تجزیه و تحلیل مورفولوژیک SSM آل 4.5% وزنی مس، اندازه گیری شده توسط شاخص کیفیت rheocast

There is a direct relationship between grain size obtained from macrostructure characterisation and the globule size and shape factor, obtained from microstructure characterisation: the smaller the grain size, the smaller the globule size and shape factor. Due to this a rheocast quality index (View the MathML source) was used to evaluate the Al–4.5 wt.% Cu alloy produced by strain induced melting activation (SIMA). The structure that presents the smallest grain size, the smallest shape factor, and the most homogeneous and globular size of the primary phase, has the best RQI and consequently the best behaviour in the semi-solid forming. Four different as-cast structures were cold rolled to obtain 20 and 40% deformation and were partially melted at 635 °C at holding times of 5 min. Both, macro- and microstructure were characterised.

Only rheocast structures produced by cold deformation followed by partial melting (SIMA—strain induced melting activation), or those produced by spray casting, present a completely spherical shape [1] and [2]. Rheocast structures, which present minor grain size, minor shape factor, and most homogeneous and globular size of the primary phase, have the best behaviour in the semi-solid forming [3] and [4]. Despite, these routes being the most expensive to obtain rheocast structures, these materials are the most suitable for thixo-forming. This paper presents the rheocast quality index (RQI) characterisation method as a powerful tool in the semi-solid characterisation area. The most common raw material production methods—mechanical or electromagnetic stirring, ultra-refining and super-cooling—assure a very refined equiaxial dendrite structure. This structure, when re-heated to the semi solid state, presents several three-dimensional interconnections within the primary phase branches. Ito et al. [5] presented these connections in 1992. Due to these interconnections, some difficulties to correctly characterise these materials were reported [6] and [7]. Some authors focused their characterisation work on the shape and size of the primary phase in microstructure; others used different shape factors as well as globule/grain size determination [8], [9] and [10]. But, few researches were focused on the macrostructure characterisation. There is a significant difference between macrostructure and microstructure for the A356 alloy, as shown in previous work [11], [12], [13] and [14], and this difference could help in the rheocast characterisation. The relationship between macro- and microstructure shows that the ratio between the size of the primary phase and the size of the grain can help to quantify the several interconnections mentioned above. Small and more globular grains tend to have few branches and a more globular shape. In this case, a grain in the macrostructure tends to have the same size and shape as the grain in the microstructure, i.e. one grain is one globule. The most common structures present a quasi-globular or rosette-shaped grain. This paper will show the difference between grain and globule by comparing the macro- and the microstructure of the Al–4.5 wt.% Cu rheocast alloy produced by SIMA.

The effect of the deformation process previous to the partial melting is more effective in producing the rheocast structure than the initial as-cast grain size. Increasing the previous deformation decreases the final rheocast grain size and a more homogeneous and globule shape structure is formed. Both the RQI as well as the RQI for primary and secondary phase (RQIP&S) are powerful tools to evaluate the rheocast structures. Increasing the previous deformation in the as-cast original structure increases the globularisation of the structure, and the RQI reflects this change in the morphology very well. The best globularisation, i.e., the best rheocast material obtained was produced by partial melting from the as-cast raw material with 39 μm and deformed 40% by cold rolling: RQI=0.98 or 89% and View the MathML source or 63%. The relationship between RQI and the thixo-forming behaviour will be discussed in a future work.