An Overview on the Process Development and the Formation of Non-Dendritic Microstructure in Semi-Solid Processing of Metallic Materials

Abstract

Semi-solid metal (SSM) processing has been an attractive method for manufacturing near-net-shape components with high integrity due to its distinct advantages over conventional forming technologies. SSM processing employs a mixture of solid phase and liquid metal slurries and/or non-dendritic feedstocks as starting materials for shaping. Since the original development from 1970s, a number of SSM processes have been developed for shaping components using the unique rheological and/or thixotropic properties of metal alloys in the semi-solid state, in which the globular solid particles of primary phase are dispersed into a liquid matrix. In this paper, the progress of the development of shaping technologies and the formation of non-dendritic microstructure in association with the scientific understanding of microstructural evolution of non-dendritic phase are reviewed, in which the emphasis includes the new development in rheomoulding, rheo-mixing, rheo/thixo-extrusion and semi-solid twin roll casting, on the top of traditional rheocasting, thixoforming and thixomoulding. The advanced microstructural control technologies and processing methods for different alloys are also compared. The mechanisms to form non-dendritic microstructures are summarised from the traditional understanding of mechanical shear/bending and dendrite multiplication to the spheroidal growth of primary phase under intensively forced convection. In particular, the formation of spheroidal multiple phases in eutectic alloys is summarised and discussed. The concluding remarks focus on the current challenges and developing trends of semi-solid processing.

Effect of Semisolid and Heat Treatment Process on Microstructural Refinement of Al–7Si Alloy

“…The utilisation of semi-solid metal (SSM) enhanced the engineering properties through the microstructural change from coarse dendritic α-Al grains to a globular shape as well as the change from flaky Si particles to an acicular shape. Furthermore, these techniques are expected to serve as alternatives to traditional casting processes because they offer the possibility of constructing defect-free, heat treatable and near-net shape components at low cost [ 8 , 9 ]. This approach produces materials that possess homogeneous globular microstructures that are characterised by decreased segregation and porosity [ 10 , 11 ].…”

Study on the Semi-Solid Thixotropic Forging Forming Process for the Low-Carbon Steel Claw Pole

“…During the SSTFF process, the semi-solid billets can be filled into the preheated die steadily under a small forging load, because the semi-solid billets are a mixed structure with fine, spherical solid grains and small surrounding liquid phases [ 12 ]. Moreover, the SSTFF process has a higher solid phase ratio and lower forming temperature than the rheocasting method, which can significantly reduce the thermal impact of the die, improve the service life of the die, and reduce the oxidation degree of materials in the forming process [ 13 , 14 , 15 , 16 ].…”

Venting Systems in Semi-Solid Processing of Aluminium

“…These routes of semi-solid processing can generally be divided into two routes, thixocasting and rheocasting [3]. The difference between these two routes is that thixocasting requires the material to be heated from the solid state to the two-phase region, whereas rheocasting achieves the semi-solid state by controlled cooling and treatment directly from the liquid melt [4].…”