Abstract
A new method of near-liquidus squeeze casting (NLSC), which obtains globular crystal, has been developed. The research focuses on the formation and growth mechanism of globular crystals of ADC12 alloy by NLSC. Globular crystal transforms into rosette crystal, average grain size (AGS) increases, and average globular coefficient (ASC) decreases with the rise of pouring temperature. But AGS decreases to 17.8 μm directly and ASC increases directly to 0.46 when the pouring temperature is 650°C. NLSC globular crystal of different sizes (AGS is 22.5 μm; ASC is 0.73) distributes uniformly in matrix. Heterogeneous nucleation grows on the solid particles of metal walls and fusing grains. The formation and growth modes of globular crystal include normal grain growth, annexation of small globular crystal, and fracture and spherulization of rosette crystal. NLSC globular crystal critical radius is inversely proportional to degree of supercooling. The pressure has very little influence on critical radius by increase of melting point and thermal conductivity, but it affects critical radius greatly by changing the degree of supercooling. NLSC globular crystal has no enough time to exceed the critical radius.
Highlights
Semisolid processing is an advanced near-net shaping technology [1, 2]
In order to distinguish between the dendrite crystal and globular crystal at microstructure of two dimensions, three vertical planes of near-liquidus squeeze casting (NLSC) sample were observed by metallographic microscope
The grain which grows by homogeneous nucleation tends to aggregation
Summary
Semisolid processing is an advanced near-net shaping technology [1, 2]. The key of semisolid processing is to prepare semisolid slurries [3]. Under the conditions of right and short time, semisolid slurries can be made via near-liquidus method. Sun et al [12] researched the simulation of heat flow and solidification of squeeze casting A356 aluminum alloy (liquidus temperature was 615∘C) and the pouring temperature 700∘C was chosen. Youn et al [13] researched the relationship between squeeze casting parameter and mechanical behavior of AZ91-Ca magnesium alloy (liquidus temperature was 595∘C) and the pouring temperature was 700–800∘C. The advantage of NLSC is that parts are composed by globular crystal with no heat preservation of molten metal and no preparation of semisolid slurries or billets. The research focuses on the formation and growth mechanism of globular crystals under near-liquidus (590∘C)
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