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Acta Metall Sin  2017, Vol. 53 Issue (1): 114-122    DOI: 10.11900/0412.1961.2016.00178
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Forming Limit of FSW Aluminum Alloy Blank Based on a New Constitutive Model
Guannan CHU1,Yanli LIN1(),Weining SONG2,Lin ZHANG1
1 School of Materials Science & Engineering, Harbin Institute of Technology at Weihai, Weihai 264209, China;
2 Weihai Northern Electric Group Company Limited, Weihai 264209, China
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Automobile lightweight can effectively save fuel consumption and reduce CO2 emissions. Aluminum and its alloys are desirable for the automotive industry due to their excellent high-strength to weight ratio. However, due to the introduction of the welding seam, it has brought new changes to the forming process, especially to the forming limit. To establish a reasonable forming limit curve (FLC) analysis method of friction stir welding (FSW) aluminum alloy blank, a new theoretical model was proposed based on the new second order function constitutive model. The main idea is using the differences in mechanical property between the welding and heat affected zone substitution for the hypothesis of geometry groove in the classic M-K theoretical model. The new second order function constitutive model was applied to M-K theoretical model. Eventually, a new FLC theoretical model for FSW aluminum alloy blank was established. Such theoretical model also overcomes the low strain hardening exponent of aluminum alloy material, which leads to a poor regression accuracy by power-exponent function model. The forming limit test for FSW aluminum alloy blank was performed, and the real-time strain was measured by three-dimensional digital speckle strain measurement system (XJTUDIC). Finally, the results of experiments and the theoretical analysis are compared. Compared with the traditional power law, the regression result of the new second order function constitutive model on the stress-strain curve no matter in the initial yield stage or in late deformation stage has a good fitting precision. The maximum fitting error of the power law on the stress-strain curve is more than 12%, but the fitting error of the new second order function constitutive model is less than 1%. The theoretical prediction based on the new second order function constitutive model is significantly better than the theoretical predictions based on power law in predicting the forming limit of FSW aluminum alloy blank. The prediction error of the first principal strain based on the new second order function constitutive model is less than 0.01. While the maximum prediction error of the first principal strain based on the power law is 0.14.

Key words:  aluminum alloy      friction stir welding      forming limit      stress-strain curve      M-K model     
Received:  10 May 2016     
Fund: Supported by National Natural Science Foundation of China (Nos.51405102 and 51475121), China Postdoctoral Science Foundation (No.2015M570286), Fundamental Research Funds for the Central Universities of China (No.HIT.NSRIF.2016093), and the Scientific Research Foundation of Harbin Institute of Technology at Weihai (No.HIT(WH)201414)

Cite this article: 

Guannan CHU,Yanli LIN,Weining SONG,Lin ZHANG. Forming Limit of FSW Aluminum Alloy Blank Based on a New Constitutive Model. Acta Metall Sin, 2017, 53(1): 114-122.

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Fig.1  Classical M-K theory (a) and modified M-K model (b) (A represents the uniform area of the blank, B represents the weak zone of the blank, σ1 is the stress which is perpendicular to the direction of the weld, σ2 is the stress which is parallel to the direction of the weld, tA0 and tB0 are the initial thicknesses of the zones A and B, respectively)
Fig.2  Specific iterative calculation algorithm of the improved M-K model
Fig.3  Aluminum alloy sheet after FSW (FSW—friction stir welding)
Fig.4  Rectangular specimens with and without notches (unit: mm, r—radious of motch, B—minimum width of notch)(a) tensile specimen(b) biaxial specimen
Fig.5  Specimens after experiments (a) and strain distribution (b) (HAZ—heat affected zone)
Fig.6  OIM images of FSW tube after heat treatment(a) welding seam (b) base metal
Zone Point Stress / MPa Strain
Welding Initial yield point 111.7 0.0018
Maximum stress point 194.7 0.2128
Middle point A 166.4 0.0529
Heat Initial yield point 111.7 0.0018
affected Maximum stress point 180.9 0.1193
Middle point A 165.4 0.0397
Table 1  Experimental points selected to do the regression by second order function model
Zone X1 X2 X3
Welding 1.45×10-4 0.57 22.43
Heat affected 2.09×10-4 0.57 72.47
Table 2  Coefficients obtained by second order function model
Fig.7  Experimental stress-strain curves of welding seam (a) and heat affected zone (b) reproduced by different functions
Fig.8  Comparison of theoretical predictions and experimental results
Fig.9  Comparison of theoretical predictions and experimental results
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