text: in order to demonstrate the advantages of the present invention , a series of experimental tests were conducted in which an aluminum alloy was heated up to a temperature above the liquidus temperature to fully melt the alloy . the molten alloy was cooled to precisely its liquidus temperature ( still in the fully liquid state ) and cast into a steel mould at room temperature . fig1 shows a photomicrograph of the structure obtained for an aluminum alloy 2618 . about 450 g of this alloy was heated to above the liquidus temperature of 638 ° c . and then cooled to precisely 638 ° c . the molten alloy was then cast into a cylindrical steel mould having an outer diameter of 120 mm and an outer length of 140 mm . the mould cavity of this mould had a diameter of 50 mm and a length of 80 mm . the mould was at ambient temperature when casting commenced . as mentioned above , fig1 shows a photomicrograph of the as cast structure obtained from this experiment . misorientation measurements between the particles and three - dimensional imaging of the particles in fig1 indicate that they are independent globules separated by eutectic phase . fig2 shows a photomicrograph of the as - cast structure obtained using similar apparatus but with casting conducted in the conventional manner in which the melt is cast at a temperature of about 50 - 60 ° c . above the liquidus temperature . as can be seen from fig2 a dendritic structure is obtained . this structure is not especially suitable for thixoforming . the microstructure obtained in 2618 alloy has been reproduced in other wrought and casting alloys , such as 2011 ( wrought alloy ), 7075 ( wrought alloy ), and a356 ( casting alloy ) by casting those alloys from their liquidus temperature . a total of over 54 experiments were carried out to produce materials of various compositions ( i . e . commercial compositions of aluminum alloys 2618 , 2011 , 7075 and a356 ) and microstructures ( i . e . as - liquidus - cast , as conventional - cast , as - reheated and as - formed ). the microstructure of the materials was examined by optical microscopy and image analysis to ascertain the non - dendritic structure . the thixotropic structure was further confirmed by three dimensional modelling and by measuring the orientations of adjacent grains by using scanning electron microscopy and electron back scattered pattern . the liquidus - cast 2618 slugs were reheated to semisolid temperature range to have about 60 % solid and 40 % liquid . the slugs were then either cut through using a knife edge or compressed manually by a flat ceramic tool to show the ability of the material to deform easily . ( it was impossible to do so with the conventionally cast material ). this shows the suitability of the material for use in thixoforming . in further experiments , the as - liquidus - cast alloy was subsequently reheated to the desired processing temperature in the solid - liquid region followed by thixoforrning into near - net - shape . the microstructure after reheating of a liquidus cast 2618 alloy is shown in fig3 ; the fine , non - dendritic structure is retained . the microstructure after thixoforming is shown in fig4 ; the flow is homogeneous . in order to further demonstrate the present invention a number of further experiments were conducted in which solidified metal , obtained from casting molten metal at substantially the liquidus temperature to produce a billet of appropriate dimensions , was brought to the semi - solid regime using an induction heating furnace . the billet was held in the induction heating furnace for a time of 2 - 3 minutes . the resulting semi - solid billet was then shaped under desired forming conditions using a commercial 500 tonne hydraulic press . in the thixoforming process used in the following examples , the billet of thixotropic material is introduced into open dies which subsequently close to form the article , and then open again for the ejection of the finished component . this is opposed to and believed to be unique over known thixoforming processes that use forming processes that are akin to die casting in which a thixotropic material is injected into a set of closed dies through a shot sleeve . however , the open die thixoforming process must not be confused with conventional open die solid forging . in fact , the open die thixoforming process of the present practice is more akin to conventional closed die solid forging , in the same way in which thixocasting can be likened to liquid die casting , for example . a typical thixoforming cycle as practiced in the present “ open die ” approach used in the following examples and shown schematically in fig5 consists of the following steps : ( a ) billet reheating , in which billet 10 is supplied to induction heating means 12 and heated to a temperature between the solidus and liquidus temperature to produce a thixotropic material . the thixotropic material is preferably self - supporting ( b ) billet transfer to open dies , 14 , 16 ( c ) forging stroke initiation and forming , and ( d ) removal of thixoformed component ( not shown ). just as a billet , 10 being reheated to a desired semisolid condition , is ready for transfer to the press , 13 the dies 14 , 16 open and the billet 10 is removed from an induction heating coil case 12 and seated in the lower die 16 . once the semisolid ( thixotropic ) billet 10 is positioned , the press operator initiates the forming cycle ( defined by the pre - set forming speed , and final forming load ) and the billet 10 is thixoformed upon the approach of the top die 14 and its closure onto the lower die 16 . during the forming cycle the thixotropic billet 10 is forced to follow the contours of the dies . the dies stay closed for a predetermined time ( dwell time ). after the elapse of the dwell time the dies open , the thixoforged article 18 is removed by the press operator , and the dies are closed again so that they are maintained at the correct temperature by a gas ring heater . a typical forming cycle , defined by the time from semisolid billet removal from the heating coil , and including its placement in the press and forming by die closure , is less than 20 seconds . using the above process , the commercially available aluminum alloy 2618 was thixoformed under various forming conditions ( to be discussed hereunder ). the billet of solidified metal was obtained by casting molten metal from the liquidus temperature . the demonstration article thixoformed in the examples is an automotive clutch hub component 20 as shown in fig6 . this component has previously been manufactured from steel by conventional forging methods . in conventional forging methods for producing this component , the component was made with the use of two die sets , blocker and finisher die and was subject to finishing / machining operations . when the article was produced by thixoforming in accordance with the present invention , only the finisher die was required to arrive at a near net shape compound in a single step . the starting thixotropic material provided to die 14 is a self - supporting cylindrical billet having a ratio of height to diameter ( h / d ) of about 1 . 4 . the thixoforming step used to produce the clutch hub shown in fig6 reduces the height of the billet to about 40 % of the original height in the central region of the hub and to about 11 % of the original height in the peripheral flange portion of the hub . the final diameter of the hub is approximately 2 . 4 times the diameter of the cylindrical billet of thixotropic material . it can be seen from fig6 that while the cross - section of the clutch hub 20 is a relatively simple symmetrical shape ( essentially a flat plate 22 extending radially from a centrally located hub region 24 of a wider cross - section ), the detail of the flange periphery 26 shows that good flow characteristics are required to faithfully reproduce this particular feature of the article . the clutch hub 20 was successfully thixoforged from alloy 2618 under a wide range of thixoforming conditions . process parameters investigated were ( a ) semisolid condition , ie . fraction of solid phase in the starting billet ; ( b ) die temperature ; and ( c ) forming speed . all of these are related to the ease ( or lack thereof ) with which an article can be thixoformed . generally , the lower the fraction of the solid phase and the higher the fraction of the liquid constituent , the less viscous is the semisolid slurry charge and the easier it is to deform . the resistance of a semisolid slurry system to applied force increases steeply with increasing fraction of solid in the material , for the range of fractions solid that can be practically applied in semisolid forging ( ie . 0 . 8 & gt ; fraction solid & gt ; 0 . 5 ). die temperature is most often related to surface finish of thixoforgings , where higher die temperatures tend to produce better surface finish and also prevent premature freezing of the semisolid slurry charge on contact with the dies . the forming speed is related to the rate of shearing ( deformation ) of the semisolid charge , and generally the higher the shearing rate , the lower the resistance of the semisolid slurry system to the applied load . another obviously important variable is the applied load necessary for the deforming ( shaping ) of the semisolid charge , and this may be several orders of magnitude less in thixoforming than is required in conventional forging . in a total of 25 thixoforming trials , clutch hubs were successfully thixoformed at solid fractions in the semi - solid material of 0 . 8 & gt ; fraction solid & gt ; 0 . 6 , at die temperatures from 150 - 300 ° c ., and forming speeds of 0 . 1 - 0 . 2 m / s . the forming load was 350 tons ( well below the press capacity ), and it was obvious that a much smaller forming load would suffice . the full forming load was only applied as the final clamping load , which is produced only after the top and bottom dies come in contact and fully close . the fraction solid of the semisolid charge was varied from low ( 60 % solid ) to medium ( 70 % solid ) to high ( 80 % solid ), and for each fraction solid the forming speed was varied from slow (˜ 0 . 1 m / s ) to medium ( 0 . 2 m / s ). low ( 150 ° c .) or high ( 300 ° c .) die temperature did not at all affect the surface finish quality of the thixoforged clutch hubs . in all of the thixoforging trials fully dense , near net clutch hubs were produced to the shape dictated by the forging dies . in summary , the solidified metal produced by casting a billet from molten metal at the liquidus temperature showed extremely favourable thixoforming characteristics . a part ( automotive clutch hub ) which required a substantial change in dimensions from the starting billet to the final article , was easily thixoformed in a single step , to near net shape as dictated by the forging dies , under a wide range of thixoforming conditions . the conditions included quite a high solid fraction of the semisolid charge ( 80 % solid ), quite cool forging dies ( 150 ° c . ), and only moderate forming speeds . preliminary results from the die filling characteristics during thixoforging show that the load - stroke profile for the deformation of a semisolid charge of alloy 2618 is very close to a profile obtained when the press is run with empty dies ( ie . demonstrating minimal flow resistance of the semisolid charge ). some typical windows of processing conditions for prior art thixoforging ( as distinguished from thixocasting ) can be compiled from data available in literature . it is claimed that for very simple test components ( eg . a flat disc ) a component can be thixoforged at fractions of solid of 40 - 80 %, at forming velocities of 0 . 1 - 0 . 5 m / s , and with dies at 150 - 300 ° c . for thixocasting , lower fractions of solid ( 40 - 60 %) and much higher forming speeds (& gt ; 1 m / s ) are necessary . the size of the processing window , or the flexibility of the thixoforming process is in either case heavily dependent on the quality of the thixoforming feedstock , and also on the complexity of the article to be produced . this again highlights the quality of the starting material produced by casting from substantially the liquidus temperature because fully dense and fully formed parts are easily produced at the outer limits of processing conditions as indicated in literature , under which a sound article can normally be produced ( ie . high fractions of solid in the starting slurry , low die temperatures , and slow forming speeds ). in fact , it would seem from published results that such outer limits as demonstrated here in the thixoforming of the clutch hub , are not usually viable for the production of realistic articles , with the exception of overly simplistic test components as mentioned previously . the microstructure of the thixoformed components as ( thixo ) formed by the present ‘ open die ’ thixoforging process is somewhat unusual . the microstructural characteristics across the section of the clutch hub components are summarised in fig7 , 9 and 10 . in fig7 the regions of heavy flow and recrystallisation are denoted by flow lines 30 . it can be seen from fig7 and 10 that there are regions where the primary particles ( the solid constituent during the forming of semisolid slurry charge ) are deformed , indicating the direction of flow of the semisolid charge during forming . it is seen that the grains in the ‘ flow ’ region are substantially changed from the globular grains in the original billet . the flow regions are found in the central part of the hub and along the periphery of the flange . in other regions , the primary globular grains remain largely unchanged from how they appear in the original reheated thixotropic billet just prior to forming ( see fig9 ). the extent of the flow regions is found to be dependent on the initial fraction of solid in the starting billet . at higher fraction of solid phase , these regions are found to increase ( as shown in fig7 ), and at lower fraction solid they are reduced ( as shown in fig8 ). the location of the flow regions is the same irrespective of the fraction of solid phase in the starting billet , only their extent changes according to solid fraction , as mentioned previously . in the view of results presented in thixoforging literature 1 , 2 , 3 this is an unexpected result , as there is no mention of flow patterns in thixoforged components . microstructures presented in literature are akin to those in the unchanged regions without flow in the above example . it can also be postulated on the basis of the above results , that the flow patterns are likely to disappear if the fraction solid in the starting billet is substantially low . it would seem from the results obtained so far that negligible flow pattern or no flow pattern at all could be obtained at fractions solid of less than 60 %. the mechanical properties of clutch hub parts thixoformed from alloy 2618 are quite encouraging and are summarised in table 2 . tests were carried out strictly according to standard astm e8 - 96 . tensile properties of thixoforged parts are markedly improved by heat treatment . the results compare very favourably to those presented for thixoforgings in literature , as shown in table 3 below , although a direct comparison is not possible since there are no other results concerning alloy 2618 . in terms of ultimate tensile and yield strengths the present results are at least as good as but mostly better than those presented for comparable alloys ( from the 2000 series ) and for comparable heat treatment regimes ( t5 or t6 ). the tensile elongation is also similar to results presented for thixoforgings in literature . it is also very encouraging that the mechanical properties of the present thixoforgings are close to or better than those expected from conventionally ( solid ) forged components . in the t6 condition conventional alloy 2618 forgings are expected to have ultimate tensile strength of 400 mpa , yield strength of 310 mpa , and tensile elongation of 4 %. it should be noted at this stage that the present thixoforging process is not yet fully optimised , and further improvements in properties of thixoforgings are expected . it is very interesting to note that the excellent tensile properties of the thixoformed clutch hubs can be related to the unique microstructure of the thixoformed parts . samples were taken from regions of the components that contained the “ flow patterns ” and from those without . regions with flow patterns showed the excellent tensile properties mentioned above , where as regions where flow of material was not observed showed somewhat lower ( but still impressive ) tensile strengths and elongations . it can therefore be concluded that the flow structure is a desirable outcome of the present thixoforming process , which has a positive bearing on the tensile properties of the thixoformed parts . it will be appreciated that the invention described herein is susceptible to variations and modifications other than those specifically described . it is to be understood that the invention encompasses all such variations and modifications that fall within its spirit and scope . 1 . hirt , g ., winkelmann , a ., witulski , t . and ziligen , m ., “ third international conference on processing of semi - solid alloys and composites ” ( ed . kiuchi , m .) 107 - 116 ( institute of industrial sciences , university of tokyo , tokyo , japan , 1994 ). 2 . kenney , m . p ., et . al ., in “ metals handbook , volume 15 : casting ” ( eds . cubberly , w . h .) 327 - 338 ( asm international , ohio , 1988 ). 3 . in “ fourth international conference on semi - solid processing of alloys and composites ”, chapter 5a : industrial applications : component manufacture ( eds . kirkwood , d . h ., and kapranos , p .) 204 - 256 ( the university of sheffield , england , 1996 ). 4 . in “ metals handbook , volume 2 : properties and selection : nonferrous alloys and pure metals ” ( eds . cubberly , w . h . et . al .) ams international , metals park , ohio , 1990 .