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Near Net-Shaping Aerospace Alloys by Thixoforming P. Kapranos & M. Farnsworth The University of Sheffield, Advanced Manufacturing Research Centre (AMRC/Boeing), Advanced Manufacturing Park, Wallis Way, Catcliffe, Rotherham, S60 5TZ, UK <p.kapranos@sheffield.ac.uk> Abstract Thixoforming, is the method of shaping metal components in a semi-solid state, utilizing non-dendritic alloy feedstock heated to a semi solid state, before being injected into a tooling cavity. The major advantages of this process are the lower energy costs due to lower temperatures involved, the removal of molten metal handling, reduced shrinkage porosity and gas inclusions, fine grain structure and improved mechanical properties, and the ability to cast near-net shape components considerably reduces the need for machining, hence reducing the amount of waste material produced. A201 is a high strength aluminum-casting alloy but it is also a difficult and an expensive alloy to cast. The application of the thixoforming process to the A201 would allow this alloy to be reliably produced with mechanical and metallurgical properties superior to other cast aluminum alloys, and with substantially reduced fabrication defects. Key words (thixoforming, non-dendritic microstructures, mechanical properties).

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Introduction Thixoforming is the near net shaping of metals in the semi-solid state, i.e. within the freezing or melting range between the fully solid and fully liquid states. During thixoforming, when the material is in the semi-solid state, it exhibits thixotropic properties, i.e. the unsupported material remains stiff (consistency of butter) and holds its shape so it can be readily handled, but rapidly thins and flows like a liquid when sheared. It is this behaviour that is the key to the thixoforming process where material flows as a semi-solid slurry into a die, as in conventional die-casting. This behaviour is exhibited in all metal alloys that possess a specific microstructure; namely one that consists of metal spheroids e.g. α-Al in aluminium alloys, surrounded by a contiguous layer of eutectic liquid when heated to the semi-solid state. This microstructure is the key to producing successful thixoformed parts. There are various routes for obtaining the necessary spheroidal microstructures, but until recently, commercially this was done either by continuous casting with magneto-hydrodynamic stirring of the melt during solidification (MHD), or through the New Rheocasting (NRC) route where the melt is cooled down into the semi-solid state prior to thixoforming. The need to be able to re-cycle material in-situ forced the development of the NRC route, based on the original Rheocasting technology developed by MIT back in the 1970’s. Currently more companies appear to develop parallel production routes that allow in-situ re-cycling, such as the SSR (Semi-Solid Rheocasting by Idra) and other developments are on the way by the aluminium manufacturer Alcan (Vortex technique) and the die-casting machine producers Buhler. The thixoforming process has advantages over both the casting and forging processes, such as fine, uniform and virtually free of porosity microstructures, products that may be heat-treated to give mechanical properties superior to those of casting, reduced energy consumption and reduced die thermal shock and therefore longer die-lives, due to the lower heat content of the semi-solid material. In addition, higher melting point alloys, such as hypereutectic aluminium-silicon alloys with very high silicon contents (~25-40%), superalloys or tool steels, which cannot be easily be die cast, may nevertheless, be thixoformed. As a relatively new process, before proving its value as a commercial success, thixoforming has had to exploit alloys that were already available. However this phase of development is now over and millions of thixoformed automotive parts are now in every day use in the cars we drive. An expanding portfolio of alloys specifically designed to exploit the potential of the thixoforming process is required to strengthen its market potential, but also address the urgent needs in the aerospace industry for near net shape, high strength, aluminium products. Alloy development work at Sheffield involves developing an understanding of the key scientific principles on which alloy design and development for semi-solid processing must be based, and to produce aluminium alloys specifically tailored to exploit the thixoforming process and with

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performance approaching that of the wrought specification aluminium alloys. This work looks at the A201 copper containing casting alloy with additional small quantities of magnesium, silicon and silver. Although this alloy is difficult to cast, it has a particularly high response to age-hardening [1] and therefore offers mechanical properties close to the wrought 2014 alloy. Experimental Conventional DC-cast dendritic material was re-cast using a cooling slope (CS) of length 200mm positioned at an angle of 450, see Figure 1, in order to obtain feedstock having the necessary non-dendritic, near spheroidal, microstructure for thixoforming [2]. A number of billets were cast using the CS, with the A201 alloy superheated 40°C above the release temperature of 670°C, and typical resulting microstructures are shown in Figure 2. These billets were machined into slugs with length and diameter of 60mm to be used for thixoforming flat products for further mechanical testing using the die shown in Figure 3. The thixoformed fingers were x-rayed and hardness measurements were obtained in the as thixoformed condition as well as the post heat treatment condition. For alloy A201, a conventional T6 heat treatment (two step solution 2 h at 5130C and 17 h at 5270C followed by water quenching and then ageing 20 h at 1530C) has been used and in addition a T7 treatment that differs only in the ageing treatment; 5h at 1900. From each thixoforging it was possible to obtain two tensile test specimens from the single finger die, see figure 4. Results The microstructures of both the as-thixoformed and the T6 heat-treated samples can be seen in figure 5. The as-thixoformed microstructure displays a small liquid fraction however, after the T6 heat-treatment there is clearly a change within the eutectic phase of the alloy. Some liquid entrapment is visible in both samples. When referring to the results of the Vickers Hardness tests (Figure 6), it is interesting to note how the thixoformed and heat-treated A201 recovers to an average value of 150vH, which is comparable to a value expected for as cast A201 from Alcan. Another point to consider is that the Vickers Hardness is relatively consistent along the length of the thixoforging finger, a distance of over 60mm. The mechanical test results for the thixoformed and heat-treated A201 are very encouraging, see figure 7. The target value for ultimate tensile strength, that of the standard as cast A201, has been matched by the thixoformed alternative, at a value of 490 MPa UTS and 7% E and also the target value of wrought 2014 at 480 MPa UTS and 13%E [3]. More significantly though, the majority of the thixoformed samples show greatly improved percentage elongation as compared to the as-cast target, in the region of 50% in some cases.

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Discussion It is clear from the investigations on the thixoformability of alloy A201 that this alloy appears to behave well under thixotropic conditions, i.e. it can develop the appropriate non-dendritic microstructure through the usual routes, and it holds its shape whilst in the semi-solid state and flows as a viscous liquid when sheared and develops high mechanical properties after appropriate heat treatment. The UTS, YS and E% of the thixoformed A201 products appear to have better properties than the casting versions of this alloy, especially the near doubling of elongation values in the T6 & T7 heat treated conditions. These values are comparable with the values of wrought alloy A2014. In addition, initial fatigue tests of thixoformed A201 specimens have yielded very promising results which we hope to present quite soon. However, these tests have been carried out on polished specimens that have been machined out of thixoformed ‘fingers’ such as those shown in Figure 8 and the next task for us is to manufacture near net-shape fatigue specimens (Figure 9) that will be tested for fatigue performance in the as thixoformed condition in order to obtain representative fatigue data for aerospace designers. In addition, the current series of mechanical properties data for thixoformed A201 has been obtained by using non-dendritic feedstock generated by the non-efficient cooling slope process. It is expected that the next series of specimens that will be generated by utilizing feedstock obtained by the Magnetohydrodynamic stirring (MHD) and the New Rheocasting (NRC) processes can only result in even better properties bringing the acceptance of thixoformed A201 alloy parts by the aerospace industry one step closer to reality. Conclusions A201 feedstock for thixoforming was cast using the cooling slope method. These billets were then thixoformed into flat products suitable for mechanical testing. This initial research has shown that the thixoforming process could be an excellent alternative to the standard casting method, with the results of mechanical tests showing comparable ultimate tensile strength to that of as-cast A201, and improvements in percentage elongation of nearly 50%. It is therefore hoped that the application of the thixoforming process to the A201 alloy will allow this alloy to be reliably produced with mechanical and metallurgical properties superior to other cast aluminum alloys, with substantially reduced fabrication defects. The technology could provide the potential to convert numerous machined parts to A201 castings at a substantially reduced cost. References

1. D. Liu et al, ‘Effect of heat treatment on properties of thixoformed high performance 2014 and 201 aluminium alloys’, J. of Mat. Sci., 39, 2004, pp 99-105.

2. T. Haga and P. Kapranos, ‘Billetless simple thixoforming process’, J. of Mat. Proc. Tech., 130-131 (2002), pp 581-586.

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3. P. Kapranos, ‘Thixoforming Wrought Alloys’, Die Casting & Technology, No 31, Sept. 2004, pp 44-51.

Acknowledgements

The authors would like to acknowledge the UK Engineering and Physical Sciences Research Council (EPSRC) for financial support under Grant GR/M89096- ‘Alloy Development for Thixoforming, and also thank Mr. Worawit Jirattitichaorean for his contribution to the cooling slope experiments as well as the die design.

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Figures

Figure 1. Cooling slope arrangement used for re-casting the A201 alloy.

Figure 2. Microstructure of A201 billets cast using the Cooling slope.

Figure 3. Die used for thixoforming, featuring a steel base and replaceable inserts; a single finger cavity was used in this work.

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Figure 4. Shows the location of tensile specimens within the forged finger.

Figure 5. Microstructure of A 201 as-thixoformed and after T6 heat-treatment.

Figure 6. Results of Vickers Hardness tests for as-thixoformed and thixoformed plus T6 heat-treated A201.

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Figure 7. UTS vs Elongation of A201 alloys compared with the as cast and also the wrought A2014 alloy target values. �

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Figure 8. Thixoformed A 201 specimens, showing the effect of the scrapping ring in the gate area.

Figure 9. Proposed near net-shape fatigue specimen die.

Mechanical Properties of Thixoformed 201 Casting Alloy

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