double twin parallel channel angular extrusion as a development of the ecap process, a step towards industrialization

In the last few decades, scientists have made many efforts to make materials with extraordinary properties through severe plastic deformation (spd) methods. despite many lab-scale successes, there has not been a significant achievement in the industry. this is mainly due to the limitations in the tool’s strength, lifespan, and energy consumption, especially in producing samples with high aspect ratios. therefore, new designs to moderate stresses in the tools and increase their lifespan and stability are among the recent challenges of researchers. equal channel angular pressing (ecap) is one of the earliest developed spd techniques, and many of the most fundamental studies in this area are conducted using ecap. in this regard, it takes much courage to argue about the problems associated with ecap. however, based on the spd literature and our experience, some difficulties can be enumerated for ecap, including the developed tail after each pass which must be removed before the next pass, as mentioned in [1], and low aspect ratios of the specimens processable by ecap. the main reason for the latter is due to the buckling and further fracture of the punch as the length of the specimen, and hence the punch length is increased. to overcome the former problem, raab proposed ecap in parallel channels [2] or, equivalently, single parallel channel angular extrusion (spcae). it is shown that this method successively reduces the amount of waste material. as a further development, abdi & ebrahimi [3] developed the twin parallel channel angular extrusion (tpcae) technique by which spcae is doubled by mirror symmetry. by tpcae, two specimens would be loaded in the die and pressed simultaneously. the samples move along until they reach the first shear plane, where they become separated; since there is no relative motion between the specimens, the frictional force at the plane of contact between them vanishes. additionally, the governing symmetry in tpce moderates the stress distribution in the die and hence increases the die’s lifespan [4]. in this investigation, a modification of tpcae is presented and named double twin parallel channel angular extrusion (dtpcae) (fig. 1). by this modification, four specimens are loaded in the die in a way that there are two planes of symmetry. this modification intensifies the benefits of tpcae in terms of lowering the processing load (per sample), increasing the punch stability, and increasing the ability to process specimens with higher aspect ratios. one of the main issues in any severe plastic deformation method is the proper die design; here, some key improvements that could be brought into the die design are presented and discussed. the relative extrusion pressure of spcae, tpcae, and dtpcae are compared with each other, and it is concluded that the decrease in the relative extrusion pressure by dtpcae is due to the elimination of the 8 (out of 16) frictional surfaces. on the other hand, while the dtpcae design increases the total load by a factor less than 2 compared to the tpcae, the minimum moment of inertia of the punch increases by a factor of 8, and hence the punch stability improves by a factor of more than 4. this improvement means that specimen length could be increased by a factor of more than 2.