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Acta Metall Sin  2014, Vol. 50 Issue (10): 1237-1243    DOI: 10.11900/0412.1961.2014.00157
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EFFECT OF Mn ON HOT CRACKING TENDENCY OF Mg-6.5Zn ALLOYS
LI Haoyu, BAI Yuanyuan, ZHANG Haitao, WU Xin, ZHANG Zhiqiang, LE Qichi()
The Key Laboratory of Electromagnetic Processing of Materials, Ministry of Education, Northeastern University, Shenyang 110819
Cite this article: 

LI Haoyu, BAI Yuanyuan, ZHANG Haitao, WU Xin, ZHANG Zhiqiang, LE Qichi. EFFECT OF Mn ON HOT CRACKING TENDENCY OF Mg-6.5Zn ALLOYS. Acta Metall Sin, 2014, 50(10): 1237-1243.

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Abstract  

Mg-Zn-Mn alloy has high hot cracking tendency (HCT), but few researches focus on its hot cracking behavior and mechanism. The effect of Mn on the HCT of Mg-6.5Zn-xMn alloys was studied by the designed equipment which can measure and record the subtle changes of temperature, shrinkage displacement and shrinkage stress during solidification in this study. The results indicate that the larger the maximum contract rate (vmax) and the stress accumulating coefficient (k), which are put forward to evaluate HCT, the higher the HCT is, and there is higher HTC when vmax or k presents at high fraction of solid. The vmax of Mg-6.5Zn-xMn alloy increases with the increase of Mn content, however its position move towards to lower fraction of solid, and the k reaches the maximum value and presents at high fraction of solid at 0.35%Mn, which means the greatest HCT in this composition. The hot cracks of these alloys initiated and propagated at final stage of solidification (with higher fraction of solid), and the intergranular feeding channels could be observed. The thicker the liquid film around grains formed by the low melting point phases and the finer the grains, the less the HCT of the alloy is. After dendritic separation, interdendritic bridging formed by the jointing of dendrite arms could enhance the adhesive force between grains at final stage of solidification. However, the break of interdendritic bridging due to the hindrance to grain contraction would result in the hot cracks.

Key words:  Mg-6.5Zn alloy      Mn      magnesium alloy      hot cracking     
Received:  02 April 2014     
ZTFLH:  TG146.22  
  TG113.26  
Fund: Supported by National Basic Research Program of China (No.2013CB632203)

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2014.00157     OR     https://www.ams.org.cn/EN/Y2014/V50/I10/1237

Alloy Zn Mn Mg
Mg-6.5Zn 5.94 - Bal.
Mg-6.5Zn-0.04Mn 5.90 0.04 Bal.
Mg-6.5Zn-0.35Mn 6.73 0.34 Bal.
Mg-6.5Zn-1.00Mn 7.21 1.18 Bal.
Table 1  Chemical compositions of tested alloys
Fig.1  Schematic diagram of free contraction displacement tested module
Fig.2  Schematic diagram of blocked contraction stress tested module
Fig.3  Schematic diagram of pouring and temperature collection
Fig.4  Temperature-fraction of solid (fs) curves of tested alloys
Fig.5  Free contraction time-temperature/displacement/displacement change rate (dl/dt) curves of Mg-6.5Zn-0.35Mn alloy (T—temperature)
Fig.6  Maximum contract rate (vmax) and fraction of solid of tested alloys
Fig.7  Blocked contraction time-temperature/stress curves (a) and temperature-stress/stress change rate (ds/dt) curves (b) of Mg-6.5Zn-0.35Mn alloy
Fig.8  Macrograph of hot cracking fracture of Mg-6.5Zn-0.35Mn alloy
Fig.9  Effects of Mn on stress accumulating coefficient (k)(a) and fraction of solid of stress accumulating (b)
Fig.10  OM images of the last part of solidification of Mg-6.5Zn-0.35Mn alloy (a) and Mg-6.5Zn-1.00Mn alloy (b)
Fig.11  OM image of Mg-6.5Zn-0.35Mn alloy near the hot crack fracture (Arrows A show microcracking; arrows B show feeding channels)
Fig.12  SEM images of hot crack fracture morphologies of Mg-6.5Zn-0.35Mn alloy

(a) the liquid films

(b) the interdendritic bridgings and warty drapes

  
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