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Cheng Y.-S.,Harbin Institute of Technology | Chen Q.,Southwest Technique and Engineering Institute | Huang Z.-Q.,Harbin Institute of Technology | Huang S.-H.,Southwest Technique and Engineering Institute
Transactions of Nonferrous Metals Society of China (English Edition) | Year: 2010

The semi-solid thermal transformation (SSTT) route was used to obtain semi-solid billets for thixoextrusion. Microstructural evolution during partial remelting was studied. Mechanical properties of thixoextruded billets were determined. The results show that the solid grain size decreases initially and then increases as the isothermal holding time increases. Furthermore, shape factor measurements show that the solid grains tend to become increasingly spheroidal after 10 min isothermal holding. With longer isothermal holding time, the degree of spheroidization slightly decreases. Middle ultimate tensile strength (263.3 MPa) is obtained for thixoextruded AZ91D alloy produced by the SSTT route, with a yield strength of 120.0 MPa and an elongation to fracture of 7.19. © 2010 The Nonferrous Metals Society of China. Source


Zhao Z.,Southwest Technique and Engineering Institute | Chen Q.,Southwest Technique and Engineering Institute | Tang Z.,Nanjing University of Aeronautics and Astronautics | Hu C.,Southwest Technique and Engineering Institute
Journal of Alloys and Compounds | Year: 2010

Cyclic closed-die forging (CCDF) was introduced into the strain induced melt activation (SIMA) route to replace conventional upset or extrusion. The microstructure evolution of CCDF formed AM60B magnesium alloy heated into the semi-solid state has been investigated. Tensile mechanical properties for AM60B magnesium alloy thixoformed from starting material produced by the SIMA route were determined. The results show that the SIMA route (four-pass CCDF and partial remelting) produced ideal, fine semi-solid microstructure, in which completely spheroidal primary solid grains had a little amount of entrapped liquid. With increasing the equivalent strain, the solid grain size decreased and the degree of spheroidization tended to be improved. Furthermore, the rate of liquation was also slightly increased. Increasing the reheating temperature was favorable for obtaining spheroidal semi-solid microstructure and decreasing the probability of coalescence among solid grains. However, prolonging holding time resulted in grain coarsening. The tensile mechanical properties of the thixoformed components were influenced by the amount of equivalent strain during CCDF. Good elongation to fracture (15%) was obtained for thixoformed AM60B magnesium alloy component, with a yield strength of 199.2 MPa and a tensile strength of 306.7 MPa. © 2010 Elsevier B.V. All rights reserved. Source


Zhao Z.,Southwest Technique and Engineering Institute | Chen Q.,Southwest Technique and Engineering Institute | Chao H.,Harbin Institute of Technology | Huang S.,Southwest Technique and Engineering Institute
Materials and Design | Year: 2010

Semi-solid processing of magnesium alloys is generally based on conventional magnesium-based casting alloys such as Mg-Al series. However, these casting alloys do not give such high mechanical properties as the alloys that are conventionally wrought such as Mg-Zn series. In this paper, a ZK60 magnesium alloy with the addition of Y was thixoforged. The semi-solid thermal transformation (SSTT) route and the recrystallisation and partial melting (RAP) route were used to obtain the semi-solid feedstocks for thixoforging. Microstructural evolution during partial remelting was studied at temperatures for times. Tensile mechanical properties of thixoforged components at room temperature were examined. Results show that a fine spheroidal microstructure can be obtained by the RAP route. Compared to the RAP route, the SSTT alloy shows coarsened solid grains with a relatively high proportion of intragranular liquid droplets. With prolonged holding time, the solid grain sizes of the SSTT alloy and the RAP alloy increased. Coalescence was dominant in the SSTT alloy and Ostwald ripening was dominant in the RAP alloy. Thixoforging for the SSTT alloy and the RAP alloy resulted in successful filling of the die. The tensile properties of the thixoforged RAP alloy were satisfactory and exceeded those of the thixoforged SSTT alloy. However, the mechanical properties of both the thixoforged SSTT alloy and the thixoforged RAP alloy decreased with prolonged holding time. © 2009 Elsevier Ltd. All rights reserved. Source


Zhao Z.,Southwest Technique and Engineering Institute | Chen Q.,Southwest Technique and Engineering Institute | Chao H.,Harbin Institute of Technology | Hu C.,Southwest Technique and Engineering Institute | Huang S.,Southwest Technique and Engineering Institute
Materials and Design | Year: 2011

An as-cast Mg-Al-Y-Zn alloy was successfully processed by equal channel angular extrusion (ECAE) in the temperature range of 225-400°C, and the influences of processing temperature on the microstructure and mechanical properties were investigated. The use of back pressure during one-pass ECAE of Mg-Al-Y-Zn alloy was favorable for eliminating the undeformed area in the billet. At the processing temperature below 250°C, the microstructures were characterized by unrecrystallised structure and the precipitated phase Mg17Al12 was elongated along the extrusion direction. With increasing processing temperature to 350°C, a large number of recrystallised grains were obtained. Increasing processing temperature promoted workability but led to decrease in the strength of Mg-Al-Y-Zn alloy. Then billets of as-cast Mg-Al-Y-Zn alloy were extruded at different numbers of ECAE passes. It was found that the microstructure was effectively refined by ECAE and mechanical properties were improved with numbers of ECAE passes increasing from one-pass to four passes. However, strengths decreased slightly after five passes though the grain size decreased considerably. © 2010 Elsevier Ltd. Source


Chen Q.,Southwest Technique and Engineering Institute | Shu D.,Southwest Technique and Engineering Institute | Zhao Z.,Southwest Technique and Engineering Institute | Wang Y.,Southwest Technique and Engineering Institute | Yuan B.,Hefei University of Technology
Materials and Design | Year: 2012

The Mg-5.3. wt.%Zn-1.13. wt.%Nd-0.51. wt.%La-0.28. wt.%Pr-0.79. wt.%Zr alloy prepared by direct chill casting is subjected to hot extrusion. The effects of extrusion ratio and temperature on microstructure and tensile mechanical properties have been studied. The results indicate coarse grains of as-cast alloys are refined with extrusion ratio increasing from 0 to 9. The eutectic constituents are elongated along extrusion direction. However, further increase of extrusion ratio has a little influence on grain refinement and the improvement of mechanical properties of the alloy. Dynamic recrystallisation is the main mechanism of grain refinement during hot extrusion. Raising extrusion temperature results in grain coarsening. Grain shape becomes more equiaxed-like with raising extrusion temperature. At the same time, mechanical properties decrease with the increase of extrusion temperature. © 2012 Elsevier Ltd. Source

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