Scaling up of High-Pressure Sliding (HPS) for Grain Refinement and Superplasticity

Abstract

The process of high-pressure sliding (HPS) is a method of severe plastic deformation developed recently for grain refinement of metallic materials under high pressure. The sample for HPS is used with a form of sheet or rod. In this study, an HPS facility with capacities of 500 tonnes for vertical pressing and of 500 and 300 tonnes for horizontal forward and backward pressings, respectively, was newly built and applied for grain refinement of a Mg alloy as AZ61, Al alloys such as Al-Mg-Sc, A2024 and A7075 alloys, a Ti alloy as ASTM-F1295, and a Ni-based superalloy as Inconel 718. Sheet samples with dimensions of 10 to 30 mm width, 100 mm length, and 1 mm thickness were processed at room temperature and ultrafine grains with sizes of~200 to 300 nm were successfully produced in the alloys. Tensile testing at elevated temperatures confirmed the advent of superplasticity with total elongations of more than 400 pct in all the alloys. It is demonstrated that the HPS can make all the alloys superplastic through processing at room temperature with a form of rectangular sheets.

Low-Temperature and High-Strain-Rate Superplasticity of Ultrafine-Grained A7075 Processed by High-Pressure Torsion

“…Here, the slope corresponds to m. Particularly, in the superplastic regions (EL > 400% at the temperature range of 250350°C), the m values result in the range of 0.30.4. These m values are fairly comparable to 0.5 expected from the rate-controlling mechanism based on grain boundary sliding (GBS), 16) and in fact several studies reported the m values of 0.30.4 for the superplasticity at higher temperatures. 3641) The occurrence of GBS is verified from the SEM observation as in Fig.…”

“…It is thus demonstrated that the HPT process can introduce the advent of HSRS and LTSP in the A7075 alloy. 3.4.2 Strain rate sensitivity (m) and activation energy (Q) High-temperature deformation can be described by the following constitutive equation 16) and it is useful to understand the deformation mechanism for the deformation.…”

“…Superplasticity as exceeding 400% in metallic materials may appear when the grain size is small (<10 µm) and stable at elevated temperatures (> half the melting point). 16) To make use of the superplasticity in metal forming process, it is important that the superplasticity should occur at strain rates as higher than at least 10 ¹2 s ¹1 , which are defined as the high strain rate superplasticity (HSRS), and close to commercial hot working rates of ³10 ¹1 to 10 2 s ¹1 . 79) In order to meet this HSRS condition, the grain size should be refined to be very small according to the constitutive equation for superplastic flow.…”

“…79) In order to meet this HSRS condition, the grain size should be refined to be very small according to the constitutive equation for superplastic flow. 16) Because the grain size dependence of the strain rate is proportional to 1/d p , where d is the grain size and p the grain size exponent equal to 23, 16) even a small reduction of the grain size leads to a significant increase in the strain rate. In addition, the constitutive equation for superplastic flow suggests that the grain refinement lowers the temperature for the advent of superplasticity.…”

“…It is well established that processing through severe plastic deformation (SPD) can produce ultrafine grains in the submicrometer or nanometer range. Several SPD processes are available such as equal-channel angular pressing (ECAP), 11) high-pressure torsion (HPT), 1214) high-pressure sliding (HPS), 15,16) accumulative roll bonding (ARB), 17) and multi-directional forging 18) and friction stir processing (FSP). 19) It was demonstrated that ultrafine grains were produced in Al alloys by such SPD processes and they led to HSRS and low-temperature superplasticity (LTS).…”