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Achieving high ductility and strength in magnesium alloy through cryogenic-hot forming

Journal of Magnesium and Alloys 2023年第9期11卷 3130-3140页

作者： Kai Zhang Zhutao Shao Joseph Robson Yan Huang Jinghua Zheng Jun Jiang Department of Mechanical Engineering Imperial College LondonLondon SW72AZUK Department of Mechanical Engineering University College LondonLondon WC1E 7JEUK Department of Materials The University of ManchesterManchester M13BBUK BCAST Institute of Materials and ManufacturingBrunel University LondonUxbridge UB83PHUK College of Material Science and Engineering Nanjing University of Aeronautics and AstronauticsNanjing 211106China

Magnesium alloys are the lightest structural alloys and have attracted substantial research attention in the past two decades. However, their mechanical properties, including ductility and strength, are limited after forming due to the formation of coarse grains and strong texture. This study proposes and proves a new cryogenic-hot forming process concept. Cryogenic deformation is imposed before the hot deformation. The effect of the cryogenic step has been compared with a conventional direct hot deformation process. The mechanical properties, microstructure,and texture of both the novel and conventional process routes have been compared. The cryogenic-hot deformed sample exhibits the highest ductility and fracture strength(ultimate tensile strength: 321 MPa, ductility: 21%) due to effective grain refinement and texture weakening by cryogenically formed twin-twin interaction induced recrystallisation. The proposed cryogenic-hot forming process can be a potential innovative manufacturing method for producing high-performance magnesium components.

关键词： Magnesium alloys dynamic recrystallisation Twinning Cryogenic deformation

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Microstructure and mechanical properties of curved AZ31 magnesium alloy profiles produced by differential velocity sideways extrusion

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Journal of Magnesium and Alloys 2023年第2期11卷 493-508页

作者： Wenbin Zhou Jianguo Lin Trevor A.Dean Department of Mechanical Engineering Imperial College LondonLondon SW72AZUK Department of Mechanical Engineering University of BirminghamBirmingham B152TTUK School of Science and Engineering University of DundeeDundee DD14HNUK

Lightweight curved profiles are widely utilised in the transportation industry considering the increasing need for improving aerodynamic efficiency,aesthetics and cutting *** this paper,curved AZ31 Mg alloy profiles were manufactured in one operation by a novel process,differential velocity sideways extrusion(DVSE),in which two opposed rams were *** of extrusion temperature and velocity(strain rate) on curvature,microstructure,and mechanical properties of the formed profiles were *** curvature was found to be more readily controlled by the velocity ratio of the bottom ram v2to the top ram v1,whereas extrusion temperature(T=250,300,350℃)and extrusion velocity(v_(1)=0.1,1 mm/s) slightly affect curvature for a given velocity ratio.A homogeneous microstructure with equiaxed grains(~4.5 μm) resulted from dynamic recrystallisation(DRX),was observed after DVSE(v_(2)/v_(1)=1/2) at 300 ℃ and v_(1)=0.1 mm/s,where the initial billet had an average grain size of ~25 *** extrusion temperature leads to grain growth(~5 μm) at 350 ℃ and v_(1)=0.1 mm/*** is incomplete at the relatively low temperature of 250℃(v_(1)=0.1 mm/s),and higher strain rate with v1=1mm/s(T=300℃),resulting in inhomogeneous bi-modal necklace pattern grains ranging in size around 1-25 μm for the former and 2-20μm for the *** refinement is attributed to DRX during the severe plastic deformation(SPD) arising in DVSE,and initiates at the prior boundaries of coarse grains in a necklace-like *** with the billet,micro-hardness and ultimate tensile strength of the profiles have been enhanced,which is compatible with grain ***,an obvious increase in tensile ductility was ***,yield strength slightly decreases except for the complete DRXed case(300℃,v_(1)=0.1 mm/s),where a slightly higher value was found,indicating strengthening by grain refinement is greater than softening caused by texture *** initial billet had a strong b

关键词： Extrusion Magnesium alloy AZ31 Curved profiles/sections Bending Grain refinement dynamic recrystallisation

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DWTT in Thicker Gauge Hot Rolled Coils for Line Pipe Application

DWTT in Thicker Gauge Hot Rolled Coils for Line Pipe Applica...

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“第十届中国钢铁年会”暨“第六届宝钢学术年会”

作者： Pradeep Agarwal Gunna Venkata Ramana Devasish Mishra Ashish Chandra Gajraj Singh Rathore JSW Steel Ltd. Vijayanagar Works.

Ductile failure in DWTT（Drop Weight Tear Test） at subzero temperatures is one of the key requirements for line pipe steels. As the temperature drops or thickness increases, the ductile failure mode is difficult to achieve in DWTT testing. JSW Steel has successfully developed new generation Nb-microalloyed line pipe steels for heavy gauges（up to 20mm） for grades up to API X-80 grade for the first time in India through hot strip mill. It has been observed that prior austenite grain boundaries act as major crack arresters due to the high angle of misorientation at the grain boundaries. In order to achieve ductile DWTT failure, the crack propagation by brittle Griffith cracks needs to be arrested by high angle boundaries with their interspacing lesser than that of the critical Griffith crack length. Hence refinement of prior austenite grain size which requires optimization of multiple dynamic and static recrystallisation phenomena coupled with reduced grain growth at rolling temperatures, leads to improved DWTT performance. This paper describes the various aspects of steel chemistry design and the hot rolling process optimization to achieve subzero temperature DWTT in thicker gauge line pipe steels. Multiple online trials have been conducted at JSW Steel, Vijayanagar works for achieving the same, such that the effects of chemistry and process variables on final microstructures and mechanical properties have been established. Further, a correlation table has been prepared for predicting DWTT transition temperature based on the largest prior austenite grain size in the transfer bar microstructure before entering into the finishing mill. This table has proven to be very useful in alloy design and in optimizing hot rolling process window to achieve superior DWTT performance for different end thicknesses in line pipe steels.

关键词： DWTT(drop weight tear test) dynamic recrystallisation static recrystallisation prior austenite grain size

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