A method for preparing Mg-Zn high-strength alloy based on multi-scale nano-precipitation phase strengthening

Abstract

The invention belongs to the technical field of magnesium alloy processing, and specifically relates to a method for preparing a Mg-Zn series high-strength alloy based on multi-scale nano-precipitation phase strengthening. First, Ca, Al and Mn elements are added to the Mg-Zn binary alloy during smelting to perform microalloying modification treatment, and then a homogeneous magnesium alloy test rod is obtained by casting, solidification and homogenization treatment in sequence; the magnesium alloy test rod is subjected to positive extrusion deformation, with an extrusion ratio of 25 / 1, an extrusion speed of 0.4 mm / s, an extrusion temperature of 220°C and a pressing angle of 90°; the invention can obtain multi-type and multi-scale nano-precipitation phases without subsequent heat treatment, so that nano-precipitation phases with smaller sizes are precipitated inside the grains, and precipitation strengthening is provided by suppressing dislocation movement; nano-phases with larger sizes are precipitated on the grain boundaries, and fine grain strengthening is provided by suppressing grain boundary migration, thereby preparing a high-strength novel Mg-Zn series alloy.

Description

Technical Field

[0001] The invention belongs to the technical field of magnesium alloy processing, and relates to a method for preparing a Mg-Zn series high-strength alloy based on multi-scale nano-precipitation phase strengthening. Background Art

[0002] Mg-Zn alloys are one of the most widely used magnesium alloys in commercial applications, and have broad application prospects in the fields of automobiles, aerospace, 3C products, etc. However, the strength of existing Mg-Zn alloys is difficult to meet the requirements of high-strength magnesium alloys. In particular, due to the coarse grain size, the tensile strength of Mg-Zn binary alloys is usually less than 100MPa. Alloying can refine the organization and improve the structural parameters of the precipitated phase (type, content, morphology, scale, etc.). As a low-cost and high-yield method, it has been greatly used in the development of high-strength magnesium alloys. At present, the development of low-cost, high-strength new Mg-Zn alloys has become one of the hot spots in the research field of high-strength magnesium alloys.

[0003] An obvious advantage of Mg-Zn alloys is that nano-precipitates can be obtained through plastic deformation to refine the alloy structure and improve the alloy strength. Usually, the nano-precipitates of Mg-Zn alloys are mostly needle-shaped, rod-shaped or plate-shaped, and the distribution is uneven, so it is difficult to exert the best strengthening effect. Therefore, how to obtain uniformly distributed nano-precipitates and effectively suppress dislocation movement by regulating their type, content, morphology, scale, etc. is a breakthrough point for the preparation of high-strength Mg-Zn alloys. In addition, compared with single-type and single-scale precipitation, multi-type and multi-scale precipitation phases have better strengthening effects. For example, the Chinese invention patent (CN117363955A) discloses a method for preparing a heat-resistant alloy with synergistic strengthening of multi-type precipitation phases, including electroslag remelting, homogenization treatment, forging, high-temperature solution treatment and aging treatment. The Chinese invention patent (CN116516225A) discloses a method for preparing a multi-phase strengthened heat-resistant magnesium alloy, which requires the addition of at least 5 elements and 3 to 5 equal channel angular extrusions, solution treatment and aging treatment. For example, the Chinese invention patent (CN109868380A) discloses a method for preparing a multi-scale precipitation strengthened magnesium alloy by mixing heterogeneous debris, 1 to 20 reciprocating extrusions or equal channel angular extrusions, solution treatment and aging treatment. The Chinese invention patent (CN111408623A) discloses a method and system for preparing multi-scale precipitated nano-heterogeneous magnesium alloy sheets. The method prepares high-strength heterogeneous magnesium alloys through three deformation processes of rolling, rolling and hot rolling and subsequent solution treatment and aging treatment. It can be seen that the design of multi-type and multi-scale precipitation phases can achieve the preparation of high-strength magnesium alloys.

[0004] From the current research status of magnesium alloys, precipitation strengthening is still one of the most important strengthening methods. For example, the Chinese invention patent (CN113755730A) precipitated high-density Mg in Mg-Al-Ce-(Nd) alloy through reverse extrusion deformation. 17 Al 12 , Al2Ce and Mg 12 Nd nanophase has achieved significant precipitation strengthening effect. The Chinese invention patent (CN114395667A) prepared a high-strength and corrosion-resistant magnesium alloy based on coherent precipitation phase regulation through extrusion deformation and subsequent solid solution and aging heat treatment. However, the current preparation technology requires multiple hot processing, special mold structure, and specialized machinery and equipment, which is difficult to use for industrial applications. Summary of the invention

[0005] The present invention overcomes the shortcomings of the prior art and proposes a method for preparing a Mg-Zn series high-strength alloy based on multi-scale nano-precipitation phase strengthening. Compared with the above-mentioned prior art, the present invention performs Ca, Al, and Mn microalloying on the Mg-Zn alloy, adopts positive extrusion deformation with greater deformation stress, and does not require subsequent heat treatment, so that multi-type and multi-scale nano-precipitation phases can be obtained, so that nano-precipitation phases with smaller sizes are precipitated inside the grains, and precipitation strengthening is provided by suppressing dislocation movement; nano-phases with larger sizes are precipitated on the grain boundaries, and fine grain strengthening is provided by suppressing grain boundary migration, thereby preparing a high-strength new Mg-Zn series alloy, in order to expand the practical application of Mg-Zn series alloys.

[0006] The present invention is achieved through the following technical solutions:

[0007] A method for preparing a Mg-Zn high-strength alloy based on multi-scale nano-precipitation phase strengthening comprises the following steps:

[0008] S1. Microalloying and modification of Mg-Zn binary alloy: adding Ca, Al and Mn elements during smelting of Mg-Zn binary alloy, wherein Zn: 0.8~1.2wt.%, Ca: 0.05~0.15wt.%, Al: 0.05~0.15wt.%, Mn: 0.05~0.15wt.%, and the rest is Mg;

[0009] S2, the smelted alloy is sequentially subjected to casting, solidification and homogenization treatment to obtain a homogeneous magnesium alloy test bar;

[0010] S3. The magnesium alloy test bar is subjected to positive extrusion deformation with an extrusion ratio of 25 / 1, an extrusion speed of 0.4 mm / s, an extrusion temperature of 220°C, and a pressurizing angle of 90°.

[0011] Preferably, the raw materials used for alloy smelting are: a magnesium block with a purity of 99.99wt.%, a zinc block with a purity of 99.99wt.%, a Mg-20Ca intermediate alloy block, an aluminum block with a purity of 99.99wt.%, and a manganese particle with a purity of 99.99wt.%.

[0012] Preferably, the alloy is smelted at 400°C, the preheated magnesium block and the Mg-20Ca intermediate alloy block are placed in a crucible and completely melted; then the preheated zinc block is added, and when the temperature rises to 750°C, the preheated aluminum block is added, and then the preheated manganese particles are added, and after keeping at 750°C for 20 to 25 minutes, the temperature is lowered to 730°C, the slag is skimmed, and a refining agent is sprinkled into the magnesium melt for refining.

[0013] Preferably, the casting and solidification are carried out by keeping the temperature at 750°C for 25 to 35 minutes, then lowering the furnace temperature to 720°C, removing the slag, and casting the magnesium melt into a preheated metal mold; after the alloy melt is completely solidified in the air and naturally cools to room temperature as the mold temperature, the sample is taken out of the mold.

[0014] Preferably, the homogenization treatment is to first set the temperature to 350° C. and homogenize for 2 hours; then increase the temperature to 450° C. and homogenize for another 2 hours.

[0015] More preferably, after the homogenization treatment is completed, the magnesium alloy test bar is subjected to water quenching treatment to obtain a homogeneous magnesium alloy test bar.

[0016] Preferably, the forward extrusion deformation adopts a vertical extruder, and the extrusion die includes an extrusion die, an extrusion gasket, an extrusion rod and an extrusion sleeve.

[0017] Preferably, the magnesium alloy test bar is water quenched after positive extrusion deformation.

[0018] The present invention aims at the problem that a single precipitate phase and a single scale precipitate phase have poor precipitation strengthening effect on magnesium alloys, performs Ca, Al, and Mn microalloying modification on Mg-Zn binary alloy, and obtains uniformly distributed multi-type and multi-scale nano-precipitates through positive extrusion deformation at a specific extrusion angle and extrusion temperature, only through one extrusion deformation and without subsequent heat treatment, including Ca2Mg6Zn3 phase (200~600nm) on the grain boundary, Al8Mn5 phase (20~50nm) on the grain boundary, and Al8Mn5 phase particles (3~8nm) inside the grain, thereby preparing a new high-strength Mg-1Zn-0.1Ca-0.1Al-0.1Mn magnesium alloy. At the same time, the present invention studies the influence of extrusion angle and extrusion temperature on recrystallization behavior by setting extrusion comparison tests with different extrusion angles and extrusion temperatures.

[0019] The beneficial effects of the present invention compared with the prior art are as follows:

[0020] (1) The present invention can obtain a fine recrystallized structure under the specific conditions of an extrusion temperature of 220°C and an extrusion angle of 90°.

[0021] (2) Under the specific conditions of an extrusion temperature of 220°C and an extrusion angle of 90°, the present invention can precipitate a large amount of uniformly distributed multi-type and multi-scale nano-precipitate phases through only one extrusion deformation and without subsequent heat treatment.

[0022] (3) In the present invention, a new high-strength Mg-Zn magnesium alloy is prepared under the specific conditions of an extrusion temperature of 220°C and an extrusion angle of 90° through fine grain strengthening of fine recrystallization and precipitation strengthening of multi-type and multi-scale nano-precipitates.

[0023] (4) The new high-strength Mg-Zn magnesium alloy prepared by the present invention has low cost, simple process, simple mold structure, and does not require special machinery and equipment, and can be used in industrial applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of thermal processing of cast Mg-1Zn-0.1Ca-0.1Al-0.1Mn alloy in Example 1 and Comparative Example 1.

[0025] Figure 2 Schematic diagram of the extrusion die with two extrusion angles of 30° and 90° in Example 1 and Comparative Example 1.

[0026] Figure 3 Mg-1Zn-0.1Ca-0.1Al-0.1Mn alloy test bars obtained after extrusion of Example 1 and Comparative Example 1.

[0027] Figure 4 Electron backscatter diffraction (EBSD) microstructure photos of the extruded Mg-1Zn-0.1Ca-0.1Al-0.1Mn alloy in Example 1 and Comparative Example 1;

[0028] Figure 4 (a) In comparative example 1, the extrusion temperature is 250°C and the pressing angle is 30°, (b) In comparative example 1, the extrusion temperature is 250°C and the pressing angle is 90°, (c) In comparative example 1, the extrusion temperature is 235°C and the pressing angle is 90°, and (d) In example 1, the extrusion temperature is 220°C and the pressing angle is 90°.

[0029] Figure 5 The scanning electron microscopy (SEM) photos of the extruded Mg-1Zn-0.1Ca-0.1Al-0.1Mn alloy in Example 1 and Comparative Example 1;

[0030] Figure 5(a) In comparative example 1, the extrusion temperature is 250°C and the pressing angle is 30°, (b) In comparative example 1, the extrusion temperature is 250°C and the pressing angle is 90°, (c) In comparative example 1, the extrusion temperature is 235°C and the pressing angle is 90°, and (d) In example 1, the extrusion temperature is 220°C and the pressing angle is 90°.

[0031] Figure 6 This is a transmission electron microscopy (TEM) photograph of the extruded Mg-1Zn-0.1Ca-0.1Al-0.1Mn alloy in Example 1;

[0032] Figure 6 (a) Ca2Mg6Zn3 phase on the grain boundary, (b) Al8Mn5 phase on the grain boundary, and (c) Al8Mn5 phase inside the grain.

[0033] Figure 7 Room temperature tensile stress-strain curves of the extruded Mg-1Zn-0.1Ca-0.1Al-0.1Mn alloy of Example 1 and Comparative Example 1. DETAILED DESCRIPTION

[0034] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention clearer, the present invention is further described in detail in conjunction with the embodiments and the accompanying drawings. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. The technical solutions of the present invention are described in detail below in conjunction with the embodiments and the accompanying drawings, but the scope of protection is not limited thereto.

[0035] Example 1

[0036] This embodiment proposes a method for preparing a Mg-1Zn high-strength alloy based on multi-scale nano-precipitation phase strengthening; first, the Mg-1Zn (mass ratio, wt.%) binary alloy is subjected to 0.1wt.%Ca, 0.1wt.%Al, 0.1wt.%Mn microalloying modification, and the total amount of alloying elements does not exceed 0.5wt.%, to obtain a high-quality cast Mg-1Zn-0.1Ca-0.1Al-0.1Mn alloy base material. Then, the cast Mg-1Zn-0.1Ca-0.1Al-0.1Mn alloy is subjected to one pass of positive extrusion deformation under specific conditions (extrusion angle and extrusion temperature) to obtain uniformly distributed multi-type and multi-scale precipitation phases, thereby overcoming the problem of poor strengthening effect of single precipitation phase and single-scale precipitation phase, thereby synergistically improving the strength of the Mg-1Zn alloy.

[0037] It includes the following two units:

[0038] S1, Mg-1Zn binary alloy microalloying modification unit

[0039] S1.1. Alloy preparation

[0040] According to the mass percentage (wt.%) of the Mg-1Zn-0.1Ca-0.1Al-0.1Mn alloy, the required raw materials are weighed and prepared, including magnesium blocks (Mg) with a purity of 99.99wt.%, zinc blocks (Zn) with a purity of 99.99wt.%, magnesium-calcium (Mg-20Ca) intermediate alloy blocks, aluminum blocks (Al) with a purity of 99.99wt.%, and manganese particles (Mn) with a purity of 99.99wt.%. The actual chemical composition range of the alloy is: Zn: 0.8~1.2wt.%, Ca: 0.05~0.15wt.%, Al: 0.05~0.15wt.%, Mn: 0.05~0.15wt.%, and the rest is Mg. After the raw materials are polished to remove the oxide layer on the surface, they are placed in a drying oven together with covering agents, casting molds, slag removal and stirring tools for drying and preheating.

[0041] S1.2 Alloy smelting

[0042] When the temperature of the resistance furnace rises to 400°C, the preheated magnesium block and the magnesium-calcium intermediate alloy block are placed in the crucible, and the preheated salt covering agent is evenly sprinkled on the surface. The crucible containing the magnesium ingot and the magnesium-calcium intermediate alloy is clamped into the resistance furnace, and argon (Ar2) is introduced into the furnace. In the present invention, the addition of calcium can have a certain flame retardant effect, and the salt covering agent + Ar2 composite protection method can prevent the oxidation and combustion of the magnesium melt, thereby obtaining a high-quality clean magnesium alloy melt. When the temperature of the resistance furnace rises to 720°C, the temperature is kept constant for 30 minutes to ensure that the magnesium and the magnesium-calcium intermediate alloy can be completely melted. After the magnesium block is melted, the slag is removed, the preheated zinc block is added, and the covering agent is evenly sprinkled and the temperature is raised. When the temperature of the resistance furnace rises to 750°C, the slag is removed, the preheated aluminum block is added, and the covering agent is evenly sprinkled and the temperature is raised. When the temperature of the resistance furnace rises to 750℃ again, remove the slag, add the preheated manganese particles, stir and sprinkle the covering agent, raise the furnace temperature to 750℃ and keep it warm. After keeping it warm at 750℃ for 20 minutes, lower the furnace temperature to 730℃, remove the slag, and sprinkle the refining agent into the magnesium melt for refining. After refining, raise the furnace temperature back to 750℃, and keep the magnesium melt warm at 750℃ for 30 minutes.

[0043] S1.3 Alloy casting and solidification

[0044] After keeping at 750℃ for 30 minutes, the furnace temperature was lowered to 720℃, the slag was removed, and the magnesium melt was cast into a preheated metal mold (200℃). After the alloy melt was completely solidified in the air and naturally cooled to room temperature with the mold temperature, the sample was knocked out of the mold to obtain a Mg-1Zn-0.1Ca-0.1Al-0.1Mn cast magnesium alloy test bar with a diameter of 42mm and a height of 150mm.

[0045] S2, Mg-1Zn-0.1Ca-0.1Al-0.1Mn alloy forward extrusion deformation unit

[0046] S2.1. Homogenization

[0047] The homogenization treatment is carried out in a tubular furnace. The cast magnesium alloy test rod is covered with MgO powder and placed in the tubular furnace. It is then evacuated and Ar2 is continuously introduced for gas protection. In the present invention, in order to prevent the magnesium alloy structure from being overburned, a two-step temperature increase homogenization treatment is adopted. First, the temperature is set to 350°C and the time is set to 2 hours; then the temperature is increased to 450°C and the time is set to 2 hours. After the set time is completed, the power is turned off, and the magnesium alloy test rod is quickly taken out of the furnace for water quenching to obtain a homogenized magnesium alloy test rod.

[0048] S2.2, positive extrusion deformation

[0049] A vertical extruder was selected. The extrusion die included an extrusion die, an extrusion gasket, an extrusion rod and an extrusion sleeve, with a maximum extrusion force of 500 tons. An extrusion die with an extrusion ratio of 25 / 1, an extrusion speed of 0.4 mm / s, a die extrusion angle of 90°, an extrusion temperature of 220°C, an extrusion billet diameter of 40 mm, and a height of 35 mm were selected. Before extrusion, the homogeneous magnesium alloy test bar was placed in a heat treatment furnace for preheating, and the extrusion die was preheated in a heat preservation furnace provided on the extruder workbench. After the test bar and the extrusion die were kept at an extrusion temperature of 220°C for 1 hour, they were extruded at the set extrusion temperature of 220°C and an extrusion speed of 0.4 mm / s. The extruded magnesium alloy test bar was water quenched to obtain an extruded test bar with a diameter of 8 mm.

[0050] Comparative Example 1

[0051] Positive extrusion deformation comparison test:

[0052] The present invention designs three groups of single-pass extrusion comparison tests with different extrusion temperatures and different pressurization angles. The step parameters different from those in Example 1 are as follows (the rest of the steps are the same):

[0053] A vertical extruder was selected. The extrusion die included an extrusion die, an extrusion gasket, an extrusion rod and an extrusion sleeve, with a maximum extrusion force of 500 tons. An extrusion die with an extrusion ratio of 25 / 1 was selected, the extrusion speed was 0.4 mm / s, the extrusion angles of the die were 30° and 90°, the extrusion temperature was 250°C and 235°C, the extrusion billet diameter was 40 mm, and the height was 35 mm. Before extrusion, the homogeneous magnesium alloy test bar was placed in a heat treatment furnace for preheating, and the extrusion die was preheated in a heat preservation furnace provided on the extruder workbench. After the test bar and the extrusion die were kept at the extrusion temperature (250°C, 235°C) for 1 hour, they were extruded at the set extrusion temperature (250°C, 235°C) and extrusion speed (0.4 mm / s), and the extruded magnesium alloy test bar was water quenched to obtain an extrusion test bar with a diameter of 8 mm.

[0054] There are three groups of comparative tests in the extrusion deformation unit, and the extrusion conditions are:

[0055] (1) The extrusion ratio is 25 / 1, the extrusion speed is 0.4 mm / s, the extrusion temperature is 250 °C, and the pressure angle is 30°;

[0056] (2) The extrusion ratio is 25 / 1, the extrusion speed is 0.4 mm / s, the extrusion temperature is 250 °C, and the pressing angle is 90°;

[0057] (3) The extrusion ratio is 25 / 1, the extrusion speed is 0.4 mm / s, the extrusion temperature is 235 °C, and the pressure angle is 90°;

[0058] For comparative analysis, see Figures 4 to 7 :

[0059] (1) In Example 1, under the specific conditions of an extrusion temperature of 220°C and an extrusion angle of 90°, a fine recrystallized structure can be obtained. In Comparative Example 1, under the same extrusion temperature (250°C), the extrusion angle is increased from 30° to 90°, and the recrystallized grain size can be reduced from 6.65μm to 3.37μm. Further, under the same extrusion angle (90°), the extrusion temperature is reduced from 250°C to 220°C, and the recrystallized grain size can be reduced from 3.37μm to 1.42μm.

[0060] (2) Example 1 Under the specific conditions of an extrusion temperature of 220°C and an extrusion angle of 90°, a large number of uniformly distributed multi-type and multi-scale nano-precipitates can be precipitated through only one extrusion deformation without the need for subsequent heat treatment, including Ca2Mg6Zn3 phases with a size of 200~600nm and Al8Mn5 phases with a size of 20~50nm distributed on the grain boundaries, and Al8Mn5 phases with a size of 3~8nm distributed inside the grains.

[0061] (3) Example 1: Under the specific conditions of extrusion temperature of 220°C and extrusion angle of 90°, a new high-strength Mg-1Zn-0.1Ca-0.1Al-0.1Mn magnesium alloy was prepared through fine grain strengthening of fine recrystallization and precipitation strengthening of multi-type and multi-scale nano-precipitates. Its yield strength, tensile strength and elongation reached 265 MPa, 345 MPa and 26.5%, respectively.

[0062] The above content is a further detailed description of the present invention in combination with a specific preferred embodiment. It cannot be determined that the specific embodiments of the present invention are limited to this. For ordinary technicians in the technical field to which the present invention belongs, several simple deductions or substitutions can be made without departing from the present invention, which should be regarded as belonging to the present invention and the scope of patent protection determined by the submitted claims.

Claims

1. A method for preparing a Mg-Zn high-strength alloy based on multi-scale nano-precipitation phase strengthening, characterized in that: The following steps are involved: S1. Microalloying and modification of Mg-Zn binary alloy: adding Ca, Al and Mn elements during smelting of Mg-Zn binary alloy, wherein Zn: 0.8~1.2wt.%, Ca: 0.05~0.15wt.%, Al: 0.05~0.15wt.%, Mn: 0.05~0.15wt.%, and the rest is Mg; S2, the smelted alloy is sequentially subjected to casting, solidification and homogenization treatment to obtain a homogeneous magnesium alloy test bar; S3. The magnesium alloy test bar is subjected to forward extrusion deformation with an extrusion ratio of 25 / 1, an extrusion speed of 0.4 mm / s, an extrusion temperature of 220°C, and an extrusion angle of 90°.

2. The method for preparing a Mg-Zn high-strength alloy based on multi-scale nano-precipitation phase strengthening according to claim 1, characterized in that: The raw materials used for alloy smelting are: magnesium blocks with a purity of 99.99wt.%, zinc blocks with a purity of 99.99wt.%, Mg-20Ca master alloy blocks, aluminum blocks with a purity of 99.99wt.%, and manganese particles with a purity of 99.99wt.%.

3. The method for preparing a Mg-Zn high-strength alloy based on multi-scale nano-precipitation phase strengthening according to claim 2, characterized in that: The alloy smelting is to put the preheated magnesium block and Mg-20Ca intermediate alloy block into the crucible and melt them completely at 400℃; then add the preheated zinc block, and when the temperature rises to 750℃, add the preheated aluminum block, and then add the preheated manganese particles. After keeping at 750℃ for 20-25 minutes, cool it down to 730℃, skim off the slag, and sprinkle refining agent into the magnesium melt for refining.

4. The method for preparing a Mg-Zn high-strength alloy based on multi-scale nano-precipitation phase strengthening according to claim 1, characterized in that: The casting and solidification are as follows: keeping the temperature at 750°C for 25 to 35 minutes, lowering the furnace temperature to 720°C, removing the slag, and casting the magnesium melt into a preheated metal mold; after the alloy melt is completely solidified in the air and naturally cooled to room temperature as the mold temperature, the sample is taken out of the mold.

5. The method for preparing a Mg-Zn high-strength alloy based on multi-scale nano-precipitation phase strengthening according to claim 1, characterized in that: The homogenization treatment is to first set the temperature to 350° C. and homogenize for 2 hours; then raise the temperature to 450° C. and homogenize for another 2 hours.

6. The method for preparing a Mg-Zn high-strength alloy based on multi-scale nano-precipitation phase strengthening according to claim 5, characterized in that: After the homogenization treatment is completed, the magnesium alloy test bar is subjected to water quenching treatment to obtain a homogenized magnesium alloy test bar.

7. The method for preparing a Mg-Zn high-strength alloy based on multi-scale nano-precipitation phase strengthening according to claim 1, characterized in that: The forward extrusion deformation adopts a vertical extruder, and the extrusion die includes an extrusion die, an extrusion gasket, an extrusion rod and an extrusion sleeve.

8. The method for preparing a Mg-Zn high-strength alloy based on multi-scale nano-precipitation phase strengthening according to claim 1, characterized in that: The magnesium alloy test bars were water quenched after positive extrusion deformation.

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