The manufacture of aluminium-bronze castings L. R. VAIDYANATH T HE development of aluminium-bronze alloys has SYNOPSIS basically been due to a combination of useful pro- perties particuarly appealing to the designers in Employing practices and equipment generally, available various engineering fields, the most important applica- for copper allv.r castings like gun metals and phosphor- tion being in the marine and hydraulic field. Important bronze, attempts have been made by Indian foundries to properties of aluminium-bronzes include the follo%%ing manufacture aluminium-bronze castings trithout under- standing its basic principles. The paper has tried to deal (1) Useful combination of mechanical properties with frith factors that should be considered for economic pro- ability to retain these properties at moderately duction of aluminium-bronze castings. elevated temperature Briefly explaining the metallurgy of the alloy, the (2) Excellent resistance to corrosion and cavitation parameters governing foundry practice of aluminium- erosion bronze have been discussed. Factors like the effect of (3) High impact value, and short freezing range, structure, effect of casting and (4) High wear resistance. mould variables on mechanical properties, control of composition and gas absorption, etc. are explained. In the design of castings. weight plays an important Concluding, suggestions have been made to adopt such part both from the economical and functional points techniques that would give a sound casting. of view and castings of aluminium-bronzes have ful- filled these requirements admirably. Perhaps this is the only alloy in the copper alloy series where almost 80% tic knowledge of the alloy through experience is of the test bar strength can he achieved in castings. lacking and there is not much published literature Although the properties of aluminium-bronze have for the guidance of the founder . It is imperative that been known for a long time, the use of the alloy in attempts in casting of this alloy will require consi- various fields-have not been appreciable but followed derable experimental work and therefore due allowance closely improvements in its casting and fabrication should be made for lapses until complete confidence techniques. Castings of aluminium-bronze are generally is obtained for the manufacture of defect-free castings. employed as components of pumps, valves for water The main bottleneck in the growth of the aluminium- and steam, gear wheels, helical gearings of certain type, bronze industry was due to lack of understand- chemical and marine fittings and in food industries. Of ing of the alloy . Metallurgically the alloy was treated the two principal alloys available, the AB-I type of in the manner of standard brasses and bronzes and alloy containing nominally 10% Al and 2.5° , Fe is economically it was considered unfavourable compared used now mostly for die-casting. Bulk of the sand cas- to brass and steel. A better evaluation of the casting tings are made from the second AB-2 alloy, which has problems of the alloy can be made when the metal- a similar aluminium content but with additional quanti- lurgical principles involved are properly understood. ties of 5°,%, each of nickel and iron. This alloy with slight compositional modifications is used in the manufacture Theoretical considerations of marine propellers, water turbine runners, pelton wheels, impellers and in many applications where resistance to The problem of manufacture of aluminium-bronze cavitation erosion is important. Resistance to pickling casting is generally associated with lack of adequate by sulphuric acid led to its use in pickling hooks, technical and foundry control of the process parti- chains, etc. cularly in the case of complex design and when Although the alloy was available for casting pur- service conditions are severe. Compared to the more poses for many years, the difficulties involved in common foundry alloys like gun metals and phosphor- economic production of the alloy, free from casting bronzes, the aluminium-bronze has different metallurgical defects, severely restricted its rapid development. Speci- characteristics. A foundry, well conversant with manu- Dr L. R. 'aidhanath, General Manager, Indian Copper Inform- facture of gun metal castings, will therefore be ill- ation Centre , Calcutta. advised to attempt aluminium-bronze castings in a 72 Vaidyanath : The manufacture of aluminium bronze castings 12001 mation of $ phase with martensitic structure which has lower elongation. The -(/fl alloys have favourable LlaUI D combination of strength and corrosion resistance. The eutectoid structure of which has a lower electro- 1100 chemical potential corrodes at a higher rate and has therefore to be avoided.' The decomposition of 9 to k+y,, which occurs during slow cooling or reheating 1000 in the temperature range between 550 C and 350°C, has to be avoided. Cooling rates have therefore to be drastic. The most important aspect is the eutectoid transfor- mation in which the phases are 1. -c Al bronze having f. c. c. lattice similar to iron in its working characteristics. 2. d Al bronze with a b. c. c. structure. 3. /3 Al bronze which at the eutectoid transfor- 700 mation of 525°C transforms to c--ye. Role of added elements 600 The role of alloying elements like nickel, iron and manganese which tend to stabilize f3 and effectively permit slower cooling rate, is very important. When 500 iron and nickel are present at a nominal level of 5% each, they modify the structure of aluminium- bronze and instead of the normal a.-i 9 an a.-K 400 structure with small but tolerable amounts of /3 is 0 2 4 6 8 10 12 14 formed. The complex alloys--notable for their high WEIGHT % ALUMINIUM--- strength, corrosion and erosion resistance-can be cast 1 Simplified copper-aluminium equilibrium diagram easily without the influence of the eutectoid structure. This is the main reason that where die-casting is intended, the economical AB-l type of alloy is pre- similar manner, without understanding certain metal- ferred. In case of complicated sand casting where lurgical concepts of the alloy. requisite mechanical and other properties like corro- Aluminium-bronze alloys can be categorized in three sion are important and desirable, the choice is of distinct series as can be seen from the binary equi- the AB-2 type of alloy, even though it may be an librium diagram of copper-aluminium system. They expensive product. In order to understand the role constitute the -c series, the -c-1-fl series and <- yz of alloying additions a knowledge of the effect of series, possessing properties entirely different in nature. each of the elements is required. The wide range of properties that can be obtained by suitable alloying addition and heat treatment has Aluminium opened out immense possibilities for its application in various engineering fields. In fact, the role of alu- Commercial binary alloys of aluminium - bronze usually minium has been compared with that of carbon in contain about 81,', Al, but the best combination of steel and comparison of properties that can be properties are obtained in the range of 9 to 1 I% Al. obtained with aluminium bronze has often been In this range of binary aluminium , advantage can be made.' taken of the characteristic eutectoid transformation in The x aluminium-bronze mostly in the solid solu- which phase changes occur. This is very important in tion range contains about 9.4°%o aluminium, which can respect of engineering alloys where suitable heat treat- be obtained in casting only on very slow cooling. It ment can confer desired properties in castings. is difficult to avoid the hard /3 formation under nor- The -c aluminium - bronze having an f. c. c. structure mal conditions of cooling, even with 7.4% aluminium is suitable for such application where high corrosion in the alloy. From the equilibrium diagram, Fig. I, it resistance is important . The aluminium - bronze with a is clear that the ^ structure is stable only at high b. c. c. structure rarely exists by itself as a phase, temperature and undergoes an .-y., transformation at but appears as a duplex x /3 structure which confers about 565°C'. Being a sluggish process, it is not a combination of properties like corrosion resistance possible for the reaction to take place in castings and strength . At 575-C /3 transforms to a.+y,, like undergoing normal cooling rate. gamma brass . It is important to stabilize /3 to avoid The 0 phase gives a high hardness combined with complete formation of 7, as its formation leads to relatively high mechanical properties under normal corrosion at higher rates . The effect of aluminium conditions, but severity of cooling results in the for- content and cooling rate on corrosion resistance of 73 Vaidyanath : The manufacture of aluminium bronze castings GOOD CORROSION RESISTANCE OIATED 100 Z+6 VARIABLE BEHAVIOUR EMI -CONTINUOUS DANGEROUS STRUCTURE (+CONT . 10 0 5 z o NCE F, rn GOOD CORROSION RESISTA d-+ ,A 2 1 9.0 9.2 9.4 RA 8.8 2 Influence of aluminium content and cooling rate on the corrosion resistance of binary PERCE NT AIJIMINIJIM copper-aluntiniunt alloys binary aluminium is shown in Fig. 23 which clearly nickel produces a Kappa phase', which has the same indicates the conditions to obtain acceptable structure. structure as the P, aluminium bronze. In alloys containing less than I I" aluminium, the decomposition of ^ pro- Iron duces -c.. cc, when nickel and iron are also present. The size and disposition of Kappa can be controlled from The addition of about 10/ iron improves the mechani- fairly massive to fairly dispersed forms. By regulating cal properties mainly due to its effect on grain refine- the speed of cooling the transformation of /3--(+K call ment. Althouuh additions up to 5 5" of- iron is permit- then he arranged to obtain two very important effects-- ted, the influence of additions above 1 .2°„ does reflect (I) hardening by precipitation (of Kin ,c) and (2) sim- appreciably improved mechanical properties like tensile ple martensitic straining to obtain relatively softer phase strength and hardness but lowered ductility.