Shear Damage Suppression Model of Magnesium Alloy Plates

: By shearing Q235 steel, aluminum, and AZ31 magnesium alloy at room temperature, the shear area of Q235 steel and aluminum is found to be relatively flat whereas that of AZ31 magnesium alloy exhibits many defects, such as potholes and cracks. The influence of temperature and strain rate on the critical fracture strain of AZ31 magnesium alloy was obtained using isothermal compression experiment. Results show that high temperature and larger strain lead to large and small critical fracture strains. Therefore, based on the isothermal compression experiment and the effects of temperature and strain rate on the critical fracture strain of AZ31 magnesium alloy, the magnesium alloy plate is heated to 100, 200, 300, and 400 °C, and shearing was conducted after 30 min of heat preservation. Based on the cross-sectional shape and the degree of damage, the optimum shear temperature ranges from 160 °C to 260 °C. At this temperature, the sheared magnesium alloy plate not only obtains an improved cross-sectional shape but also has a small shear corner area. Simultaneously, the shearing basic process model of Q235 steel plate is also obtained based on the industrial test. Furthermore, the shearing basic process model of AZ31 magnesium alloy was acquired based on the elongation ratio of magnesium alloy and Q235 steel under the same process conditions.


Experimental and establishment of shearing process model 2.1 Shear experimental
At room temperature, shear the Q235 steel, aluminum, and as-cast AZ31 magnesium alloy plate with a thickness of 5mm using the disc shear with a small shear deformation zone, and observe the shape and damage of the shear section by scanning electron microscope. The differences between shear deformation of magnesium alloy and other metal materials were analyzed. The as-cast AZ31 magnesium alloy was cut into cylinders with a diameter of 7 mm and a height of 7 mm. At 250, 300, 350, and 400 °C, the critical fracture strains were obtained by using Gleeble-3800 thermal simulator at strain rates of 0.005, 0.05, and 0.5.The AZ31 magnesium alloy plate was heated to 100, 200, 300, and 400 °C by vacuum heating furnace for 30 min, and the magnesium alloy plate was hot-sheared by disc shear. The ZEISS scanning electron microscope was used to observe the appearance of fractures, the shear experimental shown in Fig. 1. Simultaneously, the ultrafine field microscope is used to observe the range of the corner area of the shear section and obtain optimum shear temperature of AZ31 magnesium alloy, providing the theoretical basis for the shear of magnesium alloy plates.  As-cast AZ31 magnesium alloy

Isothermal compression experiment
The shear deformation ability is mainly related to the plasticity of the material, and the elongation is one of the indicators that reflect the plasticity of the material. The shearing basic process model of Q235 steel plate is also obtained based on the industrial test. Furthermore, combined with the change law of magnesium alloy shear damage with temperature, the shearing basic process model of AZ31 magnesium alloy was acquired based on the elongation ratio of magnesium alloy and Q235 steel under the same process conditions.

Sheared profiles at room temperature
The Q235 steel, 5052 aluminum, and as-cast AZ31 magnesium alloy were sheared at room temperature, and their sheared profiles are shown in Fig. 2.
The sheared profiles of Q235 steel and aluminum comprise corner, smooth, and tearing areas. Q235 steel has smaller corner and smooth areas, and larger tearing areas than those of the aluminum. However, the damage degree of the tear area is evidently smaller than that of aluminum. Given the high hardness of the Q235 steel, the deformation range on both sides of the deformation area is small, the deformation resistance of the plate end is strong, and the corner area is small during shear deformation. Tearing occurs when the shear depth is small due to the large deformation resistance. Given the low hardness of aluminum, the metal on both sides of the deformation area also deforms in the event of shear deformation; thus, the corner area is large. The tearing phenomenon of the profiles will only occur when the shear deformation is deep due to its weak deformation resistance. Large-scale cracks occur in AZ31 magnesium alloy during shear deformation at room temperature due to its poor plastic deformation capability. The fracture mainly demonstrates brittles when subjected to shear force. The brittle fracture of the material should have an improved cross-sectional morphology, but the fracture between crystals of magnesium alloy material at low temperatures cannot easily occur. When subjected to the shearing force, some of the small crystals in the matrix are extracted from the matrix of the material, resulting in many cracks and pothole defects. Moreover, the internal viscosity of the material is reduced at low temperature; the damage cannot be weakened by the metal flow after the damage; therefore, the shear damage is serious at room temperature.

Critical fracture strain of AZ31 magnesium alloy
At 250, 300, 350, and 400 °C, the critical fracture strains of AZ31 magnesium alloy at strain rates of 0.005, 0.05, and 0.5 were obtained by isothermal compression, as shown in Table 1. The critical fracture strain of the material increases with the temperature. The critical fracture strain is only reflected after the brittle deformation of materials because their plastic deformation process does not affect the critical fracture strain. When the temperature is raised from 300 °C to 350 °C, the increase in critical fracture strain is the largest; the minimum and maximum increases are 0.09 and 0.12, respectively. When the temperature rises from 250 °C to 300 °C and 350 °C to 400 °C, the increase in critical fracture strain is small; the maximum and minimum increases are 0.05 and 0.02, respectively. When the isothermal deformation temperature of cracks corner area corner area magnesium alloy is increased from 300 °C to 350 °C, the plastic deformation capability of the material is considerably improved; thus, the critical fracture strain mostly changes during this temperature range. When the temperature is lower or higher than this temperature, the temperature change has slight effects on the plastic deformation of the material. Therefore, the changes in critical fracture strain are small. The strain rate also has an evident effect on the critical fracture strain of AZ31 magnesium alloy. Overall, with the increase in strain rate, the critical fracture strain is small, but the specific range of change is also related to the deformation temperature. When the strain rate is 0.005, 0.05, and 0.5 isothermal compressions, the changes in critical fracture strain at 250 and 400 °C are small and those at 300 and 350 °C demonstrate considerable changes. This phenomenon shows that the strain rate has slight effects on the critical fracture strain at low and high temperatures whereas that at medium and high temperatures has a considerable influence on the critical fracture strain.

Sheared profiles of AZ31 magnesium alloy
The shear corner area of AZ31 magnesium alloy at 100, 200, 300, 400 °C is shown in Fig. 3. deformation capability of magnesium alloy is substantially improved due to the high temperature change of magnesium alloy, and the height of the shear corner area evidently increases with the temperature. When the recrystallization temperature is approximately 250°C, the plastic deformation capability considerably changes. Therefore, the height of the corner area near this temperature shows remarkable changes.
The width of the corner area increases when the temperature is 100 °C to 200 °C and decreases first and then increases when the temperature is 200 °C to 400 °C. When the temperature is lower than the recrystallization temperature, the effect of temperature change on the plastic change of plate metal is unclear. Therefore, the deformation resistance of the plate metal is large when subjected to shear force, leading to the large width of the corner area of the plate. When the temperature rises above the recrystallization temperature, the plastic deformation capability is fully improved, and the deformation resistance of the plate is reduced. During shear deformation, the main deformation direction is plate thickness direction, which leads to a decrease in the width of corner area.
The damage of shear deformation profiles of magnesium alloy at 100, 200, 300, and 400 °C is shown in Fig. 4.
Compared with the sheared profiles at room temperature, the shear of magnesium alloy plate can obtain improved cross-sectional morphology after heating. Moreover, no large-scale cracks occurred, and some micro-cracks, potholes and bubble-like bulges were observed. The magnesium alloy quickly dissipates heat due to the removal of the plate metal from the heating furnace to the complete shearing time. Therefore, the temperature described herein is the tapping temperature, and the actual shear temperature is approximately 40 °C to 60 °C lower than the tapping temperature.

Basic model of precise shearing
When the magnesium plate is directly sheared at room temperature, brittle fracture may occur after the shear force of the plate shearing machine. Numerous secondary crack damage defects also appear in the sheared profiles. The deformation does not easily occur due to the special crystal structure of the magnesium alloy plate, and the fracture phenomenon easily breaks when the magnesium alloy plate is subjected to large applied shearing stress. Moreover, the poor deformability of the potholes cracks plate can lead to secondary damage during the shearing fracture process. This phenomenon can cause numerous defects in the shear profiles of the plate.
The quality of the shear profiles can be considerably improved by heating the magnesium alloy plate before shearing.
Obtaining improved shear profiles is easy with the increase in temperature. The sheared profile quality is the best when the temperature is between 160 °C to 260 °C due to the absence of crack defects in the sheared profiles. The worse deformation capability of magnesium alloy is observed at low temperatures. The occurrence of inter-crystal fracture is difficult, and the brittle fracture is the main fracture situation. When the temperature is high, the internal viscosity of the plate increases and the shearing profile quality is poor. Therefore, the sheared profile quality is best obtained after the temperature is between 160 °C to 260 °C.
1)The mathematical model of shearing edge gap and steel plate thickness is as follows. Table 2 shows the optimized disc-shearing process parameters to track the accumulation of production practice for half a year in a medium-sized plate factory of a steel company.

Tab 2 Measured disc shearing process parameters(Q235)
Steel plate thickness(mm) 10  The multinomial curve fitting is conducted by using MATLAB software based on the measured data to achieve the rapid initial setting of disc shearing process parameters when the metal specifications of different materials are changed.
The analysis results show that the primary function is the most ideal, and the mathematical model of disc shear overlap corresponding to the aforementioned measured data is: , where S is overlap, and h is thickness of the plate.
The same method is applied to obtain the mathematical model of the lateral shear gap of disc shearing edge corresponding to the above measured data results in the following: , where is the lateral gap of shearing edge, and h is thickness of the plate.
2)The mathematical model of shearing edge gap and cumulative shear is as follows.
A new shearing edge will wear with the increase in cumulative shear amount. When shearing a steel plate of the same thickness, gradually reducing the shearing edge gap is necessary. Suppose a new shearing edge can shear the steel plate with the cumulative area of mm 2 as the amount to be consumed. The total area of the sheared profiles is A, and can be regarded as the residual shearing capacity coefficient. Therefore, the mathematical model between the optimal shearing edge gap and the shear profile area A after the accumulation of a certain amount of shear profile area is as follows: , where is the best clearance for the new shearing edge.
A and are determined by tracking records in production practice for specific equipment, and is a constant.
Taking the average value according to practice records, the GAP of any time can be calculated.
3)The mathematical model of shearing edge gap and shear material properties is as follows: During the metal shearing, the metal near the point of action of the shear force will slip with the increase in shear force.
When the plastic deformation of the metal on the upper and lower surfaces of the shearing edge reaches the limit-fracture strain, cracks will appear at the point of shear force. With the further approach of the upper and lower tool holder, cracks will also merge in the internal metal fiber. The fracture crack will propagate along the line between the upper and lower shear force points in the adjacent metal, reaching the fracture strain value. When the two broken lines meet, the shear undergoes fracture.
Therefore, the plasticity of shear material is mainly exhibited in the shear process. The two indexes reflect the plasticity of the material elongation and area reduction, which can be transformed into each other. The mathematical model of shearing edge gap and shear material properties can be expressed as follows: , Where k 1 is ratio of the elongation of the metal to be sheared to Q235.
4)The mathematical model of shearing edge gap and shear temperature is as follows.
When the steel plate is sheared, the temperature increases, the strength reduces, and shear defects, such as shear deformation and glitches, are likely to occur. When the temperature is high to affect shear quality, the shearing edge gap can be adjusted properly. (1), (1), (2): : adjustment model of the overlap optimal shearing edge; : adjustment model of the optimal shear edge lateral gap.

4.Conclusions
The following conclusions can be drawn from the present study: (1) When the magnesium plate is sheared directly at room temperature, the brittle fracture easily occurs after the shearing force of the plate, and many secondary crack damage defects appear in the sheared profiles. The critical fracture strain of AZ31 magnesium alloy plate increases with the temperature in the range of 250 °C to 400 °C.
The critical fracture factor decreases with the increase in strain rate in the range of 0.005 to 0.5, and the temperature range of maximum variation is in the range of 300 °C to 350 °C.
(2) The quality of the shear profiles can be considerably improved by heating the magnesium alloy plate before shearing. With the increase in temperature, obtaining improved shear profiles is easy. When the temperature is between 160 °C to 260 °C, the quality of the shear profiles is the best, and no crack defects are observed in the shear profiles.
(3) Based on the above experimental results and industrial tests, a basic shearing process model for magnesium alloy plates was established: (1), (1), (2): : adjustment model of the overlap optimal shearing edge; : adjustment model of the optimal shear edge lateral gap.