Forming limits of an age hardenable aluminum sheet after pre-straining and annealing

Abstract A two-stage stamping with an intermediate annealing method was developed to improve formability of age hardenable aluminum alloys, which are more challenging than the non-age hardenable aluminum alloys because of the effects that intermediate annealing will have on dissolution of solutes, precipitation of strengthening particles, and the subsequent mechanical behavior of the materials. The process could readily be applied locally to highly strained areas of a complex panel for 6000 series aluminum alloys in order to form a good part without fracture; however, there will be a decrease of precipitation strengthening of those locally treated, highly strained areas. In this study, the strain-based forming limit curves (FLC) of as-received AAx610-T4PD alloy sheets were shown to increase after pre-straining to ~15% and annealing at 425 °C for 10 s. However, since strain-based FLCs are very sensitive to the pre-strain history, both stress-based FLCs and polar-effective-plastic-strain (PEPS) FLCs, which are path-independent, were used to evaluate the forming limits after pre-straining and annealing. Due to the change of mechanical behavior during annealing, this paper describes the procedure to calculate the stress-based FLC in which a residual-effective-plastic-strain (REPS) is determined by overlapping the hardening curve of the pre-strained and annealed material with that of material heat treated without pre-strain. After converting the strain-based FLCs using the constant REPS method, it was found that the stress-based FLCs and the PEPS FLCs of the post-annealed materials are quite similar and both tools are applicable for evaluating the forming limits of Al–Mg–Si alloys for the two-step stamping process with intermediate annealing heat treatment.

[1]  Philip Eyckens,et al.  Marciniak–Kuczynski type modelling of the effect of Through-Thickness Shear on the forming limits of sheet metal , 2009 .

[2]  K. Inal,et al.  The effects of anisotropic yield functions and their material parameters on prediction of forming limit diagrams , 2012 .

[3]  R. Logé,et al.  An anisotropic stress based criterion to predict the formability and the fracture mechanism of textured zinc sheets , 2013 .

[4]  Albert Van Bael,et al.  Prediction of forming limit strains under strain-path changes: Application of an anisotropic model based on texture and dislocation structure , 1998 .

[5]  S. Hu,et al.  The effect of prestrain and subsequent annealing on the mechanical behavior of AA5182-O , 2011 .

[6]  Thomas B. Stoughton,et al.  A general forming limit criterion for sheet metal forming , 2000 .

[7]  Jingjing Li,et al.  Structure–property characterization of an age hardenable Al–Mg–Si alloy after straining and flash annealing , 2014 .

[8]  Thomas B. Stoughton,et al.  A non-associated flow rule for sheet metal forming , 2002 .

[9]  William F. Hosford,et al.  The influence of strain-path changes on forming limit diagrams of A1 6111 T4 , 1994 .

[10]  Postanneal Mechanical Properties of Prestrained AA5182-O Sheets , 2011 .

[11]  P. Zavattieri,et al.  Tips and tricks for characterizing shape memory wire part 5: Full-field strain measurement by digital image correlation , 2013, Experimental Techniques.

[12]  Jian Cao,et al.  Prediction of forming limit curves using an anisotropic yield function with prestrain induced backstress , 2002 .

[13]  F. J. Humphreys,et al.  Recrystallization and Related Annealing Phenomena , 1995 .

[14]  S. R. MacEwen,et al.  Hardening laws, surface roughness and biaxial tensile limit strains of sheet aluminium alloys , 1996 .

[15]  W. Hosford,et al.  Effect of changing strain paths on , 1993, Metallurgical and Materials Transactions A.

[16]  A. Barata da Rocha,et al.  An experimental and theoretical analysis on the application of stress-based forming limit criterion , 2006 .

[17]  Jyhwen Wang,et al.  MODELING THE LOCALIZED NECKING IN ANISOTROPIC SHEET METALS , 2012 .

[18]  A class of plastic constitutive equations with vertex effect—IV. Applications to prediction of forming limit strains of metal sheets under nonproportional loadings , 1985 .

[19]  R. Hill.  A theory of the yielding and plastic flow of anisotropic metals , 1948, Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences.

[20]  John E. Carsley,et al.  Forming limit analysis for two-stage forming of 5182-O aluminum sheet with intermediate annealing , 2013 .

[21]  J. Signorelli,et al.  Study of limit strains for FCC and BCC sheet metal using polycrystal plasticity , 2011 .

[22]  Jeong Whan Yoon,et al.  Path independent forming limits in strain and stress spaces , 2012 .

[23]  Thomas B. Stoughton,et al.  Review of theoretical models of the strain-based FLD and their relevance to the stress-based FLD , 2004 .

[24]  A. Barata da Rocha,et al.  A theoretical study on forming limit diagrams prediction , 2003 .

[25]  M. E. Scholting,et al.  A new method for predicting Forming Limit Curves from mechanical properties , 2013 .

[26]  Manabu Gotoh.  A class of plastic constitutive equation with vertex effect—III. Applications to calculation of FLD of metal sheets , 1985 .

[27]  Gorton M. Goodwin,et al.  Application of Strain Analysis to Sheet Metal Forming Problems in the Press Shop , 1968 .

[28]  Philip Eyckens,et al.  An extended Marciniak–Kuczynski model for anisotropic sheet subjected to monotonic strain paths with through-thickness shear , 2011 .

[29]  Jeong Whan Yoon,et al.  Anisotropic hardening and non-associated flow in proportional loading of sheet metals , 2009 .

[30]  Mitsutoshi Kuroda,et al.  Path-dependence of the forming limit stresses in a sheet metal , 2007 .