A method for constructing a magnesium alloy hot working process window

Abstract

The invention discloses a method for constructing a magnesium alloy hot working process window. Firstly, based on the real stress σ and real strain ε data obtained by hot compression test of magnesium alloy, a three-dimensional hot working process diagram and a three-dimensional activation energy diagram considering the influence of deformation temperature T, strain rate #imgabs0# and strain value ε are drawn respectively. By superimposing the hot working process diagram and the activation energy diagram under the same strain value ε, a magnesium alloy hot working process window corresponding to the strain is constructed. The power dissipation coefficient η within the process window is ≥30% and the activation energy value Q is set. act The lower area is the safest area that is most conducive to the complete dynamic recrystallization of the material, and the rheological instability parameter ξ<0 or the activation energy value Q act The higher area is a dangerous area where the material is very likely to produce defects such as local flow and grain coarsening. The present invention can not only effectively avoid instability defects in the hot working process of magnesium alloys, but also obtain a completely dynamically recrystallized microstructure, which is suitable for the optimization of hot working processes of various deformed magnesium alloy materials.

Description

Technical Field

[0001] The invention relates to magnesium alloy hot working process optimization, and in particular to a method for constructing a magnesium alloy hot working process window. Background Art

[0002] Magnesium alloy is currently the lightest metal structural material with the characteristics of low density, high specific strength and specific stiffness. It is widely used in the manufacture of various aerospace and weapons and equipment parts.

[0003] Because magnesium alloys have a close-packed hexagonal crystal structure, their plastic deformation ability at room temperature is extremely poor, and deformed products are generally processed by hot processing.

[0004] However, the thermoplastic deformation behavior of magnesium alloys is extremely sensitive to deformation conditions such as deformation temperature, strain rate and strain amount. Improper selection of deformation parameters can easily cause defects such as local flow, grain coarsening and even cracks inside the workpiece during the hot working process, resulting in the product being unable to meet the use requirements.

[0005] Establishing a hot working process window for magnesium alloys that takes into account the influence of deformation temperature, strain rate and strain, and clarifying the deformation conditions that are conducive to achieving complete dynamic recrystallization without instability defects, are prerequisites for ensuring that the product is defect-free and has an ideal microstructure.

[0006] The hot working map is an effective tool for characterizing the hot working performance of metal materials. It consists of a power dissipation map and a rheological instability map. It can be used to reveal the safe deformation zone corresponding to different microstructural evolution mechanisms and identify the rheological instability zone that needs to be avoided.

[0007] At present, two-dimensional thermal processing maps have been widely used in the optimization of thermal processing technology for materials such as magnesium alloys.

[0008] Patent No. CN201710028472 discloses a new method for optimizing the hot working process of magnesium alloys; the patent superimposes the two-dimensional hot working map and the dynamic recrystallization contour map of the magnesium alloy, and selects the deformation parameters corresponding to the hot working safety zone and the common area where complete dynamic recrystallization occurs as the appropriate hot working process parameters. Since the flow stress of magnesium alloys shows an obvious dynamic recrystallization softening effect as the strain increases, the three-dimensional hot working map containing the strain has gradually been applied.

[0009] The paper "Identification of Formability Parameters and Grain Refinement Research of Hot Forging of Deformed Magnesium Alloys" draws three-dimensional power dissipation diagrams based on different material models and three-dimensional rheological instability diagrams based on different instability criteria, and selects the deformation parameters corresponding to the peak area of ​​power dissipation coefficient without rheological instability as appropriate hot working process parameters.

[0010] The above methods provide the hot working process window of deformed magnesium alloys from different angles. However, the hot working diagram does not take into account the influence of the activation energy of the metal material, which is an important physical quantity that characterizes the energy barrier of dislocation movement and deformation resistance inside the material. The higher the activation energy value, the more difficult it is to deform the material and the worse the hot working performance. This makes the areas with higher power dissipation coefficient in the hot working diagram prone to instability defects. Summary of the invention

[0011] The purpose of the present invention is to overcome the shortcomings and deficiencies of the above-mentioned prior art and provide a method for constructing a magnesium alloy hot working process window. The present invention can not only effectively identify the dangerous area where the material is prone to local flow, grain coarsening and other instability defects, but also obtain a safe area that is conducive to complete dynamic recrystallization of the material.

[0012] The present invention is achieved through the following technical solutions:

[0013] A method for constructing a magnesium alloy hot working process window:

[0014] 1) The magnesium alloy sample was heated to 523-673K at a rate of 10K / s, kept at this temperature for 5-8 minutes to allow the sample to be heated evenly, and then heated at a strain rate of 10K / s at the same deformation temperature T. The sample was isothermally compressed to a true strain ε=1.2, and the true stress σ data and true strain ε data of the magnesium alloy corresponding to different deformation temperatures and strain rates were obtained, and the σ-ε curve was obtained.

[0015] 2) According to the σ-ε curve, the true stress σ and the logarithm of the strain rate at a given deformation temperature are calculated using the cubic spline difference method. The relationship between is fitted by polynomial, and the expression is as follows

[0016]

[0017] In formula (1), a, b, c and d are the constant term, the coefficient of the first-order term, the coefficient of the second-order term and the coefficient of the third-order term respectively.

[0018] 3) According to the σ-ε curve, true stress σ and strain rate logarithm The relationship expression between them is determined in turn:

[0019] Strain rate sensitivity index Power dissipation factor And the rheological instability parameter ξ=2m-η.

[0020] At deformation temperature T, the logarithm of strain rate In the three-dimensional coordinate system composed of and strain ε, a three-dimensional power dissipation diagram and a three-dimensional rheological instability diagram are drawn respectively, and the three-dimensional power dissipation diagram and the three-dimensional rheological instability diagram are superimposed to draw a three-dimensional hot processing diagram of the magnesium alloy;

[0021] 4) Based on the σ-ε curve, describe the true stress σ of magnesium alloy and the deformation temperature T and strain rate Arrhenius high temperature constitutive model based on the relation Determine the activation energy value Q act Then at deformation temperature T, strain rate logarithm In the three-dimensional coordinate system composed of and strain variable ε, the three-dimensional activation energy diagram of magnesium alloy is drawn.

[0022] In the Arrhenius high temperature constitutive model: A and α are material parameters; n is the stress exponent; and R is the ideal gas constant.

[0023] 5) Based on the three-dimensional hot working diagram and the three-dimensional activation energy diagram, the hot working diagram and the activation energy diagram under the same strain value ε are superimposed to construct a hot working process window of the magnesium alloy corresponding to the strain.

[0024] The present invention uses the rheological instability parameter ξ<0 or the activation energy value Q in the hot processing process window. act The deformation temperature and strain rate corresponding to the higher region are the deformation parameters where the material is very likely to have instability defects;

[0025] The present invention is within the thermal processing window, the power dissipation coefficient η ≥ 30% and the activation energy value Q act The deformation temperature and strain rate corresponding to the lower region are the deformation parameters most conducive to complete dynamic recrystallization of the material.

[0026] The magnesium alloy sample of the present invention is preferably Mg-5.42Zn-0.58Zr deformable magnesium alloy, and the size is

[0027] Compared with the prior art, the present invention has the following advantages and effects:

[0028] 1. Based on the real stress σ and real strain ε data obtained from the hot compression test, the present invention plots the deformation temperature T and strain rate. The three-dimensional hot working diagram and the three-dimensional activation energy diagram under the condition of the same strain variable ε are superimposed to construct the hot working process window of the corresponding strain. Processing within the determined process window can not only effectively avoid the occurrence of instability defects such as local flow and grain coarsening, but also obtain a completely dynamically recrystallized microstructure, which greatly improves the performance quality of the product.

[0029] 2. The method of the present invention is applicable to the optimization of various deformation magnesium alloy hot working processes, effectively avoiding product scrapping due to improper selection of deformation parameters, saving product development time and reducing development costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 σ-ε curves under different deformation conditions, including (a) 523K; (b) 573K; (c) 623K; (d) 673K.

[0031] Figure 2 It is the three-dimensional power dissipation diagram with strain ε=0.2~1.2.

[0032] Figure 3 It is the three-dimensional rheological instability diagram for strain ε=0.2~1.2.

[0033] Figure 4 It is a three-dimensional thermal processing diagram with strain ε=0.2~1.2.

[0034] Figure 5 It is the three-dimensional activation energy diagram for strain ε=0.2~1.2.

[0035] Figure 6 This is the hot working process window diagram when the strain variable ε=1.2. DETAILED DESCRIPTION

[0036] The present invention is further described in detail below in conjunction with specific embodiments.

[0037] The present invention discloses a method for constructing a magnesium alloy hot working process window, and the specific implementation steps are as follows:

[0038] 1) The magnesium alloy sample was heated to 523-673K at a rate of 10K / s, kept at this temperature for 5-8 minutes to allow the sample to be heated evenly, and then the sample was heated at a strain rate of 10K / s at the same deformation temperature. The sample was isothermally compressed to a true strain of ε = 1.2, and different deformation temperatures T and strain rates were obtained. The corresponding real stress σ data and real strain ε data of magnesium alloy are obtained as follows Figure 1 The σ-ε curve shown.

[0039] The magnesium alloy specimen is Mg-5.42Zn-0.58Zr deformed magnesium alloy with a size of

[0040] 2) According to the σ-ε curve, in the case Figure 1 Under the selected strain conditions shown in the figure, the cubic spline difference method is used to calculate the true stress σ and the logarithm of the strain rate at a certain deformation temperature. The relationship between is fitted by polynomial, and the expression is as follows

[0041]

[0042] In formula (1), a, b, c and d are the constant term, the coefficient of the first-order term, the coefficient of the second-order term and the coefficient of the third-order term respectively.

[0043] 3) According to the σ-ε curve, true stress σ and strain rate logarithm The relationship expression between them is determined in turn:

[0044] Strain rate sensitivity index Power dissipation factor and rheological instability parameter ξ=2m-η;

[0045] At deformation temperature T, the logarithm of strain rate In the three-dimensional coordinate system composed of and strain variable ε, we draw Figure 2 The three-dimensional power dissipation diagram shown and Figure 3 The three-dimensional rheological instability diagram shown in the figure is superimposed with the three-dimensional power dissipation diagram and the three-dimensional rheological instability diagram to obtain the following Figure 4 Three-dimensional hot working diagram of magnesium alloy shown.

[0046] 4) Based on the σ-ε curve, describe the true stress σ of magnesium alloy and the deformation temperature T and strain rate Arrhenius high temperature constitutive model based on the relation Determine the activation energy value Q act Then at deformation temperature T, strain rate logarithm In the three-dimensional coordinate system composed of and strain variable ε, the following is drawn: Figure 5 Three-dimensional activation energy diagram of the magnesium alloy shown.

[0047] In the Arrhenius high temperature constitutive model: A and α are material parameters; n is the stress exponent; and R is the ideal gas constant.

[0048] 5) Based on the three-dimensional thermal processing diagram and the three-dimensional activation energy diagram, the thermal processing diagram and the activation energy diagram under the condition of strain ε=1.2 are superimposed to construct the following Figure 6 The hot working process window of magnesium alloy corresponding to the strain is shown. The white line represents the contour line when the rheological instability parameter ξ=0, and the rheological instability parameter ξ<0 in the enclosed area; the black line represents the contour line of the power dissipation coefficient η; different colors represent different activation energy values.

[0049] The present invention is based on the fact that within the thermal processing window, the rheological instability parameter ξ < 0 or the activation energy value Q actThe deformation temperature and strain rate corresponding to the higher regions A and B are deformation parameters where the material is prone to instability defects. Among them, region A is concentrated at 523-526K, 0.5-1s -1 ; Region B is concentrated at 590~673K, 0.12~1s -1 .

[0050] The present invention is based on the thermal processing process window, the power dissipation coefficient η ≥ 30% and the activation energy value Q act The deformation temperature and strain rate corresponding to the lower regions C and D are the deformation parameters that are most conducive to complete dynamic recrystallization of the material. Among them, region C is concentrated at 550-585K, 0.001-0.004s -1 ; Region D is concentrated at 660~673K, 0.001~0.008s -1 .

[0051] As described above, the present invention can be better implemented.

[0052] The implementation methods of the present invention are not limited to the above-mentioned embodiments, and any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principles of the present invention shall be equivalent replacement methods and shall be included in the protection scope of the present invention.

Claims

1. A method for constructing a magnesium alloy hot working process window, characterized in that The following steps are involved: Step 1: Heat the magnesium alloy sample to 523-673K at a rate of 10K / s and keep the temperature to ensure uniform heating of the sample; Then, under the same deformation temperature T, the strain rate The sample was isothermally compressed to a true strain of ε = 1.2, and different deformation temperatures T and strain rates were obtained. The corresponding real stress σ data and real strain ε data of magnesium alloy are used to obtain the σ-ε curve; Step 2: According to the σ-ε curve, the true stress σ and the logarithm of the strain rate at a given deformation temperature are calculated using the cubic spline difference method. The relationship between them is fitted by polynomial, and the expression is as follows In formula (1), a, b, c and d are the constant term, the coefficient of the first-order term, the coefficient of the second-order term and the coefficient of the third-order term respectively; Step 3: According to the σ-ε curve, true stress σ and strain rate logarithm The relationship expression between them is determined in turn: Strain rate sensitivity index Power dissipation factor and rheological instability parameter ξ=2m-η; At deformation temperature T, the logarithm of strain rate In the three-dimensional coordinate system composed of and strain ε, a three-dimensional power dissipation diagram and a three-dimensional rheological instability diagram are drawn respectively, and the three-dimensional power dissipation diagram and the three-dimensional rheological instability diagram are superimposed to draw a three-dimensional hot processing diagram of the magnesium alloy; Step 4: Describe the true stress σ of the magnesium alloy, deformation temperature T, and strain rate according to the σ-ε curve Arrhenius high temperature constitutive model based on the relation Determine the activation energy value Q act ; Then at deformation temperature T, strain rate logarithm In the three-dimensional coordinate system composed of and strain ε, a three-dimensional activation energy diagram of magnesium alloy is drawn; In the Arrhenius high temperature constitutive model: A and α are material parameters; n is the stress index; R is the ideal gas constant; Step 5: According to the three-dimensional hot working diagram and the three-dimensional activation energy diagram, the hot working diagram and the activation energy diagram under the same strain value ε are superimposed to construct a hot working process window of the magnesium alloy corresponding to the strain.

2. The method for constructing a magnesium alloy hot working process window according to claim 1, characterized in that: In step 1, the magnesium alloy sample is Mg-5.42Zn-0.58Zr deformable magnesium alloy.

3. The method for constructing a magnesium alloy hot working process window according to claim 1, characterized in that: In step 1, the insulation time is 5-8 minutes.

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