Omid Lashkari

Property Change During Fixtured Sintering of NiTi Memory Alloy

ABSTRACT Fixtured sintering of up to 5 hours at 1223 to 1323 K is successfully used to diminish the dimensional change of NiTi memory alloys produced from mixtures of elemental Ni and Ti powders packed and pressed at room temperature under 400, 500, and 600 MPa pressure. The improvements obtained can help near-net-shape technology via powder metallurgy that overcomes the traditional casting problems such as oxygen, nitrogen, hydrogen, and carbon absorption and intermetallic compound precipitation, which lower the workability and the homogeneity of the final alloy. The effects of compaction pressure, sintering time, and sintering temperature on dimensional change, porosity, hardness, and morphology of the produced TiNi intermetallic alloy are investigated. Phase transformation temperatures and mechanical properties of the sintered samples were determined and compared for the influence of compaction pressure and sintering conditions. It was concluded that suitable thermoelastic and superelastic effects are achievable via appropriate selection of fixtured sintering conditions.

Evolution of primary α-Al particles during isothermal transformation of rheocast semi solid metal billets of A356 Al–Si alloy

Abstract Semi solid metal (SSM) processing is a relatively new technology for metal forming, different from the conventional metal shaping technologies that use either solid or liquid metals as the starting material. Semi solid metal processing deals with semi solid slurries, in which non-dendritic solid particles are dispersed in a liquid matrix. The solid volume fraction could vary between 0·1–0·5 with apparent viscosity close to that of liquid state. The slurry flows easily under pressure and makes complicated shapes with high degree of die filling and integrity. There are different ways to prepare SSM slurries including the application of direct or indirect mechanical stirring, chemical treatments of the melt, and manipulation of pouring temperature. One of the important parameters controlling the mechanical and flow characteristics of SSM billets is the morphology of the primary solid phase, e.g. α-Al for Al–Si alloys. The morphological evolution of primary α-Al particles of SSM processed A356 Al–Si billets is studied with variant soaking times, 0–480 s, at 595°C, i.e. 0·33 fraction solid before the applied compression force. The effect of applied compression force of 33 kgf on the evolution of dendritic morphology was also investigated at the same temperature and within the same holding time range. The microstructure was characterised quantitatively and showed that spheroidisation and coarsening are active during the course of treatment with the application of compressive forces bringing about further refinement. Le traitement de métal à l’état semi-solide (SSM) est une technologie relativement nouvelle de formage des métaux, différente des technologies conventionnelles de façonnage des métaux qui utilisent des métaux soit à l’état solide ou liquide comme matériel de départ. Le traitement SSM traite des coulis à l’état semi-solide, dans lesquels des particules solides non-dendritiques sont dispersées dans une matrice liquide. La fraction volumique du solide peut varier entre 0·1 et 0·5, avec une viscosité apparente proche de celle de l’état liquide. Le coulis s’écoule facilement sous pression en des formes compliquées avec un haut degré de remplissage du moule et d’intégrité. Il existe différentes façons de préparer les coulis de SSM, incluant l’application d’agitation mécanique directe ou indirecte, les traitements chimiques du bain et la manipulation de la température de coulage. L’un des paramètres importants contrôlant les caractéristiques mécaniques et d’écoulement des billettes de SSM est la morphologie de la phase solide primaire, par exemple Al-α dans les alliages d’Al–Si. On a étudié l’évolution morphologique des particules primaires d’Al-α de billettes traitées en SSM d’Al–Si A356, avec des temps d’incubation variant de 0 à 480 s, à 595°C – c’est-à-dire fraction solide de 0·33 avant la force appliquée en compression. On a également examiné l’effet de la force de compression appliquée de 33 kgf sur l’évolution de la morphologie dendritique, à la même température et dans la même gamme de temps de rétention. On a caractérisé quantitativement la microstructure et l’on montre que la sphéroïdisation et le grossissement sont actifs tout au long du traitement, l’application de forces compressives amenant davantage d’affinement.

The Impact of Partial Drainage on Chemical Composition of 356 Al-Si Alloy

A novel method is introduced to investigate the macro-segregation in 356 Al-Si alloy. In this method, a portion of the remaining liquid is extracted during solidification of the alloy from the bottom orifice of a cylindrical sample. Different parts of the sample plus the extracted material are chemically analyzed and a good correlation between the chemical composition and the predicted values from binary Al-Si diagram was obtained. Comparing the chemical composition of the decanted alloy in the cast piece with those of the 356 standard range has shown that even with 15% of the liquid extraction, the composition of the alloy is still within the intended range.