Development of Non-Dendritic Microstructure of Aluminum Alloy in Semi-Solid State under Ultrasonic Vibration

To investigate the influence of refined grains on the microstructure of 7075 aluminum alloy in semi-solid state, a new strain induced melting activation (SIMA) method was put forward containing two main stages: pre-deformation with equal channel angular pressing (ECAP) method and isothermally holding in the semi-solid temperature range. The breaking up and growth mechanisms of the grains and kinetics of equiaxed grains coarsening during the semi-solid holding were investigated. The results showed that the average grain size after ECAP extrusion decreased significantly, e.g., microstructure with average globular diameter less than 5μm was achieved after four-pass ECAP extrusion. Obvious grain coarsening had been found during isothermal holding in the semi-solid state and the roundness of the grains increased with the increasing holding time. The proper microstructure of 66.8μm in diameter and 1.22 in shape factor was obtained under proper soaking condition (at 590°C for 15 min). Two coarsening mechanisms, namely, coalescence in lower liquid fraction and Ostwald ripening in higher liquid fraction contributed to the grain growth process.

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Visualization of Micro-Segregations in A356 Aluminum Alloy by a Color Etching Method

Materials Science Forum ◽

10.4028/www.scientific.net/msf.794-796.9 ◽

2014 ◽

Vol 794-796 ◽

pp. 9-14

Author(s):

Li Gao ◽

Yohei Harada ◽

Shinji Kumai

Keyword(s):

Aluminum Alloy ◽

Grain Growth ◽

Solid Phase ◽

Color Difference ◽

Water Quenching ◽

A356 Aluminum Alloy ◽

Dendritic Microstructure ◽

Spheroidal Microstructure ◽

A356 Aluminum ◽

Semi Solid

An A356 aluminum alloy billet which has a dendritic microstructure was compressed and then partially re-melted to semi-solid state before water quenching, by which the spheroidization of Al grains was realized. A color etchant called Weck's reagent was used to characterize both the dendritic and spheroidal microstructure. In both cases, distinct color differences were observed inside Al grains by normal optical microscopy. Interestingly, a dendritic-shaped structure inside the spheroidal Al grains was visualized, which should be the reflection of the original dendrite before heating and partial re-melting. Also, the grain growth during water quenching could be clearly visualized after etching with this reagent. As a result, solid fractions could be evaluated more precisely by excluding the grain growth when measuring the area of solid phase in 2-D micrographs. In order to investigate the coloring mechanism, electron probe micro-analyses were carried out to characterize the micro-segregations inside an Al grain. Results showed that the micro-segregation of Ti had a strong correlation with the color difference. Detailed investigation found that the micro-segregation of Ti could be preserved after heating and partial re-melting due to the extremely low diffusion rate of Ti in Al.

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Influence of Morphology on the Rheological Behavior of Semi-Solid A356 Aluminum Alloy

Solid State Phenomena ◽

10.4028/www.scientific.net/ssp.116-117.601 ◽

2006 ◽

Vol 116-117 ◽

pp. 601-605

Author(s):

Heng Hua Zhang ◽

Xian Nian Zhang ◽

Guang Jie Shao ◽

Luo Ping Xu ◽

Yi Tao Yang ◽

...

Keyword(s):

Aluminum Alloy ◽

Solid State ◽

Rheological Behavior ◽

Apparent Viscosity ◽

Flow Behavior ◽

A356 Alloy ◽

Solid Alloy ◽

A356 Aluminum Alloy ◽

A356 Aluminum ◽

Semi Solid

The morphology of semi-solid alloy is one of the key influence factors on the rheological behavior of slurry during die filling and the mechanical properties of formed parts. However, it is difficult to study such effect due to hard controlling of morphology in semi-solid state. In this paper, a self-developed Searle-type viscometer was used to determine the rheological behavior of A356 aluminum alloy in different morphology, which was refined with the salts mixture of K2TiF6 and KBF4. The results indicated that the flow behavior of refined A356 alloy in the semi-solid state possesses obviously thixotropic behavior under isothermal shearing condition with less time to reach steady state and lower steady apparent viscosity as compared to that of the A356 alloy. During continuous cooling at a constant shearing rate, the apparent viscosity of refined A356 slurry in the same solid fraction decreased with the content of Ti. It is shown from quantitative image analysis that the primary α-Al grain in the refined alloy evolves from dendrites to rosettes or sphericitys, and then tends to be rounder and finer in higher Ti content. The mechanism of the influence of morphology on rheological behavior was also discussed in this paper.

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