects of an exemplary embodiment; 
         FIG. 3  is a graph showing a calculated phase diagram of a HPDC aluminum alloy showing phase transformations as a function of copper (Cu) content as according to aspects of an exemplary embodiment; 
         FIG. 4  is a graph showing a calculated phase diagram of a HPDC aluminum alloy showing phase transformations as a function of silicon (Si) content as according to aspects of an exemplary embodiment; and 
         FIG. 5  is a graph showing a calculated phase diagram of a HPDC aluminum alloy showing phase transformations as a function of magnesium (Mg) content in accordance with aspects of an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Cast aluminum alloys are provided having improved mechanical properties for transmission clutch housings. 
     In  FIG. 1 , the clutch housing  10  has a generally cup-shaped open bowl configuration with an end wall (also called piston stop face)  12 , an inner axial hub  14  and an outer wall  16 . The outer wall  16  includes an interior surface  18  defining an inner diameter and an exterior surface  20  defining an outer diameter. In accordance with the operating characteristics of the clutch housing  10 , a series of longitudinal splines  22  are formed in the interior surface  18  of the outer wall  16 . The splines  22  may be formed at varying intervals, lengths and depths for operation in a predetermined manner. In  FIG. 2 , a series of shallow metal saver pockets  30  are formed in the outer back surface  32  of the end wall  12 . The splines  22  and metal saver pockets  30  may be formed at varying intervals, lengths and depths for operation in a predetermined manner. The splines  22  receive impact and torsional loading and require high strength and wear resistance especially at very high speeds (&gt;12000 rpm). In comparison to other aluminum alloys, these new alloys exhibit a lower tendency for porosity, better castability, and higher mechanical properties (see Table 1). These alloys may also exhibit improved ductility and high fracture toughness, as well as high strength and wear resistance. As a result, the scrap rate for aluminum casting and the manufacturing cost can be reduced. And warranty cost can be reduced due to crack elimination and no need for crack detection in the manufacturing process. 
     The new alloys can be applied to both permanent mold and HPDC processes. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Mechanical Properties of New Alloy and 380/390 
               
            
           
           
               
               
               
            
               
                   
                 As-Cast 
                 T5 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                 UTS 
                   
                   
                 UTS 
                 El 
               
               
                 ALLOY 
                 YS (MPa) 
                 (MPa) 
                 El (%) 
                 YS (MPa) 
                 (MPa) 
                 (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 New Alloy 
                 185 
                 310 
                 2.8 
                 251 
                 340 
                 1.9 
               
               
                 A380 
                 145 
                 240 
                 2.2 
                 146 
                 270 
                 1.6 
               
               
                 A380_mod 
                 156 
                 301 
                 3.3 
                 218 
                 330 
                 2 
               
               
                 B390 
                 177 
                 212 
                 0.2 
                 194 
                 194 
                 0.18 
               
               
                   
               
               
                 The new alloy: Al—13% Si—0.35% Mg—0.4% Fe—0.5% Mn—0.2% Zn 
               
            
           
         
       
     
     The alloy may contain at least one of the castability and strength enhancement elements such as silicon, copper, magnesium, manganese, iron, zinc, and nickel. The microstructure of the alloy contains one or more insoluble solidified and/or precipitated particles with at least one alloying element. 
     Two examples of composition ranges of the new alloy (called Version 1 and Version 2 in these examples) are listed in Table 2, compared with the other commercially available alloys for transmission clutch housings or other articles. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Chemical compositions of two versions of the new alloy and commercial 
               
               
                 alloys A380, 390, A390, and B390 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Alloy 
                 Si 
                 Cu 
                 Mg 
                 Fe 
                 Mn 
                 Zn 
                 Cr 
                 P 
                 Sr 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 A380 
                 7.5-9.5 
                 3.0-4.0 
                 &lt;0.1 
                 0.7-1.5 
                 &lt;0.5 
                 1.5-3.0 
                   
                   
                   
               
               
                 390.0 
                 16.0-18.0 
                 4.0-5.0 
                 0.45-0.65 
                 &lt;1.3 
                 &lt;0.1 
                 &lt;0.1 
               
               
                 A390.0 
                 16.0-18.0 
                 4.0-5.0 
                 0.45-0.65 
                 &lt;0.5 
                 &lt;0.1 
                 &lt;0.1 
               
               
                 B390.0 
                 16.0-18.0 
                 4.0-5.0 
                 0.45-0.65 
                 &lt;1.3 
                 &gt;0.5 
                 &lt;1.5 
               
               
                 V1 
                 12.0-16.0 
                 0.0-6.0 
                 0.2-0.5 
                 0.0-1.0 
                 0.0-1.0 
                 0.0-1.0 
                 0.1-0.5 
                 &lt;0.1 
                 &lt;0.1 
               
               
                 (wt %) 
               
               
                 V2 
                 13.0-14.0 
                 0.0-2.0 
                 0.3-0.4 
                 &lt;0.5 
                 0.3-0.5 
                 &lt;0.5 
                 0.2-0.3 
                 &lt;0.05 
                 &lt;0.05 
               
               
                 (wt %) 
               
               
                   
               
            
           
         
       
     
     Though copper is generally known to increase strength and hardness in aluminum alloys, on the downside, copper generally reduces the corrosion resistance of aluminum; and, in certain alloys and tempers, copper increases stress corrosion susceptibility. Copper also increases the alloy freezing range and decreases feeding capability, leading to a high potential for shrinkage porosity. Furthermore, copper is expensive and heavy. 
     Aluminum alloy castings known to the HPDC parts have generally been limited to temper T5 treatment aging (natural or artificial). Aging strengthens castings by facilitating the precipitation of the hardening solutes of the aluminum alloy composition. Artificial aging is to heat castings to an intermediate temperature (e.g., 160-240 degrees C.), and then holding the castings for a period of time to achieve hardening or strengthening through precipitation. Considering that precipitation hardening is a kinetic process, the contents (supersaturation) of the retained solute elements in the casting play an important role in the aging responses of the castings. Therefore, the availability and actual amount of hardening solutes in the aluminum soft matrix solution after HPDC process has an effect on subsequent aging, which depends on the alloy composition, such as Cu and Mg content, and quenching temperature after casting is extracted from the die. 
     In Al—Si—Mg—Cu based HPDC cast aluminum alloy, like A380 alloy, the strengthening precipitates include Mg2Si, Al2Cu, and Q phase (AlCuMgSi) after the casting is subject to a solution treatment at high temperature such as 490 C. However, HPDC components are not usually subject to solution treatment, the hardening efficacy and contribution of Cu may be surprisingly limited. Although typical HPDC aluminum alloys, such as A380, 380 or 383, contain 3˜4% Cu in nominal composition, the actual Cu solute remaining in as-cast aluminum matrix for the subsequent aging is actually much reduced. Excess Cu in the alloy however will form other low melting phases. The Cu-containing low-melting phases can significantly affect alloy castability and increase porosity in the castings. One of the measures for castability of an alloy is freezing range between liquidus and solidus. The larger the freezing range, the higher the shrinkage porosity and lower castability. 
     Referring to  FIG. 3  which illustrates a calculated phase diagram  50  of Al—Cu-14 wt % Si—0.35 wt % Mg-0.4 wt % Fe-0.5 wt % Mn-0.25 wt % Cr-0.25 wt % Zn based alloy with preferred alloys having a Cu content that varies from 0 to 2 weight percent. The diagram  50  includes a liquidus boundary line  55  above which alloy material remains liquid and a solidus boundary line  60  below which alloy material becomes solid. The temperature range between the liquidus  55  and solidus  60  boundary lines is the alloy freezing range  65 . The freezing range  65  is maximized and is almost kept constant when the Cu content is between 4.0 and 6.0 weight percent. When Cu content is less than 2%, the freezing range is significantly reduced. The phase diagram  50  also indicates that with the alloy composition range particularly for the Fe and Mn content proposed no beta-Fe phase (AlFeSi) formed in the casting. Beta-Fe phase (AlFeSi) looks like needle in 2D and plate in 3D, which is brittle and significantly reduces the material ductility by initiating cracks in the material. 
     Si is an important element for cast aluminum alloy. Si increases alloy castability by increasing fluidity and releasing high latent heat during solidification to reduce shrinkage and improve feeding. High Si content also reduces alloy freezing range. For example, referring to  FIG. 4 , a calculated phase diagram  70  of Al—Si-1.5 wt % Cu-0.35 wt % Mg-0.4 wt % Fe-0.5 wt % Mn-0.25 wt % Cr-0.25 wt % Zn based alloy with new alloys having a Si content that varies from 12 to 16 weight percent is provided. The diagram  50  includes a liquidus boundary line  75  above which alloy material remains liquid and a solidus boundary line  80  below which alloy material becomes solid. The temperature range between the liquidus  75  and solidus  80  boundary lines is the alloy freezing range  85 . The freezing range  85  is small and is almost kept constant when the Si content is between 12 and 16.0 weight percent. This improves alloy castability. With the preferred alloy composition, there is no brittle beta-Fe phase forming in the material which shall help improve the mechanical properties particularly ductility 
     To further improve the aging response of cast aluminum alloy, magnesium content in the new alloy for transmission clutch components should be kept no less than 0.2 wt %, and the preferred level is above 0.3 wt %. The maximum Mg content should be kept below 0.6 wt %, with a preferable level of 0.35 wt %, so that a majority of the Mg addition will stay in Al solid solution after HPDC to mainly form only Q-phase (AlCuMgSi) precipitates.  FIG. 5  illustrates a calculated phase diagram  100  of Al—Mg-14 wt % Si-15 wt % Cu-0.4 wt % Fe-0.5 wt % Mn-0.25 wt % Cr-0.2 wt % Zn based alloy with new alloys having a Mg content that varies from 0.2 to about 0.45 weight percent. The diagram  100  includes a liquidus boundary line  105  above which alloy material remains liquid and a solidus boundary line  110  below which alloy material becomes solid. The temperature range between the liquidus  105  and solidus  110  boundary lines is the alloy freezing range  115 . The freezing range  115  is small and almost kept constant when the Mg content is between 0.2 and 0.4 weight percent. The small freezing range leads to a good castability. With the preferred alloy composition, there is no brittle beta-Fe phase forming in the material which shall help improve the mechanical properties particularly ductility. 
     Phosphorous is added to refine the primary Si particle sizes and also make the primary Si particles distributed more uniformly. Strontium is added to modified and refine the eutectic Si particles and convert the eutectic Si particles from flakes to fibers so that the ductility of the material is significantly. The aluminum alloy includes less than 0.1 weight percent each of strontium and phosphorous. 
     Further additional features may be provided, such as: the iron, manganese and chromium content being provided each in an amount so that a sludge factor is less than or equal to 1.6, wherein the sludge factor is calculated by the following equation: Sludge factor=(1×wt % iron)+(2×wt % manganese)+(3×wt % chromium), and wherein the aluminum alloy may contain up to 0.5% chromium; the aluminum alloy containing iron content less than or equal to manganese content and essentially no Beta Iron Phase (β-Fe Phase). 
     The alloys described herein may be used to manufacture a permanent mold or high pressure die cast article, such as transmission clutch housings. Therefore, it is within the contemplation of the inventors herein that the disclosure extend to cast articles, including transmission clutch housings, containing the improved alloy (including examples, versions, and variations thereof). 
     Furthermore, while the above examples are described individually, it will be understood by one of skill in the art having the benefit of this disclosure that amounts of elements described herein may be mixed and matched from the various examples within the scope of the appended claims. 
     It is further understood that any of the above described concepts can be used alone or in combination with any or all of the other above described concepts. Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.