hooked on casting

HOOKED ON CASTINGDB Critchley, technical officer with the British Investment Casting Trade

Association, andRF Smart, the organisation's director, look at the flexibilityand cost advantages of the investment casting process.

Investment casting has grown year by year overthe last decade, despite the difficultiesexperienced by most other areas of the foundryindustry. Why should this be so? The answer

clearly lies in the fact that the investment castingprocess offers the designer great flexibility coupledwith cost effectiveness. Today, the industry in the UKproduces more than £200m/annum of investmentcastings, with upwards of 30% being exported.

Recent economic trends have highlighted theadvantages of those manufacturing methods thatproduce components to finished shape and near tofinished size, minimising the need for subsequent,often costly, machining and reduces materialswastage. Of these 'near net shape' forming methods,investment casting is one of the most versatile.

Investment, or lost wax, casting has a historydating back many thousands of years, but it was thedemands of the 2nd World War - and particularly ofthe aircraft gas turbine - that transformed anhistorical craft into the high technology metal formingindustry that typifies modern investment casting.

Turbine blades were required to be produced inincreasingly refractory alloys, as designers soughtincreased efficiency by the use of ever higher gasinlet temperatures. The blades were forged, firstly inalloy steel and then in more heat resistantsuperalloys based on nickel. As more refractoryalloys were developed, the difficulties of working thematerials and machining the blades increased andattention turned to investment casting.

The late 1940s and 50s saw the growth of theindustry, mainly allied to released castings for aircraftand defence applications. Thereafter, it expanded intothe general commercial market with a host ofapplications, while the range of materials castincreased dramatically. To the original alloy steel andsuperalloys were added carbon steels, tool steels,aluminium alloys, copper alloys, magnesium alloysand, more recently, titanium alloys.

The process and its advantagesInvestment casting (see Figure 1) starts with

production of an expendable pattern, usually of wax,from a pattern die. The use of an expendable patternhas usually been regarded as a distinguishing featureof the investment casting process. Patterns aremounted onto a wax runner system to form anassembly which is covered, or invested - hence thename of the process, with a fine coating of refractory.In the block mould technique, the mould is producedin a single operation by pouring a refractory slurryaround the assembly contained in a flask. In the

widely used ceramic shell technique, the pattern isinvested with successive layers of refractory until acomplete shell has been formed. The mould isdewaxed and fired to induce strength; molten metal ispoured into the hot mould and, after cooling of themetal, the mould is removed to leave castings whichare then removed from the runner system, andfinished according to customer requirements.

The process has a number of advantages. Itconverts molten metal to precision engineeredcomponents in a single operation, with minimumwastage of often expensive material and minimummachining requirement. It also has a versatilityapproached by few other metal forming processes.Intricate or re-entrant contours can be incorporated.These features offer great freedom of design, but itshould be cautioned that complex design, withoutfunctional necessity, may not be the most costeffective approach to investment casting.

Most tooling for wax pattern manufacture isrelatively cheap and can be changed or modified, ifnecessary changed design criteria. The cheap andadaptable tooling gives the ability to cope with a greatvariety of part quantities, from very small batches tovery large call-offs.

The versatility of the technique extends tomaterials since, as has already been indicated,virtually any alloy can be investment cast, with fulldevelopment of mechanical properties. Post-heattreatment is commonly carried out where mechanicalproperties require it.

General technical developmentsWhile early applications of investment casting in

turbine blading had essentially been an alternative toforged blading, the very success of metallurgists inimproving creep resistance led to problems offoregability, so that for the most advanced alloyscasting became the only practical method of forming.IN 713 (the first vacuum cast alloy), IN 100 and MM002 were examples of the trend.

Allied to these improvements in creep resistance,techniques have been developed to incorporatecooling channels in aerofoil section castings to suchan extent that current blade cooling represents a veryadvanced manufacturing technology.

Aircraft gas turbine engine development ulti-mately became limited by lack of creep ductility inblading and this was ascribed to voids in transversegrain boundaries. The limitation was overcome by thedevelopment of directionally solidified (DS) bladesand, later, by single crystal (SC) blades. The absenceof grain boundaries allowed certain grain boundary

strengthening elements to be removed from the alloycomposition and, since these elements depressedthe melting point of the matrix, their omissionincreased the inherent temperature capability of thesuperalloy. These DS and SC techniques are nowused cost effectively in volume production and thisrepresents a real metallurgical achievement. At thesame time, the bulk of precision - cast aerofoilcomponents downstream of the highest temperature/pressure stages continues to be based on equi-axedcastings.

One of the industry's most significant recenttechnical developments has been the introduction ofrobots at the moulding stage. Within the UK, robotsare now in use capable of handling pay loads of up to350kg while, in the US, the largest robots can take upto 1000kg. The use of such robots has not onlyallowed the industry to offer much larger investmentcastings than hitherto; equally important, it hasallowed better reliability and consistency in the shellcompared with the earlier hand shelling process. Tosupport these developments, the formulations ofwaxes and for refractory shell systems have beencontinually improved by proprietary development.The overall effect of these changes has been toextend the size and weight capability of theinvestment casting process as well as promoting theuse of thinner section castings. The need forcleanliness of the molten metal has given impetus toa number of important developments including awider use of vacuum melting, the use of ceramicfilters, and improved techniques for better gascontrol.

An important process to complement investmentcasting is hot isostatic pressing (or hipping), inwhich castings are subjected to high temperaturesand pressures to eliminate remaining traces of sealedporosity, thereby improving soundness and integrityand improving component performance.

Development trends in the next few years arelikely to include continued interest in the bettercontrol of the basic investment castings process andof the individual stages upon which it is based; agreater use of CAD/CAM manufacturing techniques,possibly allied to semi empirical - and ultimatelymathematical - modelling of solidification and metalflow in the mould; the growing use of hipping toproduce special, premium grade castings; and theadoption of the process to cope with new materialssuch as metal matrix composites and high strengthaluminium alloys.

The industry from its early days has operated witha high degree of quality control and quality assurance

MANUFACTURING ENGINEER 51 MAY 1989

Firing

Fig 1: a step by step guide to investment casting

and this has stood it in good stead in the 1970s and80s, so that the investment casting industry hasprogressed along the road of improved quality tomeet the more stringent requirements of thecustomer. As an example of this trend, note may betaken that the major companies in the industry,through their trade association BICTA, have beenengaged in considerable collaborative research inrecent years. This covers a variety of topics butparticular mention should be made of work on steelinvestment castings. By the use of improvedprocessing techniques, allied to hot isostaticprocessing, fatigue performance has been achievedequal to that given by forgings measured longitud-inally.

Market trendsSteel investment castings

As designers and engineers have become awareof the potential of investment castings, so the varietyof steels used and of components cast has increaseddramatically. Nowadays, one third (by volume) of allinvestment castings are made from steel.

The industrial base spans aerospace, armament,automotive, food, petrochemical, nuclear, textiles,valve and pump and other general engineeringcomponents. Applications encompass as diverse arange of products as the ubiquitous golf club head,gear box parts for automotive applications, bicyclecam forks and a variety of gears and camcomponents in various wear resistant steels.

The recent development of high integrity steelinvestment castings is opening up a new market forcastings which, in selected dynamically - loadedapplications, are replacing forgings or fabricationswith reported savings on total costs of between 40and 70%.

Aluminium alloy castingsInvestment castings are now accepted for a wide

variety of aluminium alloy components and they findcurrent use in such fields as electronics, avionics,aerospace, pump and valve applications and military

command equipment. Whereas originally light alloycastings were commonly contained within a100-200 mm.cube, now much larger sizes areregularly cast with 800-1000 mm envelopes being notuncommon. Wall thickness has also been progress-sively reduced in order to minimise weight by the useof improved shell systems and casting techniques.

Improvement in processing has given casters theconfidence to offer premium quality castings fordemanding applications, such as air frame com-ponents. While much of the work has been satisfiedby well established alloys, there are now customerrequirements for a whole range of aluminium alloys.

SuperalloysGas turbine engine blades and nozzle guide vanes

represent components exposed to very demandingapplications and they have, over the years, led tosome of the most advanced materials and processdevelopments. Such applications are now a majoroutlet for vacuum cast superalloys.

Investment cast integrally-bladed turbine wheels forsmaller turbine engines offer considerable costsavings over mechanical fabrications, but originallythey suffered from poor low cycle fatigue performance.Improved processing to refine these coarse structureshas dramatically raised properties and causedintegral-wheels to be much more widely adopted.

A major development during the last decade or sohas been the use of investment casting to producelarge and complex thin wall engine carcase parts (egdiffuser housings and combustor castings), withcastings up to 1500 mm diameter by 600 mm indepth; these are very cost effective and replace steelfabrications.

While much of the interest in superalloyinvestment castings has inevitably centred on gasturbine blading, other fields of applications shouldnot be ignored. A particularly important outlet forinvestment casting is in supercharger wheels, ofwhich millions are annually supplied. In a quitedifferent application there is a long establishedmarket for hip replacement joints made from

Past and present. A bronze cat (above) shows aconsiderable knowledge and skill in foundryprocedures in early Egypt. Similar skills makeprecision helicopter parts (below)

cobalt-based superalloys formed by investmentcasting.

Investment casting designFor optimum efficiency, the design of an

investment casting should be a joint exercise by thedesigner and the investment caster; only by thisapproach can the full benefit of the process be madeavailable to the buyer of the casting. Against thisbackground, the points made below should beregarded as general lists to be amplified bydiscussion with the caster.

AlloysVirtually any metal or alloy can be investment

cast, but in cost-effective practice the choice islimited by the need to choose a material that (a) willgive the functional properties required in thecomponent and (b) can be cast soundly and repeatedin the casting configuration chosen.

MANUFACTURING ENGINEER 53 MAY 1989

Commercial casters produce components in allnational and international specifications. It isgenerally good practice to consider selecting from acasters list of preferred alloys since these will havebeen selected because of their known ability toproduce sound castings in a range of designs.

Basic design conceptsInvestment castings should, as far as possible,

have an even wall thickness without abrupt changesof section from thick to thin. Complex internal shapescan be produced but, where this involves the use ofcores, such techniques may lead to an increase in theprice of the casting.

Fillet radii are recommended, a reasonableoptimum radius being effectively equivalent to themean of the thickness dimensions of two adjacentwalls. It may be preferable for the customer to specifythe maximum permissible radius and rely on thecaster to find the optimum within this limit.

Holes generated by investing (rather than by theuse of soluble or preformed cores) should be kept asshort as possible in relation to their diameter.

Depending on casting size, machining allowancesare usually within the range 0.5-2.5 mm.

It is best to dimension castings from centre linedatums, rather than from one end, to avoid build-upof tolerances at the extreme points of a casting. If thecasting feature does not have centre line features it

may be desirable to add tooling lugs or datum tabs toact as tooling location.

TolerancesTolerances achievable by the process can vary

significantly depending on such factors as thecomplexity and configuration of a component.However, typical quoted figures for linear tolerancesare ±0 .13 mm per 25 mm.

Improvements on tolerances quoted can beachieved, eg through development of the die duringthe sampling stage, but designers are reminded thatthe specification of unduly close tolerances willadversely affect component price.

Tolerances for flatness and straightness areinfluenced by surface area. A figure of ± 0.15 mmper 25 mm is typical in both cases; but improvementscan be made by employing manual or mechanicaltechniques. Where parallelism is a feature of thedesign, tolerances ranging from ±0 .05 mm to±0.13 mm are possible, depending upon thesurface area of the component to be cast. A minimumangular tolerance within ± i ° is typical.

Section thicknessA minimum section thickness of 1.5 mm is typical,

but thinner sections are possible depending upon thealloy and the moulding process employed. The wallthickness tolerance will depend upon alloy type,

section thickness and die construction.The investment casting industry has, in recent

years, made considerable technical advances toconsolidate its place in the forefront of castingprocesses. The ability to offer the customerconsistent near net shape components in virtuallyany chosen material gives the process many inherentattractions. Further development is expanding therange of applications which now encompasses thosein which dynamic loading conditions occur.

The UK market for investment castings is nowgreater than that for steel castings. Competitioncontinues from overseas, and from other, metalshaping processes, and this is likely to grow, butquality and cost effectiveness will continue to be thekeynotes for success by individual companies.Overall, the portents for the industry are good andcontinued growth of 5-7% per annum has beenpredicted; this could be a conservative estimate asthe advantages of investment castings are moreaccurately perceived.

This augurs well for the industry and itscustomers. It should however be reitereated that theengineer or designer who wishes to specifyinvestment castings should discuss the componentswith the caster at the earliest possible stage; closeliaison between both parties is desirable if thesoundest and most cost effective castings are to beproduced. H

All machines are equipped with CNC numerical control, palletisationsystems and the option of shuttle link between several machines.The major feature of these machines is the robotised tool handlingsystem, with its complementary and extensive managementsoftware developed by the company, and which offers acomprehensive equipment "package", totally under PEGARDPRODUCTICS'control.

• Horizontal spindle machiningcentres, in floor-type or planertype versions

• Flexible cells• Flexible manufacturing systems• Robotised tool handling and

management systems

A|ax Machine Tool Co. Limited.Stockport Road, Bredbury. Cheshire SK6 2AT. England

Telephone: 061-430 5231 Telex: 668396 Fax: 061-494 2812

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