High strain rate superplasticity by the friction stir processing (FSP), an adaptation of the friction stir welding (FSW), is summarized in this overview article. As a common severe plastic deformation (SPD) processing technique, the microstructures prepared by FSP are characterized by fine grain sizes, being homogeneous with fragmented and dispersed particles, and having a high proportion of high-angle grain boundaries. These attributes are beneficial to the superplastic forming operations at high strain rates and low temperatures. In this monograph, the principles of superplasticity are reviewed, where the importance of grain boundary sliding (GBS), strain rate sensitivity index, grain refinement, and deformation temperature on the remarkable enhancement of ductility is emphasized. Afterwards, FSP is introduced and the effects of the main processing parameters on the heat input and grain size are critically discussed. Finally, the recent progress in the application of FSP for processing of superplastic materials is thoroughly overviewed and the influence of thermal stability against grain growth, addition of alloying elements to form pinning particles, external cooling for obtaining ultrafine grained (UFG) microstructure, FSP process variables such as tool rotation rate and traverse speed, and multi-pass FSP is summarized. Accordingly, this overview presents the opportunities that FSP can offer for controlling the superplastic behavior of materials.

High strain rate superplasticity via friction stir processing (FSP): A review

Friction stir processing (FSP) is a novel solid-phase processing technique that is derived from friction stir welding (FSW). The microstructure of the base metal can be modified with the friction heat and stir function during processing. It can be used to fabricate surface composites and in situ composites by adding reinforced particles into the metal matrix via FSP. Friction stir processing can significantly improve the hardness, wear resistance, ductility, etc., while preventing defects caused by material melting. It is an ideal material processing technology and has good prospects in the field of superplastic materials and for the preparation of metal matrix composites. This paper reviews research developments into the principle, process, and applications of FSP technology as well as its future research directions and development prospects.

https://www.mdpi.com/2079-6412/9/2/129/pdf

Research Status and Prospect of Friction Stir Processing Technology

The effect of prior cold rolling on the carbide dissolution, precipitation and dry wear behaviors of M50 bearing steel

Using heat treatments, high-pressure torsion and post-deformation annealing to optimize the properties of Ti-6Al-4V alloys

Friction stir processing (FSP) is a friction stir-based material processing method for enhancement of material microstructural and surface properties. As FSP is a multi-physics problem coupled with severe plastic deformation, material flow, heat flow, and microstructure evolution, modeling of the FSP process can be very complicated and challenging. Few research work has been reported on modeling and simulations of FSP for material modification. In this study, a computation-efficient process model is developed using ABAQUS/Explicit based on coupled Eulerian-Lagrangian (CEL) formulation to simulate FSP of aluminum alloy 5083. The three-dimensional (3D) finite element model simulates the entire process of FSP including tool plunging, dwelling, and stirring phases. Simulations are performed to evaluate the effects of tool-rotational speed and tool pin profile during the FSP process. The computational efficiency of the developed model is also evaluated in comparison with other existing models for friction stir–welding processes. FSP experiment is performed with measurements of process force and temperature for model validation. This study shows that the CEL model can be a powerful numerical tool to simulate the complex process mechanics and optimize the FSP process parameters for industrial applications.

An efficient coupled Eulerian-Lagrangian finite element model for friction stir processing

An ultrafine microstructure consisting of α grains (~ 0.51 µm) and a small amount of β phase was successfully achieved in a friction stir-processed (FSPed) Ti-6Al-4V alloy. The fraction of high angle grain boundaries (HAGBs) with random crystallographic orientations reached 89.3% revealed that dynamic recrystallization was responsible for the ultra-grain refinement mechanism during friction stir processing (FSP). Low-temperature superplasticity (LTSP) of such an ultrafine microstructure was demonstrated in the temperature range of 550–650 °C and strain rates of 1 × 10−4–3 × 10−3 s−1. Specifically, an extremely superior LTSP of 1130% was achieved at 600 °C and 3 × 10−4 s−1, which was explained by means of the ultrafine equiaxed grains, a large proportion of HAGBs with random orientations as well as the presence of β phase. The predominant superplastic deformation mechanism was considered as grain boundary sliding associated with grain boundary diffusion.

Ultra-grain refinement and enhanced low-temperature superplasticity in a friction stir-processed Ti-6Al-4V alloy

We have studied here the carbon partitioning during continuous cooling at different cooling rates using dilatometer. An interesting phase transition was observed involving multiple stages of transformation during dynamic partitioning and a mechanism was proposed to explain the observations. Two morphologies of nano-sized retained austenite were observed, film-like and block-type. It was proposed that the amount of retained austenite was stable in a certain range of cooling rate. When the cooling rate was between 0.05 °C/s and 1 °C/s, the carbon partitioning was adequate and ~ 11 to 14% retained austenite was obtained. With increase in cooling rate from 5 and 10 °C/s, carbon partitioning was not adequate, which resulted in decrease in film-like austenite. The austenite adjacent to ferrite had two different transformation products, M/A (martensite-austenite island) and twin martensite, based on the degree of carbon transfer from ferrite. However, macro-hardness test showed that cooling rate had little effect on hardness and the hardness was between 398 HV and 407 HV at cooling rate of ~ 0.05 to 10 ℃/s. Additionally, samples cooled at low cooling rate indicated continuous TRIP effect and excellent combination of high strength 1030 MPa and high elongation of ~ 25% was obtained in the plate cooled at 0.1 ℃/s. On the basis of results, a strategy was proposed to obtain high performance hot rolled Q&P steels. The study confirmed the viability of implementing quenching and partitioning process in the hot rolling production line.

A novel phase transition behavior during dynamic partitioning and analysis of retained austenite in quenched and partitioned steels

Using appropriate heat treatments, different volume fractions of the constituent phases were produced in a Ti–6Al–4V alloy. Thus, in Ti64-1 there was ∼85% of an equiaxed α-phase and ∼15% of a lamellar (α+β) microstructure and in Ti64-2 there was ∼48% of the α-phase and ∼52% of the (α+β) microstructure. These two alloys were processed by high-pressure torsion (HPT) at room temperature under an applied pressure of 6.0 GPa. Both materials showed increases in the microhardness and decreases in the grain size after HPT with a stabilization after about 10 turns. Higher hardness values and smaller grain sizes were attained in Ti64-2 thereby demonstrating that greater grain refinement is achieved when processing an alloy with approximately equal volume fractions of the constituent phases. In these experiments, the measured equilibrium grain sizes after 20 turns of HPT were ∼130 nm in Ti64-1 and ∼70 nm in Ti64-2.

Influence of phase volume fraction on the grain refining of a Ti-6Al-4V alloy by high-pressure torsion

A cold-rolled Ti-6Al-4V alloy was subjected to consecutive heat treatments at 1283 K for 1 h and at 823 K for 3 h in order to produce a fully lamellar microstructure. Thereafter, the material was processed by high-pressure torsion (HPT) through various numbers of turns up to a maximum of 30. It is shown that the HPT processing leads to exceptional grain refinement with average grain sizes of ~ 70 and ~ 50 nm after 20 and 30 turns, respectively. Tensile testing was conducted at 873 and 923 K with different initial strain rates using the material processed through 20 turns of HPT and this gave a maximum superplastic elongation of 820% at the relatively low temperature of 923 K when testing with an initial strain rate of 5.0 × 10−4 s−1. The associated strain rate sensitivity for this low temperature superplasticity was estimated as m ≈ 0.5 which is consistent with flow by grain boundary sliding.

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Grain refinement and superplastic flow in a fully lamellar Ti-6Al-4V alloy processed by high-pressure torsion

a b s t r a c t A cold-rolled Ti-6Al-4V sheet was subjected to three different heat treatments prior to pro- cessing by high pressure torsion (HPT). Quantitative measurements revealed that the volume fractions were 70% equiaxedphase and 30% lamellar (� + �) (Ti64-1), 47% equiaxedphase and 53% lamellar (� + �) (Ti64-2) and 25% equiaxedphase and 75% lamellar (� + �) (Ti64-3) and the grain sizes of thephase were 7.0 ± 2 �m, 9.0 ± 1.5 �m and 9.5 ± 1.5 �m, respectively. The processing by HPT was performed at room temperature with a pressure of 6.0 GPa and a rotation speed of 1 rpm. Processing of the three heat treatment batches was conducted through a total number of revolutions, N, of 1/4, 1, 5, 10 and 20. The results show that the microhardness increases with increasing numbers of turns in HPT processing and for all three conditions stable microhardness values are reached after about 20 turns. The grain sizes after 20 turns of HPT were 115 ± 30 nm in Ti64-1, 85 ± 25 nm in Ti64-2 and 75 ± 15 nm in Ti64-3 so that the grain size decreases as the volume fraction ofphase decreases and the lamellar (� + �) increases.

Wenjing Zhang

Papers

Ultra-grain refinement and enhanced low-temperature superplasticity in a friction stir-processed Ti-6Al-4V alloy

A novel phase transition behavior during dynamic partitioning and analysis of retained austenite in quenched and partitioned steels

Influence of phase volume fraction on the grain refining of a Ti-6Al-4V alloy by high-pressure torsion

Grain refinement and superplastic flow in a fully lamellar Ti-6Al-4V alloy processed by high-pressure torsion

Influence of phase volume fraction on the grain refining of a Ti-6Al-4V alloy by high-pressure