Haw Aluminium-alloy Gravity Die astings Can Help Motor Construction
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Close Examination of Advantages Resulting from the Use of Die-castings for Commercial-vehicle Components. When Gravity Die-casting is Suitable
THE advantages accruing from the use of aluminium-alloydiecastings for components of commercial vehicles have been well appreciated for some time. The process enables an accurate and practically finished article to be produced at a low cost, whilst machining costs and the quantity of material reduced to scrap are minimized.
Where the mass production of a given component is desirable; diecasting enables bulk delivery to be. maintained, without entailiiag the employment of an undue amount of expensive skilled labour. It is, indeed, in the field of mass production that the process may best be employed, because the cost of the die is thereby distributed over a large number of articles.
For example, a tool-room cost of £30 incurred in the production of a die will represent a charge of 6s. per diecasting when the total output is 100 components, but only 1.2d. per diecasting when 6,000 articles are required.
• A Primary Consideration • Consequently,.an essential first step in deciding whether gravity die-casting is suitable for the production of a given component is to consider whether or not the number required be sufficient to justify the cost of the necessary die.
Generally speaking, articles required in batches of 1,000 or more per annum will be found to justify die-casting. This number may be considerably reduced, however, in special cases where the design of a component is such that expensive .and difficult machining would be necessary were older methods of casting employed.
For components,such as radiator and water-pump parts, covers, sumps, gearboxes and various types of housing and manifold, an alloy of the high silicon type, to B.S.S. L.33, has been widely used for a considerable time. The material possesses an excellent combination of good casting characteristics, resistance to Corrosion, ductility and high impact strength.
1320 Details of chemical composition and mechanical properties are given in Table I.
The alloy cited is of the non heattreatable type, but where maximum' mechanical prciperties are required it must be subjected to a special modification process, which produces a refinement of the eutectic structure.
A heat-treatable type, with still higher tensile properties, is also used, but is, naturally, somewhat more expensive.
Aluminium-copper alloys, to specification B.S.S. 1.8, are also suitable for gravity die-castings, whilst for pistons, alloys of the high silicon type and more complex alloys, such as the well-known " Y " alloy, are largely employed. The high silicon alloys are of special importance, in that they combine low expansion characteristics with high mechanical properties at elevated temperatures. They are, therefore, peculiarly suitable for piston construction.
A well-known alloy of this type, the Lo-Ex alloy, is employed in the form of gravity die-cast pistons in practically all types of internal-combustion engine, including compression-ignition units using pistons of diameter up to 20 ins. The alloy, in addition to the properties already mentioned, has excellent bearing qualities, and can rue satisfactorily treated by the anodic oxidation process, a property .,not generally possessed by other widely used piston alloys.
Pistons of Lo-Ex alloy are, therefore, highly resistant to corrosion and the capacity of the anodic film for absorbing lubricants (including colloiaal graphite} enables a lubricant film, applied immediately after anodising, bi be retained throughout the working life of the piston, provided that the latter is not subjected to abnormally severe abrasion.
• Production Need Recognized. • It has, however, been recognized for some time that a need exists for extending the application of the gravity die-casting process to the production of commercial-vehicle stractural components subjected to more or less severe stresses in service.
In all such vehicles it is desirable that weight be kept to a minimum, • this applying particularly to the 50-cwt. category. The use of highstrength light alloys has already been exploited by one leading chassis maker with this end in view.
The higher cost of the material is, to some extent, offset by reduced machining costs, whilst the weight saved can be usefully employed in one or more of several directions—iii improving engine power and increasing engine size, in employing a more generous size of clutch, a more robust body, or a larger tyre section, etc. The problem of using the high-strength alloys has, however, not been an easy one to solve, because, as a general rule, alloys with the necessary mechanical properties do not possess the foundry characteristics suitable for die-casting.
The design problems involved also present some difficulty, as they are concerned with requirements not only regarding ultimate tensile strength, but also with stiffness combined with ease of assembly and a capacity for dealing with momentary excess loadings. Thus, a guaranteed elongation figure, which is not a property usually possessed by the stronger casting alloys of aluminium, becomes essential, ' As a result of intensive development work, however, it has been found possible to embody in a single casting alloy the properties desired, namely, a high proof stress allied to a guaranteed elongation and .tlm ability to produce relatively complicated components by gravity diecasting, The alloy is of the heat-treatable type and contains 9-5 per cent. copper, with silicon, iron and titanium restricted to maximum percentages of 0.9, 0.7 and 0.25 respectively, and additional impurities limited to .a maximum of 0.2 per cent. By varying the heat treatment, a wide range of mechanical properties becomes possible, the proof stress ranging from 24 tons/sq. in. down to 12 tons/sq. in. and the elongation (on 2 ins.) ranging from 1.5 to 15 per cent.
As a result of this wide range of properties three Air Ministry specifications, D.T.D. 361, D.T.D. 304 and D.T.D, 293, have been drawn up to cover the matevial. In Table II are give'n the mechanical properties reached by manufacturers of the alloy, under the conditions laid down in each of the specifications, also details of the heat treatment applied in each case.
As will he readily seen from the data given in Table II, the material is capable of meeting a wide range of requirements regarding strength, stiffness, and resistance to impact and shock. Its machining qualities are excellent, and its good natural resistance to corrosion can, if necessary, be further improved by anodising.
Thus, in the form of gravity diecastings, the material has found ready employment, particularly in aircraft and motor-vehicle construction, for stressed parts for which wrought alloys had formerly been used. As a result considerable amounts of expensive machining to shape fox hole bosses and the making of attachments are avoided, because such features may be incorporated on the casting itself.
• Displacing Sheet and Extrusions • Structures built up from sheet and extrusions, involving high tool and fabricating costs, are also being substituted by die-castings in the material. A further development is the adoption of aluminium for purposes for which, owing to the high machining costs involved in the ur of wroughtaluminium alloys, steels had formerly been used.
The wide avenues open to the alloy in the field of commercial-vehicle construction can, therefore, readily be visualized, Exampies of applications already developed include operating levers, brackets, universal-joint housings, engine bearers and pump impellers.
Wheel and bearing parts of vehicle chain-track medhanism, oscillating links and levers,' and other, parts of mechanised agricultural equipment are also striking uses. With the removal
of the present restrictions on the use of aluminium, the material will be applied to a considerable additional number of important structural components for which exhaustive tests have proved it to be eminently "suitable.
It must, however, be pointed out that there are certain limitations to the use of the alloy as a die-casting material. It is not a free-flowing alloy and should not be employed for castings of widely varying thicknesses, or for castings having a section of thickness less than about 3/18th in. The feeding of a thick section through a thin one must be avoided and adequate feeds to all thick sections are imperative This need for heavy feed is• further accentuated by the shrinkage which occurs on solidification. This feature, unless adequate feed be provided, will generally result in shrinkage cavities and draws. A. certain degree of hotshortness possessed by the alloy. may also, lead to complications, unless adequate radii be provided at all corners so that the metal may flow as freely as possible and that extraction from the die may be facilitated.
• Die Design and Foundry Practice • These limitations, however, may be overcome in the majority of designs, provided that the correct foundry technique, along the lines indicated, be employed. The advantages which die-castings in the material possess for commercial vehicle construction, especially on the score of lightness and ease and economy of mass production, well justify the paying of the necessary attention to die design and foundry practice, ' In view of the extent to which the mechanical characteristics and resistance to corrosion meet the heavy-duty conditions -inevitable in the operation of such transport equipment, the popularity which the material has • already achieved may be taken as a reliable indication of the increasing use to which it will undoubtedly be put in the future design and construction of commercial vehicles of all types.