Study On Rolling Of Mg Alloy And Mechanism Of Edge Cracks

As one of the lightest metals, Mg alloys are used in many fields, especially transport and mobile equipment. Rolling is the most common process to produce Mg sheet. But it is prone to crack during deformation in particular rolling, for its crystal structure and properties. To investigate the reason for edge crack, the finite element method, damage theory, and lots of experiments including thermal-mechanical test, rolling, XRD, SEM, OM were all used. The characteristic of Mg plastic deformation, especially effects of anisotropy, damage behavior, and factors affecting edge crack were studied systematically. Some ways were found out to predict and eliminate edge crack.Study of Mg deformation and damage behavior indicates, (1) anisotropy is obvious during Mg alloy deformation. Initial texture can impact the shape of samples and the stress- strain behavior. When loading direction is perpendicular to c-axis of HCP lattice, it turns out yielding clearly. Deformation would prefer along c-axis, and that make the section of sample oval. (2) the anisotropy of Mg deformation quite depends on temperature and strain rate. The lower temperature or higher strain rate, the more anisotropic. Even at 400℃anisotropy also exists when the strain rate is very high. (3) a constitutive equationσ= C_ijε)e+σlcos xcosλconsidering crystal orientation was built. (4) initial crystal orientation, temperature, and strain rate play important role on damage. Initial texture can affect crack direction, and make the tensile fracture oval. (5) A damage constitutive equation, 0 maxd C1uexp(RQT)∫εfσε=εrelated with temperature, strain rate, and crystal orientation was established.Based on constitutive of plastic deformation and damage theory, edge crack was studied by means of experiment and finite element method. Results show, (1) initial texture plays important role in Mg sheet rollability. When c-axis of HCP lattice is vertical to ND, the Mg sheet is quality even after a rolling with reduction of 62%, while cracks appear at the edge of sheet with initial basal texture. (2) equation D=-0.124+0.09X-0.008X2 can be used to predict edge crack and study the effects of sheet shape on crack. X=width/thickness, 0<X<6,if D≥0 cracks appear. (3)The finite element model combined with deformation constitutive and damage theory can predict the influence of reduction, temperature and shape on edge crack and simulate rolling with several passes. The result is quite consistent with experiment. (4) Microstructure of Mg sheet and edge crack influences each other. Micro-crack begins at the edge where with coarse grains, and propagates crossing grain. The plastic zone at front of the crack makes grains fine. And there would be a band component of fine grains along the crack.According to the study of plastic deformation and damage of Mg alloy, rolling experiment, and finite element simulation, a rolling map considering temperature, reduction, and pass was built, as well as some experience equation and finite element model. All these can be used to direct rolling of Mg sheet and avoid edge crack. A new ND-TD-ND-TD process was invented, and it can produce quality Mg sheet within few passes and big reduction.The new theories and processes developed in this work can be used to conduct rolling of Mg sheet. It can strongly improve rollabiltiy and production efficiency. As its HCP structure and deformation anisotropy, the damage mechanism of Mg alloy is different from the materials used to study damage before. Therefore, there is still a long way to investigate the damage of Mg alloys.