Method of forming zinc-aluminum alloys with good machinability
United States Patent 1 1 Gervais et al.  
 [ 51 Apr. 29, 1975 METHOD OF FORMING ZINC-ALUMINUM ALLOYS WITH GOOD MACHINABILITY [75] Inventors: Edouard Gervais, Montreal,  
 Quebec; Pierre Chollet, Pierrefonds Quebec, both of Canada [73] Assignee: Noranda Mines Limited, Toronto, Ontario, Canada [22] Filed: Sept. 10, 1973 [21] Appl. No.: 395,433  
 Related US. Application Data [62] Division of Ser. No. 250,557, May 5, 1972, Pat. No.  
 [30] Foreign Application Priority Data July 21, 1971 Canada 118722 [52] US. Cl. ..148/l3.l; 148/13; 148/158; 75/178 AM [51] Int. Cl C2ld l/44 [58] Field of Search 148/13, 13.1, 158; 75/178 AM, 178 A [56] References Cited UNITED STATES PATENTS 1,945,288 l/l934 Morrell 75/178 AM 3,676,115 7/1972 Hare et a1 75/178 AM 3,798,028 3/1974 Gervais et al.. 75/178 AM Primary Examiner-Walter R. Satterfield [57] ABSTRACT 6 Claims, No Drawings This is a division of application Ser. No. 250,557,  
 filed May 5, 1972, now U.S. Pa t. No. 3,798,028.  
  This invention relates generally to zinc-aluminum alloys which contain, as essential alloying elements, bismuth and magnesium and which demonstrate good machinability and satisfactory corrosion resistance.  
  Many zinc-aluminum alloys are known for use in such applications as the production of wrought, die cast and gravity cast products and are known to be suitable for machining. However, alloys of the present invention have a better machinability over such known alloys. This is due to their improved chip forming characteristics.  
  The ease of machining is measuredin terms of the surface finish, dimensional control, chip length and energy consumption and is known to depend upon such features as tool design, nature of lubricant, speed of machining, feed and amount of material removed at each revolution. It also depends on the alloy composition of the workpiece, its microstructure and its properties. The microstructure is influenced by such factors as the casting conditions, the conditions of hot working such as extrusion, the amount of cold working such as drawing and heat treatments.  
  While it has been suggested to improve the ease of machining of certain alloy systems, for example, zinccopper-titanium type alloys, by adding such alloying elements as lead and bismuth, these alloying elements have not been used with zinc-aluminum alloys. One reason for this is that, at the impurity level, the elements lead and bismuth have an adverse effect on the corrosion resistance of cast zinc-aluminum alloys (for example, the commercial die casting alloys).  
  A primary object of the present invention is to provide a zinc-aluminum alloy which has improved machinability and also possesses satisfactory corrosion resistance. it has now been found that zinc-aluminum alloys of improved machining characteristics and good resistance to corrosion are those which have a zincaluminum eutectoid transformation and contain up to 10% of copper, from 0.01 to 1% of magnesium and from 0.01 to 3% of bismuth, the balance, apart from incidental impurities, being zinc and aluminum, all percentages being by weight, and wherein the ratio of magnesium to bismuth is such as to provide sufficient magnesium to combine with substantially all the bismuth to form the intermetallic compound Bi Mg For a given screw machining set-up, tool design and lubrication, it has been found that bismuth in accordance with the amounts of the present invention, improves the chip forming characteristics of zinc-aluminum alloys without affecting the other machining characteristics of the part and at the same time, maintaining an acceptable level of corrosion resistance.  
  Especially, according to the present invention, a zincaluminum base alloy with improved machining characteristics and corrosion resistance comprises 18 to 30% aluminum, 0.01 to 1% magnesium, 0.01 to 3% bismuth, to about 5% copper, the balance being zinc except for incidental impurities and the ratio of magnesium to bismuth is such as to provide sufficient magnesium to combine with substantially all the bismuth to form the intermetallic compound Bi Mg This alloy would normally have a composition in which the percentage of magnesium equals 0.175 times the percentage of his muth plus an excess of magnesium.  
 Further, according to the present invention a method is provided for heat treating a zinc-aluminum base alloy 5 which has a zinc-aluminum eutectoid transformation, to obtain improved corrosion resistance, wherein said alloy contains 0.01 to 3% bismuth, 0.01 to 1% magnesium, 0 to copper such that the percentage of magnesium equals 0.175 times the percentage of bismuth, plus an excess of magnesium, said method comprising homogenizing the alloy at a temperature below the solidus temperature and above the eutectoid temperature of said alloy and rapidly cooling the alloy to room temperature.  
  Therefore, according to the present invention, the relationship of the magnesium content to the bismuth is such that the percent magnesium equals 0.175 times the bismuth content plus an excess of magnesium. The addition of magnesium and bismuth in such a relationship is necessary to provide sufficient magnesium to combine with substantially all the bismuth to form the intermetallic compound Bi Mg (The factor 0.175 is obtained by dividing three times the atomic weight of magnesium by twice the atomic weight of bismuth). It is thought that the formation of this intermetallic compound makes possible the combination of good machinability with acceptable corrosion resistance.  
  The tensile properties of the alloys are not significantly affected by the presence of the Bi Mg intermetallic compound. However, if the amount of magne sium in the alloy is in excess of the stoichiometric amount required to form this intermetallic compound it is found that the mechanical properties of the resulting alloy, particularly its tensile and creep strength, are improved.  
  More preferred amounts of the various alloying elements used in accordance with the invention are:  
 where the magnesium content is at least 0.175 times the bismuth content plus at least 0.02% magnesium, the balance being zinc and incidental impurities.  
  A particularly preferred range of bismuth within the above broadly indicated range is from 0.2 to 0.6% bismuth with the appropriate amount of magnesium.  
  Among examples of zinc-aluminum alloys which have been found to have a good balance of physical properties are the following alloys:  
 a. 25% by weight aluminum; 1% by weight copper;  
 0.125% by weight bismuth; 0.086% by weight magnesium and the balance zinc and incidental impurities;  
 b. 25% by weight aluminum; 1% by weight copper; 3% by weight bismuth; 0.6% by weight magnesium and the balance zinc and incidental impurities;  
 c. 25% by weight aluminum; 5% by weight copper;  
 0.5% by weight bismuth; 0.14% by weight magnesium and the balance zinc and incidental impurities;  
 d. 20% by weight aluminum; 1% by weight copper;  
 0.5% by weight bismuth; 0.14% by weight magnesium and the balance zinc and incidental impurities; and  
 e. 30% by weight aluminum; 1% by weight copper;  
 0.5% by weight bismith; 0.14% by weight magnesium and the balance zinc and incidental impurities.  
  The Table 1 given at the end of this disclosure shows the effect of alloy composition on the corrosion resistance and tensile properties of some alloys falling within the scope of the invention. The corrosion resistance has been determined by suspending /s-inch diameter rods for days in air saturated by steam at 95C., and then determining the amount of corrosion on the rod. The degree of corrosion is reported as the percentage of corrosion affecting the original diameter of the rod. This steam corrosion test originated in the die casting industry and is now widely used in the zinc industry even though it has not been standardized by the A.S.T.M. It is used mainly to simulate failure by intergranular corrosion. Experience has shown that alloys whose dimensions and mechanical properties such as impact and tensile strength are not significantly altered by the test will not suffer intergranular attack in atmospheric service. The tensile tests were carried out at a crosshead velocity of 0.250 in./min. and the total elongation was calculated over a 2 inch gauge length.  
  Sample 1 in the as-extruded and cold drawn by 31% condition of Table 1 has a Mg/Bi ratio lower than 0.175/1 and was corroded 24% of the diameter. The amount of corrosion is, however, reduced to 1 1% when the magnesium content is more than 0.175 times the bismuth content as can be seen from Sample 2. A further reduction in the amount of corrosion to 5% is experienced when the excess of magnesium is still larger as can be seen from Sample 3. Sample 4 demonstrates the improved corrosion resistance obtained when the bismuth level is reduced to 0.24%. Sample 4 has improved chip forming characteristics and is corroded 5% in the as-extruded and drawn by 31% condition.  
  Table 1 also illustrates the effect of the MgBi ratio on the tensile properties. When the Mg/Bi ratio is lower than 0.175 (Sample 1) the tensile properties are relatively low indicating that most of the magnesium is associated with the bismuth and does not influence the matrix properties. If the Mg/Bi ratio is above 0.175/1 as in Samples 2 to 8, the tensile strength of the alloy is improved thereby indicating that the excess magnesium strengthens the matrix. By comparing the aluminum content of Samples 5 and 6 with Sample 2, the usefulness of the M g/Bi addition is demonstrated over a wide range of aluminum composition where the amount of bismuth and magnesium remain almost constant. Likewise by comparing thecorrosion resistance of Samples 2, 7 and 8, the suitability of using from 0 to 4.2% copper is demonstrated.  
  The corrosion resistance of the alloys of this invention can further be improved by heat treatment. Table II also given at the end of the disclosure demonstrates the effect of various heat treatments on the corrosion resistance of an alloy which contains increasing amounts of bismuth with the appropriate level of magnesium. The amount&#39;of corrosion as a percentage of the specimen diameter is the measure of the amount of corrosion. Although increasing amounts of bismuth improve the chip forming characteristics of the alloy, increasing amounts of bismuth also decrease the corrosion resistance of the alloy as can be seen from the data in each column of Table 11.  
  Each of the heat treatments listed in Table 11 improved the corrosion resistance of the alloy in comparison to the corrosion resistance of the alloy in the asextruded and drawn by 31% condition.  
  Heat Treatment No. 1 of Table II involves furnace or slow cooling from 380C. which is a heat treatment described in U.S. Pat. application Ser. No. 108,199 filed Jan. 20, 1971 now U.S. Pat. No. 3,741,819. HeatTreatment No. 2 of Table II involves furnace or slow cooling from 380C. which is interrupted by air cooling at 250C. This is a heat treatment described in our copending United States application being filed of even date herewith.  
  Heat Treatment No. 3 of Table ll involves rapid or air cooling from 380C. which is below the solidus temperature of the alloy. In the three heat treatments the alloys were homogenized for 15 minutes at the temperature prevailing before the cooling phase of the heat treatment.  
  In the case of Alloy No. 9 of Table II having 3% bismuth all three heat treatments improved the corrosion resistance to the same level (from 14 to 10%). In the case of Alloy No. 2 of Table II with 0.5% Bismuth Heat Treatment No. 3 resutled in the most improvement in corrosion resistance (1.5 from 11%) while Heat Treatment No. 2 resulted in the next most improvement in corrosion resistance (2.5 from 11%). In the case of Alloy No. 4 of Table 11 with 0.25% Bismuth Heat Treatments No. 2 and 3 demonstrated about the same degree of improvement in corrosion resistance (0.4 and 0.5% respectively from 5%). The improvement using Heat Treatment No. 2 was slightly greater than with Heat Treatment No. 3 (but the difference was not considered particularly significant).  
  However, in the Heat Treatment of Alloy No. 2 (0.5% Bi) Heat Treatment No. 3 produced significantly more corrosion resistance than Heat Treatment No. 2. Therefore, heat Treament No. 3 appeared to be the more preferred heat treatment for general application. Other experimental tests demonstrated that homogenizing for longer periods of time, such as 18 hours, further improved the corrosion resistance of samples treated by Heat Treatment No. 3.  
  In applying Heat Treatment No. 3 to alloys containing less than about 18% aluminum, the solidus temperature becomes the eutectic temperature as is known from the zincaluminum phase diagram.  
  The alloys of the present invention clearly represent a useful advance in the art which should be of benefit to industry generally.  
 TABLE 1 EFFECT OF ALLOYS COMPOSITION ON THE CORROSION RESISTAN E AND ON THE TENSILE PROPERTIES OF EXTRUDED AND DRAWN BY 3l7zTALLOYS Magnesium in Ultimate Composition (Weight 7:) excess of Mg/Bi Amount of Tensile Alloy weight ratio Corrosion Strength 71 71 Reduc- Sample No. Al Cu Bi Mg of 0.175 &#39;71 Diameter lb/in Elongation tion Area 1 25.7 1.1 0.47 0.05 0.03) 247 48 500 7 2 26.0 1.0 0.45 0.13 0.05 l 1 7: 54I200 2i 3 26.8 1.1 0.46 0.23 0.15 5% 62,700 27 5 4 25.5 0.97 0.24 0.09 0.048 571 56.200 35 5S 5 21.9 1.0 0.44 0.136 0.059 9% 61.700 27 55 6 31.0 1.0 0.46 0.140 0.06 9% 61.900 45 7 25.6 0.53 0.134 0.041 5.69? 57.100 50 8 27 4.2 0.49 0.123 0.037 7.8% 64.300 21 26 Mg deficient allo TABLE II EFFECT OF HEAT TREATMENT ON AMOUNT OF CORROSION FOR ALLOYS CONTAINING VARIOUS LEVELS OF BISMUTH All alloy rods were extruded and drawn 3 1&#39;71. The alloys contain nominally Zn,257rAl. 1% Cu plus the mentioned amount of bismuth and the appropriate level of magensium (0.175 X 7! Bi 0.05% Mg).  
 Amount of Corrosion as 7: of Specimen Diameter Under Various Metallurgical Condition Drawn 5% after Drawn 5% after furnace cooling Drawn 5% after air furnace cooling*** from 380C. intcrcooling***from Alloy As Extruded from 380C rupted by air-cool- 380C Sample and Drawn ing at 250C*** Number by 31?! (Heat Treatment No.1) (Heat Treatment No.2) (Heat Treatment No.3)  
 9(371 Bi) l4 l0 l0 l0 2(0.57( Bi) ll 7 2.5 1.5 4(0.25% Bi) 5 4 0.4 0.5  
  Heat treated according to Canadian Patent Application No. 030,768 &#34;Heat treated according to the co-pending Canadian Patent Application.  
 &#39;&#34;All alloy rods were homogenized 15 minutes at temperature before heat treatment.  
 We claim:  
 l. A method of heat treating a zinc-aluminum alloy 40 having a zinc-aluminum eutecoid transformation comprising preparing a zinc-aluminum alloy consisting essentially of about 0.01 to about 3% bismuth, about 0.01 to 1% magnesium and, 0 to about 10% copper, the balance, apart from incidental impurities, being zinc and aluminum, wherein the ratio of magnesium to bismuth is such as to provide sufficient magnesium to combine with substantially all the bismuth in the form of the intermetallic compound Bi Mg and homogenizing said alloy at a temperature below the solidus temperature and above the eutectoid temperature of the alloy and then cooling the alloy to room temperature. 2. A method of heat treating a zinc-aluminum alloy comprising preparing a zinc-aluminum consisting essentially of about 18 to 30% aluminum, about 0.01 to 1% magnesium, about 0.1 to 3% bismuth, 0 to about 5% copper, the balance being zinc and incidental impurities, wherein the ratio of magnesium to bismuth is such as to provide sufficient magnesium to combine with substantially all the bismuth in the form of the intermetallic compound Bi Mg homogenizing said alloy at a temperature below the solidus temperature and above the eutectoid temperature of the alloy and then rapidly cooling said alloy to room temperature.  
  3. The method of heat treating defined in claim 2 wherein the composition of said alloy is such that the percentage by weight of magnesium equals 0.175 times the percentage of bismuth plus an excess of magnesum.  
  4. The method of heat treating defined in claim 1, wherein said alloy is homogenized at about 380C. and wherein said cooling is effected rapidly by air cooling.  
  5. The method of heat treating defined in claim 3 wherein said alloy contains 22 to 27% aluminum, 0.5 to 1.5% copper, 0.05 to 6% magnesium. 0.2 to 1.5% bismuth, the balance being zinc and incidental impurities wherein the magnesium content of said alloy is equal to 0.175 times the bismuth content plus 0.02% magnesium and wherein the alloy is homogenized at about 380C. and then air cooled.  
  6. The method of heat treating as defined in claim 5 wherein the bismuth content of the alloys is 0.2 to 0.6%.