Patent Publication Number: US-2012027639-A1

Title: Aluminum alloy for die casting

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to aluminum alloys and more specifically aluminum alloys for casting such as for pressure die-casting. 
     Die casting is a process in which molten metal is forced into metal dies under pressure to form dimensioned components that require little or no post processing. Aluminum is a widely used base for alloys in the casting of dimensional components. For example, aluminum alloys are used extensively in die-casting due to the excellent flow characteristics and dimensional stability achieved by aluminum alloys. 
     The use of various alloying agents to improve the strength, ductility, wear resistance, corrosion resistance, and thermal conductivity of aluminum may result in significant variations in the physical properties of the resulting alloy. The alloying agents also affect the ability of the resulting alloys to be die-cast successfully and with maintenance of the dimensional stability of the resulting parts. Various alloying agents are known to be used with aluminum. For example, copper, manganese, silicon, magnesium, zinc, and even iron, in some cases, may be used to form aluminum alloys with varying properties. 
     In addition, the resulting structures may be treated after the die casting process to modify physical characteristics. Coatings such as paint or anodize may improve the strength, wear resistance, and corrosion resistance of the final products. Various heat treatments may also be used to modify the mechanical properties of the parts after the die casting process has been completed. 
     Strontium is an elemental metal that reacts with water and burns in air. Strontium has been used in magnesium alloys which also include relatively small amounts of aluminum to form creep resistant magnesium based alloys. In such applications, the strontium is used to refine eutectic silicon. For example, U.S. Pat. No. 7,108,042 discloses that strontium, sodium, or calcium may be used to refine the eutectic silicon. 
     SUMMARY 
     According to the present disclosure, an alloy for use in die casting has improved thermal conductivity and strength. The alloy includes at least about 86.0 percent aluminum by weight, from about 9.70 to about 10.70 percent silicon, by weight, from about 0.40 to about 0.70 percent iron, by weight, up to about 0.25 percent copper, by weight, up to about 0.50 percent manganese, by weight, from about 0.10 to about 0.20 percent titanium, by weight; and from about 0.010 to about 0.025 percent strontium, by weight. 
     The alloy may include up to about 90.0 percent aluminum. In some embodiments, the alloy may include up to about 0.30 percent magnesium, by weight. In some embodiments, the alloy may include impurities. For example, the alloy may include up to about 0.25 percent nickel, up to about 0.50 percent zinc, by weight, and up to about 0.15 percent tin by weight. The alloy may be used to produce articles of manufacture. 
     Additional features and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of illustrated embodiments exemplifying the best mode of carrying out the invention as presently perceived. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detailed description of the drawings particularly refers to the accompanying figures in which: 
         FIG. 1  is a graph comparing the thermal conductivity of alloy of the present disclosure to other alloys; and 
         FIG. 2  is a graph comparing the strength of the present alloy to aluminum 365. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The aluminum alloy of the present disclosure has been shown to provide improved thermal conductivity and strength over the A413 such that the new alloy provides mechanical properties that compare to the 365 alloy with a significantly lower cost. In the special 413 alloy of the present disclosure, hereinafter referred to as SP413, strontium is added in amounts between about 0.01 and 0.02% to modify silicon included in the alloy. 
     Strontium improves the tensile strength and the thermal conductivity of the SP413 as compared to the A413. A comparison of the thermal conductivity of several alloys is shown below in Table 1. 
                     TABLE 1                  THERMAL CONDUCTIVITY (W/K-m)                                     Heat   Die Cast   Die Cast   Die Cast   SP413   SP413       Treatment   356   380   A413   w/o Sr   w/Sr                                             As cast   143   96.2   121   144   150       300 F.   144   97   124   147   150       400 F.   145   104   127   155   156       500 F.   148   116   130   158   165       600 F.   145   119   131   159   163                    
Referring to  FIG. 1 , the thermal conductivity of five different alloys is shown with various heat treatments. It can be seen that the SP413 without strontium shows improvement in thermal conductivity as compared to A413, 356, and 380. The data also shows that the addition of strontium further improves the thermal conductivity of the SP413 alloy. SP413 without strontium has about a 20% average improvement over A413 in thermal conductivity. SP413 with Strontium has about a 24% average improvement over A413 in thermal conductivity.
 
     The addition of Strontium also improves the yield strength of the SP413 alloy as shown in table 2. 
                                 TABLE 2                   Tensile Strength   Yield Strength           Material   KSI(MPa)   KSI(MPa)   Elongation %                  413 As-Cast   42.9(296)   21.0(145)   2.5       SP413 As-Cast   39.8(274)   15.6(108)   6.7       SP413 As-Cast w/Sr   44.6(308)   17.7(122)   9.9                    
The addition of strontium to SP413 improves the tensile strength of the alloy by approximately 12%, yield strength by approximately 9% and the elongation by approximately 48%. This improvement results in the SP413 with strontium to be similar to 365 in mechanical properties. As discussed below, the SP413 of the present disclosure provides this performance with a higher tolerance for impurities, thereby reducing the cost of the alloy. The relative mechanical performance is shown in Table 3.
 
                                 TABLE 3                   Tensile Strength   Yield Strength           Material   KSI(MPa)   KSI(MPa)   Elongation %                  365 As-Cast   47.6(328)   21.6(149)   8.2       SP413 As-Cast w/Sr   44.6(308)   17.7(122)   9.9                    
In one application, a test was performed to compare the performance of the SP413 with 365. A test coupon was cast with each alloy and the coupons were heat-treated under the same conditions. A compression test was then performed with the results of the tests being compared as shown on the graph in  FIG. 2 . As can be seen in  FIG. 2 , the SP413 and 365 alloys have a similar stress-strain profile until elongation. The 365 begins to elongate at a lower stress then the SP413. Once the SP413 begins to elongate the difference between the curves remains generally constant throughout the remainder of the test. Ten castings from both alloys were tested and no cracks were observed on any of the castings.
 
     The chemistry of each of the A413, SP413, and 365 alloys is presented in Table 4. 
                                 TABLE 4                          ALLOY                                 ELEMENT   A413   SP413   365               Silicon (wt. %)   11.0-13.0    9.70-10.70    9.5-11.5       Iron (wt. %)   1.30   0.40-0.70   0.15       Copper (wt. %)   1.00   0.25   0.03       Manganese (wt. %)   0.35   0.50   0.5-0.8       Magnesium (wt. %)   0.10   0.30   0.1-0.5       Nickel (wt. %)   0.50   0.25   —       Zinc (wt. %)   0.50   0.50   0.07       Tin (wt. %)   0.15   0.15   —       Titanium (wt. %)   —   0.10-0.20   0.04-0.15       Strontium (wt. %)   —   0.010-0.025   0.010-0.020       Aluminum (wt. %)   83.1-89.0   86.38-89.79   86.78-89.85                    
The use of silicon helps improve the fluidity of the alloys. The iron improves hot tear resistance and reduces the occurrence of die soldering. Copper improves the strength and hardness in the as-cast and heat-treated conditions. Keeping the levels of copper relatively low helps improve corrosion resistance. Manganese tends to reduce die soldering as well. Magnesium improves strength and hardness in the as-cast and heat-treated conditions. Titanium is used to refine the aluminum grain structure in the alloy. Strontium is used to modify and refine the silicon particles. Nickel, zinc, and tin are considered impurities in the alloy, with the SP413 tolerating reasonable amounts of the nickel, zinc, and tin, thereby reducing the costs of refining the alloy. After a review of the test coupons, it was determined that the microstructures of the SP413 and the 365 are similar.
 
     Thus, it can be seen that the SP413 alloy of the present disclosure provides an alloy having improved thermal conductivity and strength at a lower cost than known alloys. The combination of the improved thermal conductivity with improved strength provides a new option for the preparation of structural components in applications subjected to high thermal loads. 
     Although certain illustrative embodiments have been described in detail above, variations and modifications exist within the scope and spirit of this disclosure as described and as defined in the following claims.