Study On Electrochemical Cold Drawing Of In Situ Mg₂Si_p/AM60B Composite Bar

The hot extrusion of in situ Mg2Sip/AM60 B composite bars used for drawing and the subsequent electrochemical cold-drawing(ECD) were studied. Simultaneously, the hot extrusion and EC D of AM60 M alloy bars were also investigated for comparison.The results indicate that it is not conducive to obtaining bars with good surface quality when a too high or low extrusion temperature was employed. Within the extrusion temperature of 350℃~430℃, the tensile strengths of both the composite and alloy decrease as the temperature rises. because the Mg2 Si particles promote dynamic recrystallization, and thus decrease work hardening, the strength decrease of the composite is more obvious than that of the alloy as the temperature varies from 350℃ to 370℃. The sensitivity of elongation of the alloys to the extrusion temperature is relatively small. The composite has the best elongation when the temperature is within 370℃~410℃. It can be concluded that the optimal extrusion temperature for extrusion of both the materials is 410℃.During the EC D, as a result of chemomechanical effect(CME), both the drawing force and surface hardness all decrease for both the materials in comparison with drawing in air(DIA). However, the decreased drawing- force for the composite is less than that for the alloy because of the resistance against dislocations slip by Mg2 Si particles. Usually, the drawing force decreases with increasing current density. But a thick and co mpact corrosion layer will form as the current density increases, thus the friction force between the bar and the drawing die is increased and a barrier to dislocations exit is formed. So the drawing force increases when the current density rises to a point. The thickness of deformation layer in the bar surface layer for the EC D is obviously smaller than that for the DIA, and the thickness for the ECD composite bar is less than that for the alloy bar because of the decrease in plasticity caused by Mg2 Sip. In the ECD process, the increased drawing speed increases the work- hardening rate and decreases the electrochemical action time between the bar surface and the electrolyte, the drawing force thus increases as the drawing speed increases. Due to the existence of Mg2 Si particles, the work hardening rate of the composite increases and the surface dissolution rate decreases, the increase of drawing force is larger than that for the alloy. The difference in drawing forces between these two technologies decreases with the increasing drawing passes. Because for each pass, the CME maintains a constant value under the determined ECD processing parameters. But the work hardening continuous accumulates with increasing pass. So it can account for the reducing in electrochemical plasticization.