Semi-Solid Processing of Alloys and Composites XIV

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Authors: Raymond Decker, Stephen LeBeau, Bill Wilson, John Reagan, Nir Moskovich, Boris Bronfin
Abstract: The technical and commercial development of Thixomolding® of magnesium (Mg) alloys is described. Based on the MIT semi-solid casting discovery, Dow Chemical adaption to Mg and Thixomat technical findings, Thixomat commercialized this process world-wide with Japan Steel Works (JSW) and numerous licensees in 13 countries. Some 480 Thixomolding machines have been commissioned by JSW to produce a wide range of parts for application in the communication/electronic, auto, sporting goods and hand-held tool markets. The advantages over conventional casting of Mg have been established in cleanliness, safety, worker comfort, machine portability, longer die life and reduced scrap. The competiveness of Thixomolding Mg has been enhanced by more durable and less expensive machine parts, longer die life, higher raw material yields by using hot nozzles and amenability to using recycled Mg scrap. The portability of this “foundry inside a machine” and availability of pre-owned machines allows quick set-up of Thixomolding operations. Several technical advances in Thixomolding are described. Solids content was optimized. Hot nozzles increased the part yield to >90% of the granule Mg alloy feed. Special steel barrels and liners are available to substitute for expensive Superalloys. In addition to conventional AZ91D and AM60, a wide range of Mg alloys can be Thixomolded. With Dead Sea Magnesium, AM70-TH and AJX810-TH were developed for improved ductility or creep strength. Low porosity and fine grain size of Thixomolding open the door to blister-free thermomechanical processing (TTMP) to boost the tensile and fatigue strength and ductility.
Authors: John L. Jorstad
Abstract: This paper considers the industrial viability of SSM in comparison to other casting processes and the products each process is best able to produce; in such a perspective, SSM has much to offer especially in terms of cost effectiveness for thinner-walled and higher-integrity cast product lines. The ultimate success of SSM will rely on lessons learned from all past thixocacting and rheoocasting developments and experiences. Proposed is new thinking with regard to adaptation of rheocasting to conventional casting equipment; changing the semi-solid processing paramigm is key to ultimate SSM competitiveness and to opening vast new markets for for semi-solid cast products.
Authors: Luis Vanderlei Torres, Cecilia Tereza W. Proni, Eugênio José Zoqui
Abstract: In semisolid state forming the thermodynamic control of the solid liquid transition is necessary but not sufficient condition for having a good processing. For thixoforming it is critical to have a refined microstructure that during the heating to the semisolid state, achieve a mixture of small spheres immerse in liquid, which is crucial in order to get the best rheological properties for the slurry. This work compares the morphological evolution at the semisolid state of the Al-7wt%Si-2.5wt%Cu alloy produced by a) direct chill casting under electromagnetic stirring (EMS) combined with grain refinement and b) processed via one single pass in an equal channel angular pressing. EMS introduces metal flow during solidification that allows the better distribution of the nuclei provide by the grain refiner. ECAP has emerged as a promising technique capable of imposing large homogeneous deformations in metals, that could leads to a optimal grain refining. The ECAP deformation occurs in a die that contains two channels of the same cross-sectional area and form an angle of 120°. The raw materials were submitted to a heating treatment in order to reach 60% liquid fraction, at soaking times of 0s, 30s, 90s and 210s and the microstructure was evaluated via B&W and color metalography. Comparing the two production processes, it is observed that the ECAP process is highly efficient: while the traditional EMS method produced grains about 150 μm size, ECAP technique achieved grains of 60 μm, providing a fully globular structure, which exhibit favorable characteristics for the thixoforming process reaching to apparent viscosity about 2*104Pa.s.
Authors: Michael Modigell, Annalisa Pola, Torsten Volkmann, Christoph Zang
Abstract: Rheological properties, crystallization and ripening phenomena have been investigated for three different steel grades in the semi-solid state. For each steel grade detailed investigation has been performed via differential scanning calorimetry to quantify the solid fraction as a function of temperature. The experimental procedure for all samples has been as follows: the completely liquid samples were cooled with a constant rate under constant shearing to temperatures corresponding to solid fractions of 30 and 40%. Tracking the viscosity during this procedure allowed analyzing the crystallization kinetics and final ripening process, which differ for the different steel grades. Evaluating the viscosity data on the base of a combination of convection dominated ripening model and a modified Krieger-Dougherty equation gives valuable information of the microstructural evolution. This is validated by additionally quenching samples.
Authors: Jia Wang, Rong Feng Zhou, Han Xiao, De Hong Lu, Lu Li, Dan Bai
Abstract: The isothermal compression tests of semi-solid ZCuSn10P1 alloy by strain induced melt activation (SIMA) process are carried out by Gleeble-1500 thermo-mechanical simulator, and the same tests are finished to samples of as-cast ZCuSn10P1 alloy. The deformation temperature respectively is 910°C, 920°C and 930°C, and the strain respectively is 0.4 and 0.6, the strain rate is 0.5s-1, 1s-1 and 10s-1. The experimental results indicate that the deformation resistance of semi-solid ZCuSn10P1 copper alloy with smaller, more uniform and rounder solid grain is about half of the as-cast ZCuSn10P1 copper alloy. The deformation resistance of ZCuSn10P1 alloy by SIMA process decreases with the deformation temperature increasing, and the deformation resistance increases with the strain rate increasing.
Authors: Takehiko Yanagiya, Yasuhiro Kishi, Koji Kajikawa, Takeshi Yamaguchi, Ken Saito, Shinji Tanaka
Abstract: Semi-solid injection molding (Thixomolding) is a molding method for magnesium alloys. Our team has been investigating the forging of thixomolded magnesium body in order to improve its mechanical properties so that the thixomolded magnesium parts expand the application in transportation industry. Thixomolded magnesium alloy can include coarse alpha phase grain when the alloy is molded below liquidus temperature. Although it is expected that the mechanical properties of a forged thixomolded magnesium alloy are affected by the existence of the coarse alpha grains, its microstructure evolution during forging process has not been clarified yet. We studied the dynamic recrystallization (DRX) behavior of a thixmolded magnesium alloy using a cylinder compression test and microstructure observation. The microstructure observation showed that DRX occurred at the grain boundary and within the alpha phase. The Electron backscattering diffraction (EBSD) data showed that DRX is caused not only by diffusion but also by twinning in the alpha phase.
Authors: Hong Yu Xu, Ze Sheng Ji, Ye Wang, Mao Liang Hu, Hai Bo Wang, Sumio Sugiyama
Abstract: In this paper, chip recycled pressing process (CRP for short), a new method for recycling magnesium alloy chips, was put forward to prepare semi-solid billet. AZ91D magnesium alloy chips of different sizes were respectively recycled to prepare semi-solid billet by CRP process. Effect of chip size on compactness, microstructure and oxidation of CRP billet were investigated. And then CRP billet were heated to semi-solid state and its semi-solid microstructures were studied. The results showed that with the increase of chip thickness, grain refinement effect weakened, deformation uniformity aggravated and oxygen content of CRP billet decreased. Under the same CRP process conditions, the relative densities of four kind of CRP billet were 98.27%, 98.13%, 98.05% and 97.92%, respectively. Meanwhile, there was no significant difference in compactness and microstructure between them. Normally, oxygen content was not more than 1.8% and uniformly dispersed in CRP billet. Hence, the influence of size and deformation inhomogeneity of chip on compactness and microstructure of CRP billet were eliminated via CRP process. In CRP billet, microstructure and strain were uniform and this inherited during semi-solid isothermal treatment.
Authors: Annalisa Pola, Lorenzo Montesano, Ciro Sinagra, Marcello Gelfi, Marina La Vecchia
Abstract: In this paper the effect of globular microstructure on the cavitation erosion resistance was assessed and compared to that of conventional dendritic one. Three different wrought aluminum alloys in as-cast conditions were investigated. The samples were completely characterized by metallographic analyses and microhardness measurements. Cavitation erosion tests were performed according to ASTM G 32 standard. The volume loss was evaluated during the test by periodical interruptions. It was identified the damaging mechanism in case of both dendritic and semisolid microstructure. It was also found that the globular microstructure increases the cavitation erosion resistance only for one of the studied alloys.
Authors: Kang Du, Qiang Zhu, Da Quan Li
Abstract: T6 heat treatment is an effective method to improve the comprehensive properties of Al-Si-Cu-Mg series aluminium alloys. Solution treatment temperature and time, quench process and media, as well as artificial ageing temperature and time are the key factors to determine mechanical properties. Besides these factors, natural ageing, i.e. the holding time between quenching and the starting of artificial treatment at ambient temperature was observed to be significant affect mechanical properties of the aluminium alloys. This effect on semi solid processed aluminium alloys was lack of investigations as the semi solid process produces T6 treatable and weldable components. The present paper focuses on the change regularity of hardness and precipitate behaviour of semi-solid 319S aluminium alloy under different natural ageing (NA) treatment additional to standard T6. Density and morphology of hardening precipitates are analysed using TEM, and the influence mechanism of NA during T6 heat treatment will be discussed. The results show that NA has a positive influence on mechanical properties of the rheo-cast 319S alloy.

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