ABSTRACT: The basic principle and characteristics of the continuous shear deformation method for sheet metal are introduced. The microstructure of T2 copper sheet annealed at 350 °C X 40 min was analyzed after continuous shear deformation along c path for 6 passes. The results show that the microstructure of the sheet after continuous shear deformation is obviously refined, the tensile strength of the deformed plate increased from 210MPa to 330MPA, and the hardness increased from 65HVI to 120HVI.

KEYWORDS: T2 Red Copper Plate; Continuous Shear Deformation; classification number: TG386.2 document identification number: A article number: 1001-3814(2006)05-0060-03

Moderate grain refinement is an effective means to improve material performance and processing performance. Equal channel angular pressing (ECAP) is an effective method to produce ultra-fine bulk or rod materials by means of a simple shearing principle. The emergence of ECAP technology has been widely concerned, and has been deeply studied and developed. With the deep research of ECAP method, its application scope is also expanding, but it is only suitable for the preparation of Rod, block materials, but not for the plate began to show the limitations. The continuous shear deformation (CZSZ) technique is a new method developed on the basis of ECAP technique, which is specially used to prepare fine-grained plates. It effectively solves the problem that the ECAP method can not be applied to sheet metal.

1. The basic principles and characteristics of CZSZ method 

1.1CZSZ method

The basic principle of the CZSZ method is shown in figure 1. In the method, the extrusion cavity is composed of two intersecting channels with small size changes. During the test, under the action of the guide wheel, the feeding wheel generates enough friction with the plate to send the plate into the die cavity. The plate undergoes a strong shear-like deformation at the corner of the die cavity, as shown in Fig. 2. At the corner, the deformed plate is extruded from the other side of the die cavity. The thickness of the plate at the exit is the same as the raw material. Thus, the plate can be fed repeatedly into the same die

The shear strain of each pass can be added continuously, and finally the grain size of the plate can be refined and its properties improved. In addition, as can be seen from figure 1, in order to ensure adequate feeding power, die cavity at the entrance of the plate thickness will be slightly reduced. Therefore, the thickness variation must be taken into account when calculating the total strain in the extrusion process. The original ECAP method, the total strain of the calculation formula, not applicable here. seokIZJ obtained the calculating formula of the total strain of the CZSZ method after repeated experiments:

In the formula, N is the extrusion pass, K is the thickness change factor, and k = (the thickness of the plate at the entrance/the thickness of the plate at the exit) , in the middle is the Die Angle, less is the die circumferential arc angle.

Characteristics of 1.2CZSZ method

The CZSZ method, as a new effective method for preparing ultra-fine grain plates, has many unique advantages: (l) refining grain by the mechanism of approximate pure shear deformation, the obtained plate has the same advantages as the bar and block materials prepared by ECAP, such as compactness, no inclusion, high strength, etc. . (2) the length and size of the specimen are unlimited, in theory the specimen can be infinite in length; (3) continuous operation can be realized, the utility model has the advantages of high working efficiency and convenient industrialization of the experiment, without demoulding between adjacent passes. The CZSZ method has its own shortcomings, such as high precision of cavity size and long time to adjust the gap.

2. Materials and methods of test

The materials used in the experiment were Zrns. T2 Red Copper Sheet (copper content 99.91%) , annealed at 380 °C X 40 min, sample size zrns. X20 ~ X30OR. Phantom N. The C Path is adopted, that is, after each pass of extrusion, the sample is reversed 1800 into the next pass. After mechanical polishing, the processed sample needs chemical polishing again. The chemical polishing solution is a mixture of 3L [ : 33 ML ACETIC ACID + 33 ml nitric acid + 33 ml phosphoric acid, the temperature is controlled at 65-75 °c, the etching time is controlled at 35-455 °C. The corroded sample is immediately rinsed and stored in an alcohol container to prevent oxidation for further metallographic examination. The results show that the y plane of the sample is fine and deformed because of the fracture of the elongated structure, the x plane is very small and can be neglected, and the Z plane is almost unchanged. Therefore, in the study, focus on choosing the y-plane as the research object.

3. Experimental results and discussion

Fig. 3(a) is the original microstructure of T2 plate with coarse grains and obvious annealed structure. Fig. 3(b) is the microstructure of CZSZ sample after one pass of deformation. Some of the grains in the graph are finer than those in the original state, and the orientation of the grains also shows certain directionality. Fig. 3(C) is the microstructure of the sample after three CZSZ deformations, in which the grain is finer than that of the original material and the direction of shear action is more obvious due to the elongation at the corner of the die under the shear action. Fig. 3(D) is the microstructure of the sample after 6 CZSZ deformations. The grain refinement effect is obvious and the grain is uniform. The shear streamline under the action of Shear Force shows obvious flow direction. By observing the microstructure in Fig. 3, it can be found that with the increase of extrusion pass, the effect of grain refinement is further improved

The grain structure of the original annealed coarse equiaxed grains basically disappeared, a large number of grains at the corner were elongated and sheared due to the effect of Shear Force, the microstructure became fine and uniform, and the grains presented obvious shear streamline. Fig. 4(a) and (b) are the curves of tensile strength versus extrusion pass and hardness, respectively. After 6 passes of CZsZ Shear deformation, the tensile strength of the plate increased from 210 MPA to 330 MPA, and the hardness of the plate increased from 65 Hvi to 120 Hvi.

In the CZSZ process, the specimen undergoes intense shear deformation at the corner of the mold cavity. The coarse grains in the original rolling state are elongated and cut through the rotation angle, which makes the coarse grains develop towards smaller and smaller grains. The orientation of some grains is also changed because the grains rotate at the corner under the action of shearing force. With the increase of extrusion pass, the grains are further refined, and the dislocations are easy to accumulate, absorb and annihilate at the grain boundary. On the whole, the number of dislocations remains unchanged after 3 to 4 deformations. At the same time, in the post-sequence extrusion, hardening and dynamic recovery take place simultaneously. Therefore, from the Mechanical Properties Curve, after 3-4 times extrusion deformation, the tensile strength, hardness and other mechanical properties are basically maintained at a constant value.

Fig. 5 is the tensile strength curve of T2 Red Copper Plate after 6 CZSZ treatments at different annealing temperatures. As can be seen from the diagram, the strength of the alloy has little change at low annealing temperature, especially when the annealing temperature is about 100 °C, but when the annealing temperature rises to 300 °C, its strength value obviously drops greatly, and is lower than the raw material strength value. The reason for this phenomenon is that when the sample is annealed at low temperature (the annealing temperature is lower than 300 °c) , the internal stress caused by the processing can be effectively eliminated, and the grain size will not change greatly, however, when the annealing temperature is close to 300 °C, the recrystallization temperature of T2 copper is reached, and the recrystallization of the grains takes place rapidly, which makes the mechanical properties decrease greatly.

4. CONCLUSION:

 (L) after CZSZ deformation, the microstructure of the annealed T2 copper plate can be refined obviously, and the results of the related experiments are analyzed, and the grain is more uniform, showing obvious streamline shape. (2) The tensile strength of the plate deformed by CZSZ was increased from 210MPA to 330MPA, and its mechanical properties were improved obviously. (3) annealing below the recrystallization temperature of the material does not decrease the mechanical properties of the shear-deformed T2 plate. 

Source: Chinanews.com, by Cheng Shilai