technology foresight SEMI-SOLID high-strength ALUMINIUM CASTINGS through SEMI-SOLID FORMING

Lightweighting of automobiles has become a major goal for vehicle manufacturers, due to high fuel costs, strict- er emission norms, energy security worries and recycling concerns. Heavy parts have to be replaced with light alloy parts that match the excellent material properties provided by the steel varieties. The design and fabrication of such high-strength aluminium components needs new knowledge and intensive processes with precise process controls. The automotive industry is willing to accept only high volume production process- es, with low per part costs. They are also ready to accept higher initial capital investments and sophisticated processes, when it enables fast, accurate and reliable fabrication methods.

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18 www.autotechreview.com autHors INTRODUCTION Romeo, and car body components by Audi, Fiat and DaimlerChrysler, 1. In the automotive industry currently, The basis of SSF is casting in a semi- there is a strong preference for using high solid state (i.e. between solidus and liqui- pressure casting process for shaping of dus temperature), which ensures non-tur- aluminium parts, due to its high produc- bulent mould filling and better control of tion rate, low cost and the ability to form the microstructure of the cast product. Sajid Mubashir small complex parts. High pressure die The key advantages are: is Director/ Scientist F at TIFAC, Department of Science & casting’s main drawback comes from an :: It eliminates air entrapment, and hence Technology, Government of India inability to control the melt flow inside the cast product will have negligible the mould cavity. When molten liquid porosity and can be welded and heat aluminium is injected into a die cavity treated. Since the molten material is under high pressures, the fluid flow tends cast as slurry, containing a high per- to be turbulent. This leads to air entrap- centage of solid fractions, SSF operates ment, oxide inclusions and formation of at a lower temperature and hence less gas bubbles in the cast part, thus increas- expensive die moulds can be used. Prof KSS murthy ing the porosity of the cast part and mak- :: The solidification shrinkage is negligi- is Professor (Retired), Foundry Science, Metal Joining, Structure ing it difficult to machine, anodise, weld, ble. The cast parts have near net shape Property Correlation, Non- or heat treat. and high surface finish, and would not Destructive Testing at The improved or variant processes that need or surface treatments. IISc, Bangalore have been developed to overcome these :: The semi-solid slurry is thixotropic in limitations include squeeze casting, vac- nature (i.e. thinning). This prop- uum casting, and fully automated die erty enables smooth flow through thin- casting. Another approach is the develop- ner sections in the die, thus facilitating ment of new aluminium alloys that are production of components of intricate more suited to high pressure die casting shapes. processes. :: The superior mechanical properties are Suresh Babu Muttana is Scientist C at TIFAC, Department The Semi-Solid Forming (SSF) process nearly equal to forged components, of Science & Technology, is another emerging technology, which is and unlike , SSF can produce Government of India ideal for lightweighting of automobiles. It complex geometry components also. is suitable for high volume-of-production Currently, SSF utilises aluminium cast die casting technology for light alloys, and this limits the strength such as aluminium, copper and magne- properties that can be achieved. If sium. The process has been used for man- wrought alloys can be used for SSF, ufacturing chassis by automakers, includ- then the properties can match those ing Porsche, DaimlerChrysler and Alfa achieved through the forging process. pROF PRADIP DUTTA is Professor, Heat Transfer in Manufacturing, Computational Methods at IISC, Bangalore

Aluminium sub-frames owing to its properties can better divert forces autotechreview July 2012 Volume 1 | Issue 7 19 technology foresight SEMI-SOLID FORMING

rheocasting

thixocasting

Partical solidifi cation and agitation Solidifi cation

billet with reheated spherical Semisolid garains globular feedstock 1 Semi-Solid Forming routes

An example is the research collabora- casting machines. A number of applied dritic billets, and followed it up with a tion between Rio Tinto Alcan and a research groups are active in this area, cooling slope technique as a cheaper and leading Canadian SSF research group 2. simpler alternative to do the same. Subse- to modify aluminium wrought alloys to In India, a National Facility for Semi quently, they partnered with couple of make them suitable for SSF process. Solid Forming at IISc, Bangalore has been leading automotive companies for semi- So why is SSF not commercially wide- set-up with pilot production facility to solid manufacturing of some automotive spread? One reason is that it relies heavily carry out applied research. It has indige- components. on pre-processed aluminium feedstock nously developed a linear electromagnetic This article discusses the state-of-the- (billets) having appropriate crystalline stirring process for production of non-den- art SSF technology, the main technology structure. This makes it slightly more expensive. The other reason may be related to its knowledge intensive design requirements, the precise process controls and more elaborate die tool design with provision for controlling the cooling rate. Semi-solid forming is also a significant deviation from conventional die casting processes, and the multitude of process parameters makes it difficult for industries to standardise the process. It was origi- nally developed at MIT during the 1970s, and the critical material preparation tech- niques that are needed for the SSF process was protected with many patents. By 1980s, other research groups developed slightly different feedstock preparation techniques. After the patents expired, the research work has spread across the world during the last two decades. The SSF feedstock is now available from companies like SAG (Austria), Stampal (Italy), AFT & Ormet (USA) and Pechiney (France) and some companies developed new heating tech- nologies and online-controlled pressure Aluminium suspension strut mounts and V-pattern strut

20 www.autotechreview.com variants, the required alloys and die solid forming process, within a year. Applications of SSF in the moulds, and the process control issues. Thixotropy can be explained simply: automotive sector: We do not discuss, (a) the modelling/ when one wants to vigorously stir “New- Thin walled Cast nodes for new space simulation issues, which needs an under- tonian” fluids like water, it is required to castings frame chassis standing of fluid flow thermodynamics, put more effort to get to higher speed of High strength Engine mounts, knuckles, and (b) the related die-tool designs. These stirring. In thixotropic material, this is not parts suspension components, two subjects also represent the challenges required, as the viscosity of the materials wheels to be overcome, for widespread commer- decreases with increased force (or shear Wear resi- Compression pistons, bra- cial use of semi-solid forming technology. rate). So, Mayonnaise (a “non-Newto- stant parts ke drums, steering systems nian” fluid) would be highly viscous, Parts with Master brake cylinders, air when still or stirred slowly and becomes high degree conditioner compressor BASIC PRINCIPLE OF SSF less viscous, when stirred vigorously. of tightness housing and safety critical The SSF metal alloy is a slurry of solid parts

A major problem faced in conventional spheroidal particles embedded in a 3 Various semi-solid forming applications die casting is the formation of dendritic molten liquid phase, when the tempera- crystalline aggregates within the die cast ture is maintained between its solidus and part, as the molten metal cools and solidi- liquidus temperatures. Good thixotropic Potentially, the SSF process can be fies. Dendritic microstructure growth flow behaviour is obtained when the solid used for several light metals like alumin- leads to zones of weakness and reduces fragments is between 30-65 %. This semi- ium, magnesium, copper, zinc, titanium its mechanical integrity of the cast part. solid material can stand freely (like a alloys and metal-metal-composites. SSF This partly explains the high rejection rate solid), when it is not subjected to pres- material can be processed by casting, in aluminium cast industry in India, sure, and will behave like a fluid when forging or extrusion. However, its use in which stands at an astonishing 20 %! subjected to deformation forces. So, SSF aluminium casting is the most advanced In 1970, the researchers at MIT were billets can be handled as if they were commercially. Applications of semi-solid studying the viscosity of molten metal, completely solid, prior to the casting oper- forming in the automotive sector are and when they fragmented the dendrite ation. And during the casting process, the explained in 3. formation through a stirring mechanism, material flows like a viscous liquid and the resulting molten metal slurry devel- energy required to move it is quite low, oped a large fraction of solids with globu- when compared to other solid forming SSF TECHNOLOGIES lar shape. More interestingly, this mushy processes. This behaviour has something mix of globules and molten metal exhib- to do with the globular particles being The preparation of the molten slurry is a ited the fluid dynamics behaviour of able to move freely against each other in critical step in the SSF process and “thixotropy”. MIT researchers could fore- the slurry. The theoretical basis of thixot- involves vigorous stirring of the cooling see the future commercial potential for ropy in viscous metal slurry is still to be melt to break-up the dendritic structure this molten metal phenomenon, and understood fully, but it has been put to that form during solidification. The den- quickly developed and patented the semi- practical use in SSF. drite fragments circulate in the hot melt, and become the nuclei for the growth of globular structures. There are many ways to prepare the Applied research groups active in SSF feedstock: semi-solid aluminium alloy slurry in the 1 Massachusetts Institute of Technology, USA; laboratory – by using mechanical or elec- tromagnetic stirring, rheocasting, strain- 2 University of Sheffield, UK; induced melt activation (SIMA) process, 3 Aluminium Casting Research Center at Worcester Polytechnic Institute, USA; single or twin screw process, near liqui- 4 Brunel University, England; dus casting process, or the cooling slope method. However, only electromagnetic 5 University College of Dublin, Ireland; stirring and SIMA processes have been 6 RWTH University of Technology, Aachen, Germany; used for commercial production. Since 7 State University of Campinas, Brazil; this slurry preparation represents the additional cost involved in SSF, compared 8 Institute of , ETH Zurich, Switzerland; to conventional casting, researchers are 9 Swiss Federal Institute of Technology, Switzerland; keen to develop low cost methods for 10 All-Russia Institute of Light Alloys (VILS), Moscow, Russia; preparation of semi-solid aluminium alloy 11 CSIRO Manufacturing Science and Technology, Australia slurry or billet. Depending on the method of preparing Many universities across the world are also engaged in studying the SSF process, and developing the thixotropic melt fluid, there are four new techniques. different SSF technologies: rheocasting, 2 Applied research groups active in SSF feedstock thixocasting, thixomolding, and SIMA. autotechreview July 2012 Volume 1 | Issue 7 21 technology foresight SEMI-SOLID FORMING

using direct chill casting with active Liquid metal supply (mechanical or electromagnetic) or pas- sive stirring. The stirring intensity, excita- tion frequency and cooling rate have to be Stirring Chamber controlled precisely as they affect the overall solidification process, which will determine the final microstructure of the Mould billet. (b) Reheating of billets & forming opera- tions: The billets are heated accurately and homogeneously to the semi-solid con- Electromagnetic dition, usually through inductive heating. Stirrer However, conventional radiation/ convec- tion-type are also used. The semi-solid billet is formed into the final shape by injecting it into a closed die, like in high pressure die casting. Conventional real-time controlled die casting machine is used, and the shot chamber system is adapted to the semi-solid billet insert. The Water spray die filling velocity is slightly lower than cooling high pressure die casting, but there is laminar die filling and the air enclosed in the die cavity is evenly conducted away through venting channels. Properties of A356 (an aluminium alloy) by various processes under As- Billet Cast/heat treated condition are given in 5. 4 Electromagnetic Stirring There is 25 % elongation, 30 % increase in impact and 7-10 % increase in fatigue strength in semi-solid Rheocasting and Thixocasting are highly continued as the melt solidifies, till the products. This leads to formability of suitable for aluminium cast alloys, desired solid fraction is reached. thin walled aluminium alloy parts with whereas thixomolding is exclusively used good consistency. for magnesium castings. Thixocasting The effort at IISc, Bangalore was in devel- Thixomolding: Rheocasting oping the thixocasting route. It has two Magnesium alloys are the lightest struc- “Rheocasting” is an integrated process, separate steps: tural materials available to the automotive where the feedstock is prepared by partial (a) Billet production: Billets with a fine- industry. Its excellent properties include solidification of the aluminium, and grained globular microstructure is cast high specific stiffness and strength, good immediately pushed into the die mould. Mechanical or electromagnetic stirring, 4, is employed within the die casting Alloy Heat Yield Ultimate Ten- Elongation Treatment strength sile Strength % machine itself to prepare the semi-solid (MPa) (MPa) slurry. Since all the steps are carried out Casting alloys in a continuous sequence, the need for process control is more, and this is an A 356 – Sand Cast As Cast - 230 2 intense area of research, and many differ- A 356-Permanent As Cast - 189 8 ent techniques have been proposed for Mould generating the semi-solid slurry. A 356-Permanent T6 - 280 10 One example is the Swirled Enthalpy Mould Equilibration Device (SEED) process ASSF356- As Cast - 200-225 8-15 developed by Rio Tinto Alcan and NRC ASSF356- T6 250-270 290-320 5-12 Aluminium Technology Centre, Canada. A ASSF356- T5 185-200 230-250 2-5 thermal equilibrium is developed within ASSF356- T4 135-145 230-245 8-13 the solidified melt by an off-centre swirl- ing of the mould. The swirling of liquid is 5 Properties of A356 alloy under As Cast/ heat treated condition

22 www.autotechreview.com heat conductivity, high electromagnetic A) Solidification range (∆T): is defined as expected. interference shielding and damping capa- the temperature range between the D) Potential for age : In order to bilities. However, it is difficult to shape solidus and the liquidus lines of the achieve high strength, the alloys magnesium into components. alloy. Pure metals and eutectic alloys designed for SSM processing need to An SSF technology known as “Thixo- are not suitable for SSM processing, have high potential for age hardening. molding” is used to form magnesium whereas, alloys with too wide a solidi- During A T5 temper, SSM parts ejected alloys, in an inert argon atmosphere to fication range experience poor resist- from the die are quenched immedi- prevent oxidation. Magnesium alloy chips ance to hot tearing. It is therefore sug- ately in water and then artificially aged are fed into a barrel and they are heated gested that the solidification range of at a relatively low temperature. There- into the semi-solid temperature range. A an SSM alloy be between 40-130° C. fore, the potential for age hardening of screw feeder inside the barrel shears the B) Temperature sensitivity of fraction a phase can be gauged by the concen- melt to generate the slurry with globular solid: For a given alloy composition, tration difference (∆C) of the major structure, which is then pushed into a temperature sensitivity of the fraction alloying elements in the α-phase steel die, in a process similar to injection solid (fs) is defined as the slope of the between the and ageing moulding. The process is economically fs Vs T curve, i.e., it is dfs/dT, 6. In temperatures. attractive as it has a cycle time of 30 sec- order to obtain stable and repeatable Wrought aluminium alloys have better onds or less, and the ability to make net- processing conditions, the temperature mechanical properties, but they are not shape parts. sensitivity of the fraction solid should suitable for casting as they have a ten- be as small as possible in the fraction dency for hot cracking during solidifica- Alloys solid range of commercial operations tion. So, processes like forging, extrusion Since the SSF process has less reduced (ideally, fs should be 0.3-0.5 for rheo- or rolling have to be used to shape mechanical stresses during filling and the casting and 0.5 - 0.7 for thixocasting/ wrought alloys. The wrought alloys, lower alloy processing temperature, it is thixoforging). which have been used for thixoforming possible to use softer and cheaper die C)Temperature process window (∆T): are A6061, A2014, A2024, A6061, A7003, materials like graphite, easily machinable Depending on the application, for rhe- A7075. stainless and disposable one-shot ocasting, ∆T is defined as the tempera- A commonly used wrought alloy – non-metallic dies. So SSF can be used for ture difference between 0.3-0.5 fraction AA6061 – has been modified for SSF- rapid prototyping, and customised or solid, whereas, for thixoforging, ∆T is solid pressure die casting, particularly for small volume production. defined as the temperature difference automotive structural parts by Rio Tinto Cast aluminium alloys to be used in between 0.5-0.7 fraction solid. Consid- Alcan (RTA) and the NRC Aluminium the SSF process have to be selected care- ering temperature variations during Technology Centre, Canada. There is a fully, following the criteria mentioned commercial forming operations, a rela- strong demand to produce to near net below: tive large temperature window is shape components with aluminium 7xxx

Aluminium body shell can significantly reduce weight and increase structural integrity autotechreview July 2012 Volume 1 | Issue 7 23 technology foresight SEMI-SOLID FORMING

series alloys, which are currently machined from the wrought state involv- Different solubility of solute in solid and liquid ing considerable waste. TM and since C < C ==> Solute enrichment of liquid Thixoforming of higher melting iron- S L based alloys has also been investigated. Steel casting involves high temperatures, TL which cause high thermal loading of tools and dies, hence cost-efficient permanent mould casting processes for steel do not T exist. Since Thixoforming can be per- formed at lower temperatures, in future it TS might be possible to manufacture near- net-shape steel components with compli- cated geometry in a single-step permanent TE mould process. em T perature

CONCLUSION

Lightweighting of automobiles is now in CS Ci CL CE hot pursuit by auto designers all over the Concentration world, as it has been well established that lightweighting is a surer and more eco- nomical method of accomplishing desired 6 Schematic representation of temperature sensitivity of solid fraction objectives of increasing fuel efficiency and

CO2 emissions than improving engine many SSF technologies are patented. The design. In the long term, all the techno- Indian automobile industry in its desire to logical improvements achieved in weight- comply with global scenario should also saving could be applied in precisely equal collaborate with research centres in the measure to hybrid vehicles or to electric country engaged in development of SSF vehicles. technology, as without that, the automo- While acknowledging the need to bile industry in India cannot forge ahead replace heavier metal by lightweight alu- in producing state-of-the-art automobiles, minium components, the conditions especially for the export market. imposed on such a change relating to Hence, there is a need to focus on R&D quality and characteristics such as struc- efforts on developing cost effective and tural integrity, rigidity, strength, ductility simplified processing for making slurry, and thin wall sections are well accom- quality control and standardisation for plished by semi-solid forming. In automo- SSM feedstock, optimising the overall pro- biles, the performance critical applications duction, and enhancing product cycle are now being identified as the most time so as to meet the requirements of potential end-uses of semi-solid cast automotive industry for adoption of this components. technology for large scale production. As India has now become a strong manufacturing base of automobiles, both for domestic and global markets, there is an immediate need to indigenously Read this article on develop appropriate SSF technology as www.autotechreview.com

Technology Information, Forecasting and Assessment Council (TIFAC) is an auto­ nomous organisation set up in 1988 under the Department of Science & Technology to look ahead in technologies, assess the technology trajectories, and support technology innovation by network actions in select technology areas of national importance.

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