Title of Invention

A METHOD OF PREPARING A MAGNESIUM BASE ALLOY FOR HIGH PRESSURE DIE CASTING

Abstract

188/MAS/96 The present invention relates to a method of preparing a magnesium base alloy for high pressure die casting comprising the step of heating a mixture of at least 91 weight percent magnesium, 0.1 to 2 weight percent of zinc, 2 to 5 weight percent of a rare earth metal component, o to 1 weight percent of calcium, 0 to 0.1 weight percent of an oxidation inhibiting element other than calcium, 0 to 0.4 weight percent zirconium, hafnium and/or titanium; 0 to 0.5 weight percent manganese 0 to 0.001 weight percent of strontium, 0 to 0.05 weight percent of silver and 0 to 0.1 weight percent of aluminium and substantially free of undissolved iron, at a temperature higher than the melting temperature of the magnesium.

Full Text

Che automotive industry, and high purity versions of known alloya, such as AZ91 and AM60, are beginning to be used in this market because of their greatly enhanced corrosion resistance, Howavsr, both of these alloys have limited capability at elevated temperatures, and are unsuitable for applications operating much above 100°C, 10 Some of the properties considered to be desirable in an HPDC alloy are: a) Creep strength of the product at 175°C as good as AZ91 type alloya at 150°C. b) Room temperature strength of ths product similar lb to AZ91 type alloys. c) Good vibration damping. d) castability of the alloy aimilar to, or better than A7,91 type, alloya. e) Corrosion resistance of the product similar to 20 AZ91 type alloys. f} Thermal conductivity of the product preferably better than A291 type alloys, g) Cost equivalent to AZ91 type alloys 25 One successful alloy development at this stage was within the Mg-Al-Si-Mn system, giving alloys such as those known, aa AS41, AS21 and ASH; only the first of these has been fully exploited; the other two, although offering even higher creep strengths, are generally regarded as difficult 30 to cast, particularly since high melt temperatures are required. AS41 meets most of the objectives listed above, although its liquidua temperature is about 30°C higher than that of AZ91 type alloys. Magnesium Alloys This invention relates to magnesium alloys. 5 High pressure die cagt (HPDC) components in magnesium base alloys have been succegsfully produced for almost 60 years, using both hot and cold chamber machines. Compared to gravity or aand casting, HPDC is a rapid 10 process suitable for large scale manufacture. The rapidity with which Che alloy solidifies in HPDC means thae the cast product has different propercies relative to the same alloy when gravity cast. In particular, the grain size is normally finer, and this would generally be expected to 15 give rise to an increase in tensile strength with a concomitant decrease in creep reaiatance. Any tendency to porosity in the cast product may ba alleviated by the use of a "pore free" processs (PFHPDC) in 20 which oxygen is injected into the chamber and is gettered by the casting alloy. The relatively coarse grain size from gravity casting can be reduced by the addition of a grain refining component, 25 for example zirconium in non-aluminium containing alloys, or carbon or carbide in aluminium containing alloys. By contrast, HPDC alloys generally do not need, and do not contain, such component. 30 Until the mid 1960's it would be fair Co say that the only magnesium alloys used commercially for HPDC were baaed on the Mg-Al-Zn-Mn system, such as the alloys known as AZ91 and variants thereof. However, since the mid I960's increasing interest has been shown In the use of magnesium 35 base alloys for non-aerospace applications, particularly by ABSTRACT S A magnesium base alloy for high pressure die casting (HPDC), providing good creep and corrosion resistance, comprises: at least 91 weight percent magnesium; 0.1 to 2 weight percent of zinc; 10 2 to 5 weight percent of a rare earth metal component; 0 r.o 1 weight percent calcium; 0 to 0.1 weight percent of an oxidation inhibiting element other than calcium (e.g. Be); 3,5 0 to 0.4 weight percent zirconium, hafnium and/or titanium; 0 to 0.5 weight percent manganese ; no more than 0.001 weight percent strontium; no more than 0.05 weight percent silver; and 20 no more Chan 0.1 weight percent aluminium. any remainder being incidental impuriciea. For making prototypes, gravity (e.g. sand) cast and HPDC components from the alloy have similar mechanical 25 properties, in particular tensile strength. The temperature dependence of the latter, although negative, is much less ao than for some other known alloys. Another aeries ot alloys developed at: about the same time included a rare earth component, a typical example being AE42. comprising ot the order of 4% aluminium, 2% rare earth(s), about 0.25% manganese, and the balance magnesium 5 with minor components/impurities. This alloy has a yield strength which is similar at room temperature to that of AS41, but which is superior ac temperatures greater than about 150°C i.even 30, the yield strength still shows a relatively marked decreaae in value with rising 10 temperature, as will be mentioned again below). More importantly, Che creep strength of AE42 exceeds even AS21 alloy at all temperatures up to at leant 200°C. The present invention relates to magnesium based alloys of: lb the Mg-RE-Zn system (RE=rare earth). Such systems are known. Thus British Patent Specification No. 1 3 79 231 discloses magnesium based light structural alloys which comprise neoaymium, zinc, zirconium and, optionally, copper and manganese. A further necessary component in these 20 alloys is 0.8 to 6 weight percent yttrium. British Patent Specification No, 1 023 128 also discloses magnesium base alloys which comprise a rate earth metal and zinc. in theae alloys, the zinc to rare earth metal ratio 25 is from 1/3 to 1 where there is less than 0.6 weight percent of rare earth, and in alloys containing 0.6 to 2 weight percent rare earth metal, 0.2 to 0.5 weight percent of zine is present. 30 More particularly British Patent Specification Nos 607588 and 637040 relate to systems containing up to 5% and 10% of zinc respectively. In GB 607588, it is stated that "The creep resistance is not adversely affected by the presence of zinc in small or moderate amounts, not 35 exceeding S per cent for example....", and "The presence of zinc in amounts of up to 5 per cent haa a beneficial effect on the foundry properties for these types of casting where it is desirable to avoid localised contraction or: solidification and aome dispersed unsoundness would be leas 5 objectionable". A typical known gystem ia the alloy ZE53, containing a nominal 5 percent zinc and a nominal 3 percent rara earth component.. In these systems it is recognised that the rare earth 10 component gives rise to a precipitate at grain boundaries, and enhances castability and creep resistance, althouqh there may be a alight decrease in tensile strength compared to a similar alloy lacking such component. The high melting point of the precipitate assists in maintaining the 15 properties of the casting at high temperatures. The two British patents last mentioned above refer to aand casting, and specitically mention the desirability of the presence of zirconium in the casting alloy as a grain 20 refining element. To be effective tor such purpose, the neceBsary amount of zirconium ia said to be between 0.1 and 0.9 weight percent (saturation level) (OB 6075Q8) or between 0.4 and 0.9 weight percent (GD 637040). 25 As used hereinafter, by the term "rare earth" is intended any element or mixture of elements with atomic numbers 57 to 71 (lanthanum to lutetium) . While lanthanum is, strictly speaking not a rare earth element, it may or may not be present; however, "rare earth" is not intended to 30 include elements such as yttrium. The present invention provides a magnesium base alloy for high pressure die casting comprising at least 91.9 weight percent magnesium; 35 O.l to 2 weight percent of zinc; 2 Co 5 weight percent o£ a rare earth metal component; 0 to 1 weight- percent calcium; 0 to 0.1 weight percent of an oxidation 5 inhibiting element other than calcium; no more than O.001 weight percent strontium; no more than O.Ob weight percant silver; less than O.l weight percent aluminium, and substantially no undissolved iron; 10 any remainder being incidental impurities. The invention also providea a magnesium baae alloy for high pressure die casting comprising at least 91 weight percent magnesium; lb .. 0.1 to 2 weight percent of zinc; 2 to 5 weight percent of a rare earth metal component 0 to 1 weight percent calcium; 0 to 0.1 weight percent of an oxidation 20 inhibiting element other than calcium; 0 to 0.4 weight percent zirconium, hafnium and/or titanium; 0 to 0 .5 weight percent manganese; no more than C.OOl weight percent strontium; 25 no more than 0.05 weight percent silver; and no more than 0.1 weight percent aluminium. any remainder being incidental impurities. Calcium, manganese, zirconium/hafnium/titanium and any 30 element other than calcium which inhibits oxidation (for example beryllium) are optional components, and their contributions to the composition will be discussed later. A preferred range for zinc is 0.1 to l weight percent, and 35 more preferably 0.2 to O.S weight percent. Following Che ASTM nomenclature system, an alloy containing a nominal X weight percent rare earth and Y weight percent zinc, where X and Y are rounded down to the nearest integer, and where X ia greater than Y, would be referred 5 to as an EZXY alloy. This nomenclature will be used for prior art alloys, but alloys according to the invention as defined above will henceforth be termed MEZ alloys whatever their precise 10 composition. Compared with ZE53, MEZ alloys can exhibit improved creep and corrosion resistance (given the same thermal treatment), while retaining good casting properties; zinc 15 is present in a relatively small amount, particularly in the preferred alloys, and the zinc to rare earth ratio is no greater than unity (and is significantly less than unity in the preferred alloys) compared with the 5:3 ratio for ZE53. 20 Furthermore, contrary to normal expectations, it has been found that MEZ alloys exhibit no very marked change in tensile strength on passing from sand or gravity casting to HPDC. In addition the grain structure altera only to a 25 relatively minor extent. Thus MEZ alloys have the advantage that there is a reasonable expectation that the properties of prototypes of articles formed by sand or gravity casting will not be greatly different from those of such articles subsequently mass produced by HPDC. 30 By comparison, HPDC AE42 alloys show a much finer grain structure, and an approximately threefold increase in tensile strength at room temperature, to become about 40% greater than MEZ alloys. However, the temperature 35 dependence of tensile strength, although negative for both types of alloy, is markedly greater for AE42 alloys Chan for HEZ alloys, with the result chat at above about 150°C the MEZ alloys tend tO have greater tensile strength. 5 Furthermore, the crssp strength of HPDC AE42 alloys is markedly lower than that of HPDC MEZ alloys at all tamperaturee up to at leagt 177°C. Preferably the balance of the alloy composition, if any, is 10 less Than 0.15 weight percent. The rare earth component could be cerium, cerium mischmetal or cerium depleted mischmecal. A preferred lower limit to the range is 2. 1 weight percent. A preferred upper limit 15 is 3 weight percent. An MEZ alloy preferably contains minimal amounts of iron, copper and nickel, to maintain a low corrosion rate. There is preferably lose than 0.005 weight percent of iron. Low 20 iron can be achieved by adding zirconium, (for example in the form of Zirmax, which is a 1:2 alloy of zirconium and magnesium) effectively to precipitate the iron from the molten alloy; once cast, an MEZ alloy can comprise a residual amount of up to 0.4 weight percent zirconium, but 25 preferred and most preferred upper limits for this element are 0.2 and 0.1 weight percent respectively. Preferably a residue of at least 0.01 weight percent is present. Zirmax is a registered trademark of Magnesium Elekcron Limited. 3 0 Particularly where at least some residual zirconium is present, the presence of up to 0.5 weight percent manganese may also be conducive to low iron and reduces corrosion. Thus, as described in greater detail hereinafter, the addition of as much as about 0.3 weight percent of 35 zirconium tbut more commonly 0.S weight per cent) might be required to achieve an iron content of leas than 0.003 weight percent; however, the same result can be achieved with about 0.06 weight percent of zirconium if manganese is also present. An aiternative agent for removing iron is 5 titanium. The presence of calcium ig optional, but is believed to give improved casting properties. A minor amount of an element such as beryllium may be present, preferably no 10 less than 0.0005 weight percent,, and preferably no more than 0.005 weight percent, and often around O.001 weight percent, to prevent oxidation of the melt. However, if" it is found that auch element {for example beryllium) is removed by the agent (for example zirconium) which ia 15 added to remove the iron, substitution thereof by calcium might in any case be necessary. Thus calcium can act as both anti-oxidant and Co improve casting properties, if neceeaary. 20 Preferably there is less than 0.05 weight per cent, and more preferably substantially no aluminium in the alloy. Preferably the alloy contains no more than 0.1 weight percent of each of nickel and copper, and preferably no more Chan 0.05 weight percent copper and 0.005 weight 25 percent nickel. Preferably there is substantially no strontium in the alloy. Preferably the alloy comprises substantially no silver. As cast, MEZ alloys exhibit a low corrosion rate, for 30 example of less than 2.50 mm/year (lOO mila/year) (ASTM B117 Salt Fog Test) . After treatment T5 U4 hours at 250'C) the corrosion rate is still low. As cast, an MEZ alloy may have a creep resistance auch that 35 the time co reach 0.1 percent creep strain under an applied stress of 46 MPa ac 177°C is greater than 500 hours; after treatment T5 the time may atill be greater than 100 hours. The invention will be further illustrated by reference to 5 the accompanying Figures, and by reference to the appended Tables which will be described as they are encountered, in the Figures; Figure 1 shows Che grain structure of gravity case ZE53 10 with high zirconium, melt DF2218; Figure 2 shows the grain structure of gravity cast ZE53 with manganese added, melt DF2222; 15 Figure 3 shows the grain structure of gravity cast MEZ with high zirconium, melt DF2220; Figure 4 shows the grain structure of gravity cast MEZ with manganese added, melt DF2224; and 20 Figure 5 shows the grain structure of gravity cast MEZ with low zirconium, malt DF2291. Figure 6 illustrates and compares the tensile properties of 25 pore free HPDC alloys MEZ and AE42; Figure 7 illustrates and compares Che tenaile properties of HPDC MEZ and pore free HPDC (PFHPDC) alloys MEZ; 3 0 Figure 8 illustrates the effect of heat treatment on the tensile properties of PFHPDC MEZ at various temperatures; Figure 9 shows the results of measuring creep resistance of PFHPDC MEZ, AE42 and 2071 under various conditions of 3 5 stress and temperature; Figure 10 shows the grain structure of PFHPDC MEZ in the aa cast (F) condition; Figure 11 shows the grain structure of PFHPDC MEZ in the T6 5 heat treated condition, and Figure 12 shows the porosity of HPDC MEZ. The condition F ie "as cast", and T5 treatment involves 10 maintaining the casting at 250°C for 24 hours. For T6 treatment the casting is held at 420°C for 2 hours, quenched 'into hot water, held at 180°C for 18 hours and cooled in air. 15 An initial investigation was made into the properties of ME2 alloys and iiE53 alloys in the gravity cast state. Table 1 relates to ZE53 and MEZ alloys, and indicates the effect of manganese or zirconium addition on the iron, 20 manganese and zirconium content of the resulting alloy. The first eight of the compositions of Table 1 comprise four variations of each of the alloys MEZ and ZE53. One set of four compositions has manganese added to control the 25 iron content, and the other set has a relatively high zirconium addition (saturation is about 0.9 weight percent) for the same purpose, and arrow bars were gravity cast therefrom. A different set of four selected from these eight compositions is in the as cast state, with the 30 complementary set in the T5 condition. Table 2 indicates the compositions and states of these eight alloys in more detail, and measurements of the tensile strength of the arrow bars. 35 Table 3 gives comparative data on creep properties of theae eight alloys MEZ and ZE53 in the form of the gravity cast arrow bars. 5 Table 4 gives comparative data on corrosion properties of the eight alloy compoaitions in the form of the gravity cast arrow bars, and illustrates the effect of T5 treatment on the corrosion rate. 10 Corrosion data on another two of the alloys listed in Table 1 is contained in Table 5, measurements being taken on a sequence of arrow bars from each respective single casting. In addition to the elements shown in the Table, each of alloys 2290 and 2291 included 2.5 weight percent rare 15 earth, and 0.5 weight percent zinc. This table is worthy of comment, since it shows that those bars which are first cast are more resistant to corrosion than those which are cast towards the end of the process. While not wishing to be bound to any theory, it seems possible that the iron is 20 precipitated by the zirconium, and that the precipitate tends to settle from the liquid phase, so that early bars are depleted in iron relative to later castings. Figures 1 to 5 show grain structures in some of these 25 gravity cast arrow bars. From this initial investigation it can be seen that while T5 treatment is beneficial to the creep properties of gravity cast ZE53 alloys, it is detrimental to gravity cast 30 MEZ alloys {Table 3). The creep strengths of ZE53 + Zr and both types of MEZ alloy are significantly greater than that of AE42 alloy, and indeed are considered to be outstanding in the case of both MEZ alloys in the aa-cast (F) condition and the ZES3 with zirconium alloy in the T5 condition. The 35 T5 treatment also benefits the tensile properties of ZE53 with zirconium, but has no significanc effect on the other three types of alloy (Table 2). It will also be seen that iron levels have a significant 5 effect on corrosion rate of all the alloys (Tableg 4 and 5) . Zinc also has a detritnental effect, and the corrosion resistance of 2E53 was found to be poor even with low iron Content. T5 treatment further reduces the corrosion resistance of all alloys. In addition, iron levela remain 10 comparatively high even in the preaence of 0.3% Mn (no Zr being present). When the amount of iron is sufficiently great as to form an insoluble phase in the alloy, corrosion is significant. 15 However, when the amount is sufficiently low for all the iron to remain dissolved within the alloy itself, corrosion is far leas of a problem, and accordingly MEZ alloys contain substantially no iron other than that which may be dissolved in the alloy, and preferably substantially no 20 iron at all. As a result of further testing, it was found that Co obtain a suitably low iron level, gay 0.003%, an addition of at least 6% Zirmax was necessary in the case of both MEZ and 25 ZE53. However, if manganese is also present, the necessary addition of Zirmax {or equivalent amount of other zirconium provider) is reduced to about 1%. Casting alloys undergo a certain amount of circulation 30 during the casting process, and may be expected to undergo an increase in iron content by contact with ferrous parts of Che casting plant. Iron may also be picked up from recycled scrap. It may therefore be desirable to add sufficient zirconium to the initial alloy to provide a 35 residual zirconium content sufficient to prevent this undesirable increase in iron (up to 0.4 weight percent, preferably no more than 0.2 weight percent, and most preferably no more Chan 0.1 weight percent). This may be found to be more convenient than a possible alternative 5 course of adding further zirconium prior to recasting. In one trial, it was found that HEZ material with 0.003% iron resulting from a 0.5% Zirmax addition underwent an increase in iron to 0.006% upon remelting, with the 10 zirconium content falling to 0.05%. However, MEZ material with 0.001% iron resulting from a 1% Zirmax addition underwent an increase in iron only to 0.002% upon remelting/ with the zirconium content remaining subetantially constant. 15 To investigate the properties of HPDC alloys, an ingot of MEZ of composition 0.3% Zn, 2.6% RE {rare earth) , 0.003V Fe, 0.22% Mn and 0.06% Zr was cast into test bars using both HPDC and PFHPDC methods. The details of the casting 20 methods are appended (Appendix A). Analysis of the bars is given in Table 6, where FCl, PC2, PC3 respectively represent samples taken at the beginning, middle and end of the casting trial. The high Zr figure of 25 the first listed composition indicates that insoluble zirconium was present, suggesting an error in the sampling technique. Table 7 and Figures 6 to 8 indicate the measured tengile 30 properties of the teat bars, together with comparative measurements on similar bars of AE4 2 alloy. It will be seen that MEZ and AE42 have similar yield strengths, but that while AE42 has a superior tensile strength at room temperature, the situation is reversed at higher 35 temperatures. There appeared to be no useful advantage Tables 10 and 11 respectively, it will be seen that there is a close resemblance between the tensile properties if the aandcast and diecast products. 5 In a separate test, a further ingot from the same batch was melted, but 6 weight percent of zirmax (33% Zr) was added using conventional magnesium foundry practice. The analysis of the resulting melt gave 0,58 weight percent zirconium. 10 A section from a sandcasting made from this melt, of the same automotive oilpan configuration as above, was tensile tested at ambient temperature. 0.2% PS was 102 MPa, UTS was 178 MPa, and elongation was 7.3%, figures which are 15 very similar to those of Tables 10 and 11. These results may be contrasted with those for the alloy AE42 {Mg-4%A1-2%RE-Mn) , not within the present invention, which may be used for applicaj:ions requiring good creep 20 resistance sit elevated temperatures. In this case, although satisfactory properties can be generated in HPDC components, as illustrated elsewhere in this specification it is impossible to generate satisfactory properties in the alloy by conventional sand casting techniques. 25 For example, an alloy AE42 (3.68* Al; 2.0% RE; 0,26 Mn) was cast into steel chilled "arrow bar" moulds, Tensile properties of specimens machined from these bars were only 46 MPa (0.2% PS) and 12S MPa (UTS). Similar bars cast in 30 an MEZ alloy gave values as high as 62 MPa (0,2% PS) and 180 MPa (UTS) (0.5% Zn; 2.4% RE; Q.2% Mn). a) MEZ PFHPPC TRIAL 5 TIME OBSERVATION 0500 Furnace l on, crucible fully charqed wich half ingor. 109 kgs) . HOC Charge tally molten 650°C, 10 1315 Melt controlling at 684°C surface somewhat drossy. 0500 Furnace 2 on, remaining melt (approx 20 kg) from pre trial melted. 1100 charge fully molten 650°C. 1315 Melt controlling at 690°C - surface gomewhat drossy. 15 Both melts protected with Air + SF. Heavy oxide/sulphide skina evident on melt surfaces. 1325 Both halves of die mould preheated with gae torch (fixed half 41°c;, moving half 40°C) . Die sleeve preheated with metal ladle poured from Furnace 2. 20 1330 Die mould further preheated by injection of metal ladle poured from Furnace 2. Three injecCiona raised die temperature fixed half to 50°C and moving half to 51°C. (FCl analysis sample ladle poured). 1335 Oxygen switched on at 100 litres/min. Bar canting 25 begins. Metal supply, ladle poured from No. 1 furnace for each shot (800g) . Die mould sprayed with graphite water based inhibited release agent throughout. 1340 Casting stopped after 3 ahots metal chilling on 30 ladle- Melt temperature raised to 700°C. 1343 Re-start casting at 683oC casting rises to 700°C. Stop coating, adjust otroke of plunger. 1350 He - start casting. No. 11 castings fractured (8 and 10mm dia baral both show good fracture. 1400 Casting stopped. 14 shots) plunger cleaned of oxide contaminar. ion. 1410 Restart casting melt temperature 7 01°C. Fixed half die temperature 701°C. Moving half die temperature 67°C. (FC 2 analysis sample ladle poured'. 1455 Casting complete after 40 shots. 120 tensile bars • 40 charpy bars. (FC3 analysis sample ladle poured). NOTE: A further 10 PFHPDC ahota wera carried out following the HPDC trial giving a total of 150 tensile bare 50 charpy bars. Identification of each bar was carried out by marking each one reapectively P-1, P2, P-3, P-4, etc, b) MEZ HPDC TRIAL TIMS OBSERVATION 1535 Melt temperature in furnace l ® 699°c. Die mould preheated with first shot and bars discarded. Fixed half die mould temperature 74°C. Moving half die mould temperature 71°C. 1536 Bar casting begins, without oxygen, but with the sane casting parameters as the PFHPDC trial, i.e. Preasure of 300 kgs/cm2 1.2 metres/sec plunger speed. 100 - 200 metres/acc at the ingate. Die locking force ot 350 ton kg/cm2. (FCl analysis sample ladle poured). 1550 Bars 8mm dia and 10mm dia from shots 11 and 12 were fractured. Very alight shrinkage/entrapped air was observed- 1600 Fixed half die mould temperature increaaee to 94°C. Moving half die mould temperature increased to 89°C. (FC2 analysis aample ladle poured after shot 21, temp 7 0 2 ° C . ) 1610 Casting stopped die mould cooled. Fixed half cooled to 83°C. Moving halt cooled to 77°c, l620 Re-Starr casting. 1550 Casting complete atter 42 shots, 120 tensile bars 42 charpy bars. (FC3 analysis sample ladle poured). NOTE: A further 10 HPDC ahots were carried out following this trial giving a total of 152 tensile bars + 52 charpy bars. Identification ot each bar was carried out by marking each one respectively 0-1, 0-2, 0-3, etc. (c) AE42 HPDC Trial 0200 Furnace on, crucible previously fully charged with ha 1f i ngot s. 1000 Meit at 680°c. Die heating begins. 1005 Die temperature at 85°C. 1015 Sleeve heating using melt aampie begins. Melt surface much cleaner than ZC7l. Casting surfaces also less discolnuTfid. 1240 Casting run begins. 1430 Casting run terminated , 1. A magneaium base alloy tar high presaure die cagcing comprising at lease 91.9 weight, percent magneaium; 0.1 r.o 2 weight percent of zinc; 2 Co 5 weight percent of a rare earth metal component. ; 0 to 1 weight percent calcium; 0 to 0.1 weight percent of an oxidation inhibiting element other than calcium; no more than O.001 weight percent strontium; no more than 0.05 weight percent silver; leas Chan 0.1 weight percent aluminium, and substantially no undissolved iron; any remainder being incidental impurities. 2. A magnesium base alloy for high pressure die casting comprising at least 91 weight percent magneaium; 0.1 to 2 weight percent of zinc; 2 to 5 weight percent of a rare earth metal component; 0 to 1 weight percent calcium; 0 to 0.1 weight percent of an oxidation inhibiting element other than calcium; 0 to 0.4 weight percent zirconium, hafnium and/or titanium; 0 to 0.5 weight percent manganese; no more than O.001 weight percent strontium; no more than 0.05 weight percent silver; and no more than 0.1 weight percent aluminium. any remainder being incidental impurities. 3. An alloy according to claim l or claim 2 wherein the balance of the alloy composition, if any, is leas than 0.:5 weiqht percent. . An alloy according to any one of claims l to 3 comprising lees than 0.005 weight percent of iron. 5. An alloy according to any preceding claim which contains no more than 0.05 weight percent aluminium. 6. An ailoy according to any preceding claim which is eubatantially free of aluminium. 7. An alloy according to any preceding claim containing no more than 0.l weight percent of sach of nickel and copper. 6. A cast alloy according to any preceding claim having a creep reeiatance such that the time to reach 0. i percent creep strain under an applied stress of 46 MPa at 177oC is greater than 500 hours. 9. An alloy according to any preceding claim which after heating to 250°C for 24 hours has a creep resistance buch that the time to reach O.1 percent creep strain under an applied stress of 45 MPa at 177°C is greater than 100 hours. 10. A cast alloy according to any preceding claim exhibiting a corrosion rate of leas than 2.5 mm/year. 11 - An alloy according to any preceding claim wherein Che rare earth component is cerium, cerium mischmetal or cerium depleted mischmetal. This invention relates to mangesium alloys. High pressure die cast (HPDC) components in magnesium base alloys have been successfully produced for almost SO years using both hot and cold chamber machines. Compared to gravity or sand casting, HPDC is a rapid process suitable for large scale manufacture. The rapidity with which the alloy solidifies in KPDC means that the cast product has different properties relative to the same alloy when gravity cast. In particular, the grain size is normally finer, and this would generally be expected to give rise to an increase in tensile strength with a concomitant decrease in creep resistance. Any tendency to porosity in the cast product may be alleviated by the use of a "pore free" process (PPHPDC) in which oxygen is injected into the chamber and is gettered by the casting alloy. The relatively coarse grain size from gravity casting can be reduced by the addition of a grain refining component, for example zirconium in non-aluminium containing alloys, or carbon or carbide in aluminium containing alloys. By contrast, HPDC alloys generally do not need,' and do not contain, such component. Until the mid 1960's it would be fair to say that the only magnesium alloys used commercially for HPDC were based on the Mg-Al-Zn-Mn system, such as the alloys known as AZ91 and variants thereof. However, since the mid I960's increasing interest has been shown in Che use of magnesium base alloys for non-aerospace applications, particularly by Following Che ASTM nomenclature system, an alloy containing a nominal X weight percent rare earth and Y weight percent zinc, where X and Y are rounded down to the nearest integer, and where X is greater than Y, would be referred to as an EZXY alloy. This nomenclature will be used fcr prior art alloys, but alloys according to the invention as defined above will henceforth be termed MEZ alloys whatever their precise composition. Compared with ZE53, MEZ alloys can exhibit improved creep and corrosion resistance (given the same thermal treatment), while retaining good casting properties; zinc is present in a relatively small amount, particularly in the preferred alloys, and the zinc to rare earth ratio is no greater than unity (and is significantly less than unity in the preferred alloys) compared with the 5:3 ratio for ZE53. Furthermore, contrary to normal expectations, it has been found that MEZ alloys exhibit no very marked change in tensile strength on passing from sand or gravity casting to KPDC. In addition the grain structure alters only to a relatively minor extent. Thus MEZ alloys have the advantage Chat there is a reasonable expectation that the properties of prcotypes of articles formed by sand or gravity casting will not be greatly different from those of such articles subsequently mass produced by HPDC. By comparison, HPDC AE42 alloys show a much finer grain structure, and an approximately threefold increase in tensile strength at room temperature, to become about 40% greater than MEZ alloys. However, the temperature dependence of tensile strength, although negative for both This invention relates to magnesium alloys. High pressure die cast (HPDC) components in magnesium base alloys have been successfully produced for almost 60 years. using both hot and cold chamber machines. Compared to gravity or sand casting, HPDC is a rapid process suitable for large scale manufacture- The rapidity with which the alloy solidifies in KPDC means that the cast product has different properties relative to the same alloy when gravity cast. In particular, the grain size is normally finer, and this would generally be expected to give rise to an increase in tensile strength with a concomitant decrease in creep resistance. Any tendency to porosity in the cast product may be alleviated by the use of a "pore free" process (PFKPDC) in which oxygen is injected into the chamber and is gettered by the casting alloy. The relatively coarse grain size from gravity casting can be reduced by the addition of a grain refining component, for example zirconium in non-aluminium containing alloys, or carbon or carbide in aluminium containing alloys. By contrast, KPDC alloys generally do not need, and do not contain, such component. Until the mid 1960's it would be fair to say that the only magnesium alloys used commercially for KPDC were based on the Mg-Al-Zn-Mn system, such as the alloys known as AZ91 and variants thereof. However, since the mid 1960's increasing interest has been shown in the use of magnesium base alloys for non-aerospace applications, particularly by 12. An alloy according to any preceding claim and comprising : l to 3 weight percent of the rare earth component. 13, An alltY according to any preceding claim, and compriaing in more than 1 weight percent zinc. 14 An alloy according to any preceding claim and comprising n; more than 0.6 weight percent zinc. 15. An alloy according to any preceding ciaim and compriaing substantially no aluminium and/or substantially no strontium and/or substantially no silver. 16. An alloy for high pressure die casting as claimed in claim 1 and substantially as hereinbetore described. 17. A method of producing a cast product wherein high prGseure die casting is used in conjunction with an alloy as claimed it any preceding ciaim. 19. A method according to claim 17 wherein a pore free high presaurs die casting method is used. 19. A cast product produced by the method according to ciaim 17 or :laim 18. 20. A magnesiun base alloy for high pressure die oasting. substantially as herein described and illustrated with referenoe to the accompanying drawings.

Documents:

0188-mas-1996 abstract.pdf

0188-mas-1996 assignment.pdf

0188-mas-1996 claims.pdf

0188-mas-1996 correspondence-others.pdf

0188-mas-1996 correspondence-po.pdf

0188-mas-1996 description(complete).pdf

0188-mas-1996 drawings.pdf

0188-mas-1996 form-10.pdf

0188-mas-1996 form-2.pdf

0188-mas-1996 form-26.pdf

0188-mas-1996 form-29.pdf

0188-mas-1996 form-4.pdf

0188-mas-1996 form-6.pdf

0188-mas-1996 form-9.pdf

0188-mas-1996 others.pdf