Abstract
A novel continuous process of severe plastic deformation (SPD) named continuous close die forging (CCDF) is presented. The CCDF process combines all favorite advances of multidirectional forging and other SPD methods, and it can be easily scaled up for industrial use. Keeping constant both the cross section and the length of the sample, the new method promotes a refinement of the microstructure. The grain refinement and mechanical properties of commercially pure aluminum (AA1050) were studied as a function of the number of CCDF repetitive passes and the previous conditioning heat treatment. In particular, two different pre-annealing treatments were applied. The first one consisted of a reheating to 623 K (350 °C) for 1 h aimed at eliminating the effect of the deformation applied during the bar extrusion. The second pre-annealing consisted on a reheating to 903 K (630 °C) for 48 h plus cooling down to 573 K (300 °C) at 66 K/h. At this latter temperature, the material remained for 3 h prior to a final cooling to room temperature within the furnace, i.e., slow cooling rate. This treatment aimed at increasing the elongation and formability of the material. No visible cracking was detected in the workpiece of AA1050 processed up to 16 passes at room temperature after the first conditioning heat treatment, and 24 passes were able to be applied when the material was subjected to the second heat treatment. After processing through 16 passes for the low temperature pre-annealed samples, the microstructure was refined down to a mean grain size of 0.82 µm and the grain size was further reduced to 0.72 µm after 24 passes, applied after the high temperature heat treatment. Tensile tests showed the best mechanical properties after the high temperature pre-annealing and 24 passes of the novel CCDF method. A yield strength and ultimate tensile strength of 180 and 226 MPa, respectively, were obtained. Elongation to fracture was 18%. The microstructure and grain boundary nature are discussed in relation to the mechanical properties attained by the current ultrafine-grained (UFG) AA1050 processed by this new method.
Highlights
It is well known that the microstructure plays an important role in the physical and mechanical properties of polycrystalline materials
According to the Hall–Petch relationship, which describes the dependence between the yield stress, σy, and the grain size, d, in Equation (1) [1,2], the strength of metallic materials can be enhanced by grain refinement
Following this continuous close die forging (CCDF) route, up to 16 passes could be applied to the material that followed the low-temperature pre-annealing condition and a total of 24 passes could be applied to the AA1050 samples that followed the high temperature pre-annealing
Summary
It is well known that the microstructure plays an important role in the physical and mechanical properties of polycrystalline materials. It is well known that when a SPD process, performed at low temperatures, is applied to a given metallic material, a UFG structure can be achieved where most of the grain boundaries are of the high angle type. Several SPD processes have been proposed in the literature, meeting the latter specifications and generating uniform microstructures throughout the volume of the piece This homogeneity is important to ensure the stability of the mechanical properties and subsequent forming processes. MDF is based on the repetitive application of compression to a metal, while varying the deformation axis after each pass This promotes accumulation of redundant plastic deformation, either at low or high temperatures. The new CCDF process was suitable to generate UFG microstructures regardless of the conditioning of the samples by different pre-annealing heat treatments. The pre-annealing conditions affected the number of deformation passes that could be applied to the samples and, the final grain size and mechanical properties
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