Superplasticsheet formingJ. Deschodt, J.
Vanheule and W. VanoverbergheGhentUniversity, BelgiumAbstractThispaper discusses superplastic sheet forming. First there is a conciseintroduction on the phenomenon of superplasticity. The introduction onsuperplasticity discusses the basic characteristics, the basic mechanism behindthe superplasticity and the influence of the strain rate on the superplasticflow. Some examples of material used during superplastic sheet forming will be given.The most important phenomenon of the total deformation is the grain boundarysliding, which is accommodated by the dislocation creep and grain boundarydiffusion.
A summary of advantages and disadvantages is present, as well as anexplanation for the three most important process variables. Those beingtemperature, pressure cycle and die size or geometry. Following, differentforming techniques are discussed. Every super plastic sheet forming (SPF)technique begins with the heating of the material to half its melting point. Thematerial becomes so soft that processes normally used on plastics can beapplied to metals, such as thermoforming, blow forming and vacuum forming. 1, p. 16 2Keywords Superplastic sheet forming • SPF • Superplasticity • Processparameters • Forming techniques1 INTRODUCTION Superplastic sheet forming(SPF) is a process that is mostly used to construct precise and complex parts,out of specific types of materials who possess superplastic behaviour. The term superplasticity becamecommon after 1945, but the first spectacular experiment was already conductedin 1934 by Pearson.
3, pp. 2-3An elongation of 1950% was demonstrated for tin-bismuth and 1500% for lead-tineutectic alloys. Conventional deformationtechniques typically reach an elongation of 10 to 30%, while the superplasticsheet forming technique can reach an elongation of 2000 up to 3000% at anincreased temperature. Where very low strain rates are applied, and it takesseveral minutes to hours to make one product. 4In the first section, the superplasticityphenomena will be discussed.
The text gives an answer to the neededrequirements of superplasticity and which materials can be used. Superplasticforming processes follow in the second section, described with advantages anddisadvantages, process parameters, some techniques and a couple of applications.2 Superplasticity IN GENERALThe termsuperplasticity is applicable when polycrystalline solids can undergo anextremely large elongation at high temperature. Mostly a temperature higherthan 0.5 times the melting temperature is used.
A uniaxial tension elongationof ~200% is an indication that superplasticity occurs. Some materials can reacha total elongation above 1000%. 1The deformation is performed at low strain rates in the range 10-4 upto 10-1s-1. The grain size of the materials subjected tothe deformation are below 15 µm. 1Theprevious paragraphs describe the structural superplasticity; a second type ofsuperplasticity is environmental superplasticity.
This last type of deformationis based on a phase transformation of the material. The deformation process isdivided in several steps of small deformation followed by a heat treatment. Inthe following only structural superplasticity is discussed 3, pp.
4-5. 2.1 MechanismThe microstructural requirements for the material to reachsuperplasticity are well determined, but the exact mechanism is lessunderstood. It is a combination of three phenomena, the first phenomenon isgrain boundary sliding, which is the motion of grains or groups of grains relativeto each other. A second phenomenon is dislocation creep, this is the motion ofdislocations in the lattice or in the metallic structure of the grains. Thisphenomenon results in grain elongation.
A third and last phenomenon is grainboundary diffusion. This phenomenon is the migration of atoms from high stressedzones to low stressed zones. Several studies have shown that most of the strain takes place by themotion of grains or groups of grains (grain boundary sliding) relative to eachother. 5This vision is supported by experimental studies on the motion of marker linesplaced on a superplastic deformed specimen. In experiments performed by Langdon,it has been shown that rotation of the grains occurs, but there isn’t abuild-up rotation of the grains.
Some grains rotated to the left and othersrotated to the right. The impact of the rotation on the strain is very small.Other researchers studied the impact of the motion of intergranulardislocations on the strain, but the impact on the total deformation is verysmall. When the displacement of the grains happens in a completely rigidmicrostructure, a void will occur in the microstructure. To fill this void thematerial needs to cavitate, this should be avoided during superplastic flow. 1 The totaldeformation process is the realignment of grains, where the grain boundarysliding phenomenon determines the total deformation and the other phenomena accommodatesthe grain boundary sliding. 6Themicrostructural difference between conventional plastic flow and superplastic flowis that the grains elongate in the tensile direction with conventional plasticflow, but with superplastic flow the shape of the grains doesn’t change a lot.2.
2 Strainrate sensitivityThe flow stress is a function of rate of deformation and the deformationitself, it can be expressed as:With the flowstress, k a constant, strain, thestrain rate, n the strain hardeningcoefficient and m a factor for thestrain rate sensitivity of the flow stress. A higher value of m corresponds to a material with a highresistance against neck propagation; for superplastic flow, a value of m larger than 0.3 is required and forincreased temperatures the factor n equals zero. 1The factor m dependson the strain rate, in figure 2 the flow stress isplotted in function of the strain rate. The factor m is the slope of the curve.In stage I the impact of diffusion creep increases. In stage III the slopedecreases and during the deformation process, grain elongation occurs due todislocation creep.
Both stages results in an inefficient superplasticdeformation. When the strain rates are in the range of stage II, the curvereaches a maximum slope, which results in a maximum of the strain ratesensitivity index m. In figure 3 the strain ratesensitivity index m is plotted in function of the total elongation. On this figure,the highest total elongation results in the highest strain rate sensitivityindex m. In accordance with figure 2, the total elongationwill reach a maximum in stage II. 3 12.3 MaterialsThe materials that can be super plastically deformed, must have a verysmall, stabilized grain size. For this reason, the material is modified todecrease the grain size.
This can be obtained in two ways. The first way is forpseudo single phase materials where a small contribution ofprecipitates gives a smaller grain size. This grain refinement can be executedby recrystallization of the material before the SPF process, or some othermaterials develop a smaller grain size at higher temperatures at the start ofthe SPF process. The second way is for materials with about the samecontribution of two phases; in such materials, an allotropic transition is usedto reach a smaller grain size. The temperature at which the SPF process iscarried out is about 0.5 times the absolute melting temperature.
A few examplesof superplastic materials are given in table 1. 1Table 1: materials forsuperplastic flow 7 Type material Grain Size range in ?m Strain rate range in S-1 Max. Elongation range in % Temperature range in °C Aluminium based alloys <1 to 14 10?4 to 10?1 250 to 1500 100 to 550 Titanium base alloy 6 10?4 1600 950 Copper base alloys 4 to 8 10?4 to 10?2 200 to 5500 460 to 850 3 SPF processesThe process makes use of asingle-part and -operation pressing instead of multi-operation conventionalmethod or even multi-part constructions. Many thermoplastics processes can be usedfor superplastic forming of metals. In thermoforming, a pressure causes thesheet to form the shape of a heated die. Blow forming, vacuum forming, deepdrawing and combinations with diffusion bonding are other possibilities. 53.
1 Advantagesand disadvantagesProductswith large and complex shapes or curves can be produced without joints andrivets. Processes requiring a large amount of deformation can be performed inone operation. Multistage manufacturing processes can be avoided. The obtainedprecision is very good and fine details can be reproduced with great accuracy. Thereis a high reduction of residual stresses due to the temperatures used.
By thelatter, an absence of spring back, or elastic recovery, is also present becausethe yield strength is low in that case. Neck-free elongations of many hundredpercent are achieved in contrast to conventional forming. There is a vastimprovement in formability. By making the products larger and eliminatingassembly operations often the weight of products can be reduced. There are lessholes to start fatigue cracks.
Forming pressures are also drastically reduced.Tooling and fabrication costs are lower and there is a shorter production steplead time. Close tolerances can be guaranteed which reduce machining costs. Wastageis minimized so that there is maximum use of the material. This is an importantsaving in energy intensive materials such as titanium or aluminium alloys. Theproducts have a fine and uniform grain size, which leads to better strength,ductility, and fatigue resistance, but also uniform mechanical propertiesthroughout the body of the finished product. 18Themajor disadvantage with a controlled superplastic forming process is therequired time due to the slow forming rate to maintain superplastic behaviour.Cycle times start from two minutes and goes up to two hours.
In contrast tojust a few seconds to perform conventional forming. Which is why applicationsare mostly limited to low volume products, such as those common in theaerospace industry. The process is also quite costly because of temperature andtime needed. 493.2 ProcessparametersThe process parameters having a major impact on the global achievableelongation are the strain rate sensitivity of the material and the processingtemperature. The superplastic forming process consists of multiple steps ofvariables.
The three most important process variables are temperature, pressurecycle and die size or geometry. These three main parameters have and influenceon the process time as well. 103.2.1 TemperatureTemperatureis the most important parameter for superplastic behaviour. Depending on thematerial, a specific temperature value activates and balances the grainboundary sliding, diffusion and dislocation creep relevance. A high workingtemperature increases the grain boundary sliding part in the total elongation(relative sliding of the grains) and decreases the strain hardening phenomenon,so that the mechanical characteristics of the product are better.
To improvethinning distribution, not evenly distributed temperature on a sheet can beused. A higherforming temperature value weighs on the product and installation costs on theother hand. The lifetime of dies and presses decrease with higher stressescaused by higher working temperatures. Table 1 shows the processing temperaturerange for aluminium, titanium and copper. 11Strainhardening may be completely unwanted as an event that unintentionally happensduring the manufacturing process. Making a product stronger this way is notalways wanted, especially if the material is being heavily deformed, becausethe ductility will be lower.
Also, a great deal of force is required as part ofthe process. The directional properties of the metal can be affected as well. 123.2.2 Pressure cycle ComparingSPF alloys with plastics shows a much higher temperature, but the same order ofmagnitude in pressure. This forming pressure must vary continuously in time fora constant strain rate during deformation.
The pressure must increase becauseof extending contact surface, the material flow and strain rate lower. When pressureor strain rate increases too much, then the grain boundary sliding part lowersin respect to the dislocation creep contribution (because there is not enoughtime for sliding, so stretching of the grains occurs), causing a highermaterial strain hardening and worse mechanical properties of the product. Thepressure cycle depends on the forming depth. The strain rate sensitivity first grows with the strain rate up to a maximumvalue and then decreases again ( figure 4). There also is an influence oftemperature here and total elongation is a function of m. Pressure determinesthe installation forming and working time.3.
2.3 Die size or geometryThe finalthickness distribution depends on the die size and geometry. Friction and therebylubrication plays a role. E.g. a small radius hinders material flow and reducessuperplastic behaviour. Generally, high friction forces between the sheet and dielead to the phenomenon of cavitation. This can be the result of a complex shapeand introduces microscopic holes in the material.
Seriousness of cavitationgrows with the strain rate (pressure). When the process has an uncontrolledtemperature and pressure cycle, the thickness along the surface of a die as notuniform at all.For thedesign of sheet formed parts, design guidelines are available frommanufacturers. The most important parameters are depth of draw, draft anglesand corner radii. 33.2.4 TimeTime is acrucial parameter for profitable production. It is dependent on the type ofmaterial, thickness and die shape.
A method of trial and error is still used todetermine the minimal time needed. Finite element analyses are available to determinethis parameter without real-life tests. They also determine other variableslike stress and deformation. But there is still a lack of fundamental knowledgeabout the SPF process. 133.
3 Formingtechniques3.3.1 Single sheet thermoformingManydifferent methods can be used to realize a component by means of single sheetthermoforming. Figure 5 illustrates an example of such a process.This specific process is called pressure forming or blow forming. The sheet isfixed between an upper and lower die. Inert gas under pressure is used tostretch the sheet into the die chamber. The process progresses further untilthe deformed sheet contacts the lower die.
Of course, this results inconsiderable thinning of the sheet. To ensure superplasticity the die and sheetare maintained at the same temperature within a heating press. 1Figure5Single sheet blow forming of SPF materials showing the cross-section of die andsheet: (a) initial flat sheet inserted in between upper and lower dies; (b)progression of forming under gas pressure; (c) final shaped part in contactwith lower die; and (d) removal of the part 1During theforming process, the pressure is dynamically changed. With finite elementanalysis, the pressurization profile can be calculated as well as theaccompanying thickness distribution.
The pressure control algorithm keeps themaximum strain rate in the deformation zone of the sheet. 14 The pressure controlalgorithm can be optimized for a better control of the right strain-rateinduced in the material. Because of this optimisation the thicknessdistribution is more constant. 15Besidesblow forming, single sheet thermoforming has many other variations. Thesevariations introduce a more uniform thickness distribution. In contrast to optimizingthe pressurization profile, these variations use mechanical deformation for abetter thickness distribution.
The initial mechanical deformation makes theregions that normally remain thick, already thinner. 3, pp. 231-2373.3.2 Multi-sheet forming with diffusionbondingMulti-sheetforming is a technique were many sheets are formed to a single component.Diffusion bonding (DB) is the process that connects the different sheets. Thistechnique can form complex shapes that would normally be made by several parts,for example wings with internal reinforcements.
The reduced number of parts arebeneficial for weight reduction and cost saving.In somecases, blow forming SPF can be beneficially combined with diffusion bonding,which offers a benefit to fabricating high stiffness multi-sheet structuressuch as honeycomb components. Two sheetsor more are placed in a heated press. Blow forming SPF makes sure that thesheet stretches over the die and the press assures diffusion bonding, see Figure 6.
The temperature requirements for SPFand DB are essentially the same which leads to the natural combination of bothprocesses. In addition, many of the superplastic alloys can use diffusionbonding within the small range of temperatures used for SPF. 1Figure 6 Two-sheet SPF blowforming and DB3.3.3 Recent rapid forming processesBefore SPFcan be used in the automotive industry the cycle times are required to be lowerthan a few minutes. Rapid forming processes have been in development for over15 years to make mass production possible. Two companies have described theirprocesses in open literature for closure panel production. 1• Quick Plastic Forming developed byGeneral Motors• HighCycle Blow Forming developed by Honda3.
4 ApplicationsAerospace and automotive industries make the most use of superplasticforming. Others are rail transport, architecture, medical and communicationfields. 1SPF is widelyused in aviation and aerospace for components such as: hollow blades, inletlips, wing boxes and other parts. These components are manufactured throughthree-layer titanium alloy structure bonded together with DB. These multilayered components show a great advantage in weight reduction. The large designfreedom, high load-bearing capability and fine structural integrity of SPF hasgreat significance to the design and manufacturing of aircrafts, aircraftengines, missiles and other spacecraft parts.
16Superplasticforming of aluminium alloys has become more popular in the automotive industrythrough the years. Companies are setting up their own facilities where partnumbers approach 100 000 per annum. These facilities are using a rapidforming processes like GM and Honda to form their car panels. 1Figure 7: Left: Ti–6Al–4V hollow, wide-chord fan blades on a Rolls-Royce Trentseries gas turbine engine (copyright Rolls-Royce plc) 1, Right: One piece bodysidesmade by Superform 174 CONCLUSIONSThe superplastic sheet formingis based on the phenomenon of superplasticity. This deformation process resultsin a grain realignment and almost none grain elongation.
The exact mechanismbehind the superplasticity is still less understood. In literature, it is notedthat the mechanism behind a combination of grain boundary sliding, dislocationcreep and grain boundary diffusion. Superplastic has a lot of advantages overconventional forming techniques.
The metal can be deformed with less powertherefor the dies may be cheaper. Better strength of the result is obtainedwith SPF and a conventional multi part component can be made in one part. Allthese advantages and more ensure that SPF is better for certain applicationsbut it has still some big disadvantages because of which it is not generallyused. The duration times of SPF are to long for mass production and togetherwith the high temperature leads to costly manufacturing technique. Generalmotor has made progress with quick plastic forming (QPF) on reducing the time.Other innovations in SPF are mainly about reducing heating costs and cuttingheating-plates maintenance costs whilst increasing productivity through reducedcycle time 3. The three main process parameters have been discussed and theirinfluence can be explained.
A complex temperature and pressure cycle is neededto maintain the superplastic behaviour in ideal circumstances. Therefore, a lotof research is performed to make the computer models more accurate fordifferent parameters5 AcknowledgementsThe authors would like toacknowledge the support of prof. dr. ir. Wim De Waele for the review of thispaper and thank each other for the good teamwork.6 References 1 G.
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