Browsing by Author "Chi, Chao-Ting"

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A study of weldability and fracture modes in electron beam weldments of AZ series magnesium alloys
(2006-11-05)
Chi, Chao-Ting
;
Chao, Chuen-Guang
;
Liu, Tzeng-Feng
;
Wang, Che-Chung
;
材料科學與工程學系
Given the growing need for environmental protection and lightweight construction, electron beam welding (EBW) is becoming the most important welding technology because it can compensate for the poor formability of magnesium alloys. This paper examines interactions between the properties of three AZ series magnesium alloys and welding conditions. The EBW process can yield four kinds of defect in a weld: cavities, the heat-affected zone (HAZ), undercuts, and root concavities. These defects obviously induce stress concentrations in the weld, and may seriously damage its strength. Additionally, the distribution of precipitates (gamma phase, Mg17Al12) in the fusion zone (FZ) changes from a relatively small number of scattered particles to a dense population of dendrites as the Al content of the magnesium alloy increases. Under excessive tensile stress, alloy weldments break in one of two fracture modes: an irregular FZ fracture, or a regular HAZ fracture. AZ31B usually exhibits the former mode and AZ91D the latter, while AZ61A exhibits each mode half the time. The overall weldability, which depends on the random distribution of these precipitates and defects, is found to be greatest for the AZ61A alloy. The best process window, on the other hand, is found for the AZ91D alloy. Finally, we obtain optimum parameters for the EBW process and empirical formulae for the weldment strength as a function of these parameters. These results are closely related to each other. (c) 2006 Elsevier B.V. All rights reserved.
Aluminum element effect for electron beam welding of similar and dissimilar magnesium-aluminum-zinc alloys
(2007-05-01)
Chi, Chao-Ting
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Chao, Chuen-Guang
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Liu, Tzeng-Feng
;
Lee, Chuan-Hua
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材料科學與工程學系
Defects causing stress concentration from electron beam welding are related to the Al content of the Mg-AI-Zn weldment and its effect will be most conspicuous for the welding of dissimilar metals. Precipitates which are gamma phase (Mg-17(Al,Zn)(12)) also influence the formative mechanism of pores, the heat-affected zone and the fracture mode of the weld. More noteworthy is that the Al content of 6.0 wt.% is a very important delimitation for these mechanisms. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Characterization on electron beam welds and parameters for AZ31B-F extrusive plates
(2007-02-02)
Chi, Chao-Ting
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Chao, Chuen-Guang
;
材料科學與工程學系
;
Department of Materials Science and Engineering
Recent years have seen a renewal of interest in magnesium alloys because of the extensive applications in the industry of 3C, aerospace and traffic. However, applications of magnesium alloys are still restricted by their poor formability. This can be improved by dividing the shape-complicated work piece into several easily-made parts and then combining into one. There are various kinds of defects in the weld under the interaction of AZ31B properties and welding conditions. These defects will produce obvious stress concentration and serious material damage. In this study, a home-made 11 mm-thick AZ31B extrusive plate was used. By operating and comparing the changeable parameters of electron beam welding (EBW), the condition was optimized as 100 mA, 50 kV, 60.6 mm/s and focus position at bottom. Seventy-eight percent and 91% of the base material strength were obtained by stress and non-stress concentration weldments, respectively. Meanwhile, the harmful influence of SC and grain coarsening reached at least 13% and 9%, respectively. With other worse parameters, the strength of AZ31B weldments were reduced by undercut, root concavity, cavities and heat-affected zone (HAZ). (c) 2006 Elsevier B.V. All rights reserved.
Characterization on electron beam welds and parameters for AZ31B-F extrusive plates
(2006-02-02)
Chi, Chao-Ting
;
Chao, Chuen-Guang
;
材料科學與工程學系
;
Department of Materials Science and Engineering
Recent years have seen a renewal of interest in magnesium alloys because of the extensive applications in the industry of 3C, aerospace and traffic. However, applications of magnesium alloys are still restricted by their poor formability. This can be improved by dividing the shape-complicated work piece into several easily-made parts and then combining into one. There are various kinds of defects in the weld under the interaction of AZ31B properties and welding conditions. These defects will produce obvious stress concentration and serious material damage. In this study, a home-made 11 mm-thick AZ31B extrusive plate was used. By operating and comparing the changeable parameters of electron beam welding (EBW), the condition was optimized as 100 mA, 50 kV, 60.6 mm/s and focus position at bottom. Seventy-eight percent and 91% of the base material strength were obtained by stress and non-stress concentration weldments, respectively. Meanwhile, the harmful influence of SC and grain coarsening reached at least 13% and 9%, respectively. With other worse parameters, the strength of AZ31B weldments were reduced by undercut, root concavity, cavities and heat-affected zone (HAZ). (c) 2006 Elsevier B.V. All rights reserved.
Optimum evaluation for electron beam weldments of AZ61A-F extruded plates
(2007-05-01)
Chi, Chao-Ting
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Chao, Chuen-Guang
;
材料科學與工程學系
;
Department of Materials Science and Engineering
Recently there has been a renewed interest in magnesium alloys because of their extensive applications in industries, such as computing, communications, consumer electronics, aerospace, and traffic. However, the plastic deformation of these alloys is restricted because HCP magnesium has fewer dislocation slip systems. This limitation can be circumvented by dividing shape-complicated workpieces into several more easily made parts, which are then combined into one. Due to the interaction between material properties and welding conditions, there are various kinds of defects in the welds of magnesium alloys, and these defects cause obvious stress concentrations and serious material damage. This study used a custom-made 11-mm-thick AZ61A extruded plate. By operating and comparing the variable parameters of electron beam welding (EBW), the condition was optimized for ultimate tensile strength (UTS) at 100 mA, 50 kV, 60.6 mm/s, and a focal position at the bottom of the workpiece. Under these conditions, ultimate tensile strengths of 83% and 96%, respectively, were obtained for the base metal in weldments with and without stress concentrators on their surface. For worse conditions such as 100 mA, 45 kV, 86.0 mm/s, and a focal position at the bottom, the UTS of electron beam weldments for AZ61A-F plates was reduced by the three factors of root concavity, heat-affected zone (HAZ), and cavities.
Relational analysis between parameters and defects for electron beam welding of AZ-series magnesium alloys
(2008-06-19)
Chi, Chao-Ting
;
Chao, Chuen-Guang
;
Liu, Tzeng-Feng
;
Wang, Che-Chung
;
材料科學與工程學系
In the last half century, lightweight magnesium alloy has gradually shifted from military applications to civil applications. More noteworthy is that its low melting point, high thermal conductivity, and superior fluidity are good for weld pool flow and welding parameter research. This paper presents a novel approach to these characteristics, which analyzes the influences of electron beam welding parameters on weldment strength and defect formation by linking Taguchi's method with the grey relational analysis. Not only are the parameter contribution and the defect weight individually quantified, but also the relationship between welding parameters and defect dimensions can also be obtained this way. Crown Copyright (C) 2008 Published by Elsevier Ltd. All rights reserved.