New Alloys for Zinc Castings

Jim Birch Castings Manager, Zinc Development Association, 42-46 Weymouth Street, London WlN 3LQ

Abstract The ZA family of zinc.aluminium alloys provide materials which combine the castability of the conventional zinc alloys with the higher strength and temperature and wear resistance required for more demanding applications. After an outline of the capability of the pressure die casting process to produce very close tolerance castings in zinc alloy the properties of the alloys are compared with rival materials. Areas in which ZA alloys are particularly attractive are highlighted.

Introduction Zinc alloy die casting can produce complex, three dimensional shapes to very high accuracies. The reduction or total elimination of machining and fabrication which results means the process is an economically attractive method of producing a wide variety of components. Until recently zinc alloy pressure die castings were only produced in two, very similar, alloys to BS 1004A and BS 1004B. Although these alloys still account for over 95% of the current output, new alloys have been introduced - the range of ZA alloys. These have greater strengths than the conventional alloys and are capable of operating at higher temperature. The new alloys give the engineer the opportunity of benefitting from die casting econ- omies for components not previously viable in zinc alloy.

the service conditions. The process also facilitates the use of ribbing to achieve component stiffness. Complex three-dimensional shapes can be achieved by using moving cores which slide in the die. Most external

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0 .1

C 0~

4 ul

o c 0.01 (J

Pressure die casting ef zinc alloys In the pressure die casting process molten alloy is rapidly Fig 1 injected into a split metal mould and is solidified under high pressure. The mould then opens, the casting is extracted and the cycle repeated. The process is fully automatic and very high production rates can be achieved. By use of complex tooling three dimensional components are produced in one rapid operation which otherwise could only be made by machining or by assembly of sub-components. Very little tool wear occurs when casting zinc alloy and over a million parts may be produced from a single tool.

The dies are accurately made and precisely fitted and this together with the process automation results in dimensionally consistent castings. In Fig 1 the toler- ances achievable by various forming methods are compared. A zinc alloy casting can be produced to tolerances comparable to those of a machined part. (Fig 2).

Zinc die castings can be produced with large areas of less than 1 mm wall thickness and ribs of 0.5 mm Fig 2 are common. The process allows components to be designed to the minimum sections necessary to take

[ t J ~

10 100 1000

Dimensior ls (rnm)

A comparison of the dimensional accuracy of various shape forming processes

A lawn edge trimmer which uses a train o f accurate zinc cast gears located in a strong and stiff zinc cast enclosure

MATERIALS & DESIGN Vol. 11 No. 2 APRIL 1990 0261-3069/90/020083-05 © 1990 Butterworth-Heinemann Lid 83

threads can be cast provided they are bisected by a parting line. Full threads can be produced but where possible, fiats should be incorporated at the parting line so that trimming is simplified and pitch errors accommodated. It is usually cheaper to machine internal threads since the need to rotate the core to permit extraction or the use of "loose pieces" interferes with production. For many applications the component can be left bare or given a simple chemical coat. However, a valuable characteristic of zinc die castings is the wide choice of finishes which they will take.. Electroplating produces surfaces of outstanding quality. The usual treatment is to deposit chromium on top of copper and nickel layers. However, a variety of other metals can be used, e.g. silver and gold for decorative ware and some electrical components. Satin finishes can be achieved and, by texturing the die surface, almost any pattern can be reproduced. Most types of paint and painting processes can be used with zinc die castings. Powder spraying is increasingly used as a method of application, the coatings being tough, adherent and corrosion resistant.

Conventional zinc alloys Properties For over fifty years zinc pressure die castings have been made from two alloys, both containing 4% aluminium. One alloy then has an addition of 1% copper and is the alloy commonly used in continental Europe. The copper free alloy is the one generally cast in the UK and America. Very close limits on impurities guarantee the long term soundness and stability of the metal. The properties of the traditional pressure die cast zinc alloys are given in Table ! together with the values attained by rival materials. The zinc alloy has good strength and stiffness, comparable to aluminium and magnesium, coupled with ductility and impact resistance which are properties in which the other alloys are deficient. It is not as strong as steel but it must be remembered that in an assembly of pressings the components strength is governed by the weakest joint, not by the inherent strength of the material.

Compared with typical engineering plastics eg glass reinforced nylon, zinc alloys are significantly stronger. Perhaps of greater importance is the fact that zinc is ten times stiffer than plastic. In many components it is rigidity which is the limiting factor and a plastic part would need to be at least twice the thickness of the zinc alloy component in order to achieve the same stiffness. The material volume saving possible using zinc thus means that the weight difference between a plastic and a zinc part is not as great as the density difference might imply.

The streamlining of the Rover 800 cars required very thin, yet stiff and strong, door finisher strips to retain the window glass. Sheet metal was rejected on the grounds of difficulty and cost of production. Use of plastics would have resulted in a much thicker component and the aerodynamics would have been adversely affected. The component was, therefore, made as a one millimetre thick zinc diecasting which met both the mechanical requirements and the need for an attractive surface finish. (Fig 3).

In the electronics industry interest is being shown in zinc alloys because of their thermal and electrical conductivity. The insulating properties of plastic materials can cause problems of heat dissipation and electromagnetic interference when used as enclosures. A zinc die casting, in comparison, can be produced with integral cooling fins and, being conductive, shields the electronics from interference.

Limitations The attractive material properties combined with process economies have ensured the use of zinc

Table I The properties of the conventional zinc die casting alloys compared with other die cast or moulded

materials

Material

Tensile Strength MPa

02% Proof Stress MPa

Elongation %

Hardness BHN

Impact Strength Joules

Youngs Modulus MPa X 103

Thermal Conductwit W/m/°C at 70o-140oC

Electrical Conductivity % IACS

Thermal Expansion Coefficient °C-1 X 10 _6

Specific Gravity

Zinc Alum- Mag- Plastic injure nesium

1004A 1004B LM24 AZ91 Nylon Nylon 6/6 6/6

30% GRI

283 324 285 225 60 120

NA NA 150 160 NA NA

15 9 2 2 100 6

83 92 85 75 NA NA

57 58 3 NA NA

85 85 71 42

113 109 96 67

26 25 24 12

27 27 21 26

6.7 6.7 2.8 1.8

2 10

0.17- 0.24 0.5

80-100 23-40

1.14 1.4

Fig 3 Door finisher for Rover 800 car

84 MATERIALS & DESIGN Vol. 11 No. 2 APRIL 1990

castings in a variety of applications for many years. However, there are some service conditions for which

the conventional alloys are unsuitable, in some applications the tensile strength was not adequate but more often the service temperature was too high. Under sustained loads, particularly at elevated temperatures, castings can deform. The result was distorted com- ponents and, in the case of screw threads, the loss of bolt preload. It should be pointed out that although creep may be a problem with zinc castings, the phenomenon is far more severe in plastic materials. The creep strength of even quite advanced, and therefore expensive, thermoplastics is not as good as zinc alloy. Also the conventional alloys only develop their full properties when pressure die cast, hence components which were not suited to this process, because of size, shape or production volume should not be made in zinc alloy. It was to overcome these restrictions on the use of zinc alloy that a new range of alloys, the ZA alloys were developed. These alloys have now been fully characterized and are rapidly being accepted for use in a wide range of manufacturing industries.

ZA al loys There are three alloys, ZAS, ZA12 and ZA27; the numbers corresponding to the approximate aluminium contents. The alloys were originally developed as sand and gravity casting alloys but it was subsequently found that they could also be high pressure die cast. The process chosen depends on the requirements of the casting - strength, weight, complexity, accuracy, applied finish - and of course on the total number of castings needed. Pressure die cast ZA alloy components can be held to the same tolerances as those made of the conventional zinc alloys.

Properties ZA8 is a medium strength alloy. When pressure die cast, its properties particularly at elevated temperatures, show a useful improvement on those of conventional alloys, and in addition it accepts readily the whole range of finishes applicable to conventional zinc pressure die castings.

ZA12 has a higher strength. It was initially used for prototyping as its mechanical properties when sand or gravity cast approximate to those of conventional pressure die castings. More recently ZA12 has been accepted as a useful alloy in its own right, with high strength and useful non-sparking properties.

ZA27 is the strongest member of the family, and is indeed stronger than most common non-ferrous casting alloys. It is also significantly lighter than the other zinc alloys.

Table 11 summarises the mechanical and physical properties of the zinc alloys compared with some other materials with which they would be expected to compete both technically and economically. The strengths of zinc castings depend on both the alloy chosen and the casting process used, with highest strengths achieved by pressure die casting. The lowest strength - for gravity cast ZA8 is nevertheless comparable with that of common aluminium gravity castings, whilst the highest strength - for pressure die cast ZA27 - comfortably exceeds that of most common cast metals including malleable iron (Fig 4).

The ductility of the ZA alloys, whilst lower than that of the unique traditional pressure die casting alloys, is comparable with that of many cast aluminium alloys and is better than cast iron, it is however lower than for some bronze and malleable irons. Castings in ZA27 can be heat treated to improve ductility at the expense of tensile

Table II The properties of the new zinc alloys ZA8, ZA12 and ZA2 7 compared with those of rival materials

Material

Property

ZA8

Gravity Pressure

Zinc

ZA12

Sand Gravity Pressure!

ZA27

Sand Gravity Pressure

Aluminium

LM25TF LM24M LM4M

Gravity Pressure Sand

Brass Bronze

SCB3 ~ 66O

Sand Sand Sand cast , cast cast

8.5 8.9 7,2-7.4

925-975 855-975 1 1176

20 18 f 12.1

120 59 t 50

Density g/cm 3

Melting range °C

Coefficient of i Thermal Expansion (rn/mlKxl0 ~')

Thermal conductivity W/m/'K

Electrcal conductivity % IACS

cast die cast

6.3 6.3

375-404 375-404

23.2 23.2

115 115

i 27.7

1

27.7

Tensile strength 240 375 MPa

F Yield strength (MPa] 210 290 (0.2o~ offset)

Youngs Modulus 86 86 (GPa)

Elongation 1.2 6-10 %in 2in (50 rnm)

Hardness Brinell 90 100

Impact strength J 40

cast cast die cast

6.0 6.0 6.0

377-432 I 377-432 377-432

24.1 24.1 24,1

116 116 116

28,3 28.3 28.3

300 330 400

210 260 320

82 82 82

1-2 1.5-2,5 4-7

95 90 100

25 30

cast cast die cast

5.0 5.0 5.0

375-484 375-484 375-484

26 26 26

126 126 126

29.7 29.7 29.7

440 320 425

370 260 370

78 78 78

3-6 8-11 2.0-3.5

115 95 120

44 60 12

cast die cast cast

2.7 2.8 2.7

555-615 520-580 525-625

21.5 21.8 21,5

151 96 120

39 24 32

300 285 165

240 150 90

71 71 71

4 2 2

100 85 70

8 3 1

Cast iron

Grey Malleable (F12101) (F22200)

Sand cast

7.2-7.4

1232

11.9

28 12 6

200 240

95 124

90 100

20 20

50 65

15 8

214

124

I 90-113

i_ 210

345

221

172

t0

110-156

54.88

MATERIALS & DESIGN Vol. 11 NO. 2 APRIL 1990 85

strength. The impact resistance of the ZA alloys falls with increasing aluminium content but even ZA27 is still much tougher than cast aluminium.

Temperature fimits A limitation on all zinc alloys has been their rapid loss of strength at temperatures above about 100°C. The ZA alloys have been particularly well researched in this regard, and show considerably improved properties making them suitable for continuously stressed appli. cations at moderately elevated temperatures. It is normally in continuous loading situations that creep (i.e. time dependent deformation) needs to be considered. The ZA alloys are significantly more creep resistant than the conventional zinc alloys. Alloy ZA8 in particular should be considered for applications where the conventional pressure die cast zinc alloys are not suitable. Allowable design stresses over a range of temperatures have been determined.

As a rule of thumb, compared to the 47o aluminium alloy, ZA8 offers:- - fourteen times the service lifetime, or - double the design stress, or - 30°C increase in operating temperature.

Examples of the use of ZA8 are in auto distributors and electronic enclosures. In Fig 5, ZA12 is being used for a speed transducer which is fitted into the car gearbox. The components are swaged after casting in order to retain an internal component and to seal the housing. Alloy ZA12 was chosen to avoid any possibility of these attachments loosening at engine temperatures.

Corrosion resistance The corrosion resistance of ZA alloys lies between that of the conventional alloys and aluminium. In the cast condition they have adequate corrosion resistance for most atmospheric environments. Under more aggres- sive conditions or where a decorative effect is required a range of finished may be applied. ZA8 alloy, having the lowest aluminium content, comes closest to the conventional alloys in accepting a very wide range of finishes - from a simple chemical dip, and zinc anodising, through painting and powder coating to electroplating with almost any metal. ZA12 can also be chemically treated, anodised, painted and powder coated. For electroplating, the plater needs to pay special attention to the cleaning stages of the process to obtain a satisfactory result.

ZA27 can also be chemically treated, anodised, painted and powder coated, but is not generally recommended for electroplating.

Wear resistance A feature of the ZA alloys, particularly ZA27, is their excellent bearing and wear properties. Tested against the standard bearing material, SAE 660 bronze, the alloys showed superior performance (Fig 6). In field trials ZA27 has consistently proved to have longer life than bronze for components such a slewing rings, bushes and wear plates. The ZA alloys have low coefficients of friction, better load bearing capacity and lower running temperatures. The natural lubricity of the cast component can be used to eliminate the need for

bushings and wear inserts thus lowering both material and manufacturing cost.

Hazardous environments Zinc alloys are non-sparking and are widely used in potentially explosive atmospheres. All the alloys, except

F04 The end-caps of this hydraulic implement lift are cast in ZA27. They replace a combination of bronze bushes and steel forgings

F05 An electronic speed transducer housing formed from two ZA12 castings. The parts are assembled by swaging

Fig 6 The bearing properties of zinc alloys ZA12 and ZA2 7 are similar to that of the standard bearing alloy SAE 660 bronze

86 MATERIALS & DESIGN Vol. 11 No. 2 APRIL 1990

ZA27 which has too high an aluminium content, are UK approved for use in coal mines and for use with petroleum products. Recent tests have shown that the ZA alloys have sufficient property retention after severe ageing and sufficient resistance to electrical burn through for them to be considered for applications covered by the US Underwriter Laboratory standards. In other parts of the world such as UK, zinc alloy has been widely used for many years for plug bodies, switch boxes and other electrical applications particularly those requiring robust construction (Fig 7).

Comparison with other materials Relative costs in net shape forming it is the total cost of the fully finished component which is paramount. Hence a casting in zinc could be more expensive than a casting in aluminium but the aluminium may require machining to final size and hence, in total, be more expensive. For just the zinc alloys their cost per tonne incireases progressively from Alloy 3 to ZA27. However, their density also decreases and so in terms of cost per material volume they are all about the same.

ZA vs aluminium alloys If an existing aluminium alloy pressure diecasting is giving satisfactory performance there is usually no incentive to change to ZA27 or ZA12. However, a change to a hot chamber ZA8 diecasting can bring significant cost advantages, especially if the better precision and lower draft angles available with ZA8 can be utilised to eliminate machining operations. ZA12 and ZA27 can be used in existing pressure dies intended for aluminium and will give very much higher strength components. This alloys upgrading of an existing product or avoids the necessity for design modification in unsatisfactory castings.

ZA vs steel fabrications There are undoubtedly a significant number of oppor- tunities for diecastings in this area. The amount of effort required to bring them to fruition is usually large but the rewards very worthwhile. ZA alloys serve to extend the area over which diecastings can compete with fabrications, especially those which are subject to some wear in service and those which require strengths approaching that of welded forged assemblies.

ZA vs plastics In components carrying significant stresses pressure diecastings in general are more cost effective than injection mouldings. It is only when the loads become so small as to make it practically impossible to produce diecastings sufficiently thin, that injection mouldings come into their own. Even glass reinforced nylon 6/6 does not offer any strength to weight advantage over ZA alloys. Since it is only in lowly stressed components that injection mouldings can be used, ZA alloys do not generally complete with them, ZA being at its best when the high stress capability is utilised.

ZA vs magnes ium ZA diecastings have very much superior strength and

7 Industrial 3-phase electrical switch box - a typically arduous application for tough zinc castings.

wear resistance than magnesium and correctly designed diecastings utilising these properties are much more cost effective than magnesium. However, magnesium does offer the best strength to weight ratio of any pressure diecastable alloy and it is chiefly where minimal weight is of paramount importance that magnesium is used. Because magnesium has better creep resistance than standard zinc alloys and it can be hot chamber diecast there has recently been a threat that small magnesium diecastings would make inroads into the zinc diecasting market, especially in elevated temperature applications. Hot chamber diecasting in creep resistant ZA8 does much to counteract this threat.

ZA vs cast iron The term cast iron covers a very wide range of mechanical properties, from low grade coarse flake graphite material, to extremely hard but extremely brittle white irons, to tough malleable iron and spheroidal graphite irons. ZA is competitive with iron because of high yield strength and good wear resistance, properties which pressure diecast aluminium cannot offer, coupled with savings in machining costs arising from the castability of ZA alloys.

ZA vs copper alloys Substitution here is largely in specialist areas where ZA can replace expensive bronze in applications where wear is important and in non sparking applications. Alloy 3 has adequate properties for most applications where brass machined parts are replaced.

Conclusions The ZA alloys can be sand and gravity die cast and can be pressure die cast to very close dimensional tolerances. Hence component machining and fabri- cation can be eliminated. Strength and wear resistance are greater than those of conventional zinc alloys and higher service temperatures can be endured. Potential uses are in areas where "cast to size" economies are required but the older alloys were not sufficiently strong, especially at elevated temperatures.

References A wide selection of publications on casting processes, alloy properties and component design data are available from the Zinc Development Association.

MATERIALS & DESIGN Vol. 11 No. 2 APRIL 1990 87