Submerged Friction Stir Processing Research Articles

Microstructures and tensile properties of submerged friction stir processed AZ91 magnesium alloy

Abstract 6 mm thick AZ91 casting alloy plates were subjected to normal friction stir processing (NFSP, in air) and submerged friction stir processing (SFSP, under water), and microstructures and tensile properties of the experimental materials were investigated. After FSP, the coarse microstructures in the as-cast condition are replaced by fine and equiaxed grains and the network-like eutectic β-Mg 17 Al 12 phases disappear and are changed into particles pinned at the grain boundaries. SFSP results in further grain refinement in comparison with NFSP, and the average grain sizes of the NFSP and SFSP alloys are 8.4 ± 1.3 and 2.8 ± 0.8 µm, respectively. XRD results reveal that the intensity of β-Mg 17 Al 12 diffraction peaks in the SFSP specimen decreases compared with NFSP. Due to significant grain refinement, the tensile strength and elongation of the SFSP AZ91 alloy are increased from 262 MPa and 18.9% for the NFSP material to 282 MPa and 25.4%, and the tensile strength (282 MPa) is nearly three times that of the BM (105 MPa). SFSP is an effective approach to refine the grain size and enhance the tensile properties of AZ91 casting alloy.

High‐Strain‐Rate Superplasticity in Mg–Y–Nd Alloy Prepared by Submerged Friction Stir Processing

Superplastic tensile behavior of Mg–Y–Nd alloy prepared by submerged friction stir processing (SFSP) is investigated. Microstructure of the SFSP alloy is mainly composed of fine‐grained magnesium grains (≈1.3 µm) and homogeneously distributed second‐phase particles. Elongations are all above 500% within a wide high‐temperature range of 708–758 K at 1 × 10−1 s−1, and an elongation of 549% at 1 × 10−1 s−1 is achieved at 758 K. The excellent high‐strain‐rate superplasticity is attributed to the fine‐grained microstructure and high thermal stability during superplastic deformation. Grain boundary sliding is the dominated deformation mechanism during superplastic deformation, and the specimens failed through cavity coalescence.

Experimental investigation on the effect of process environment on the mechanical properties of AA5083/Al2O3 nanocomposite fabricated via friction stir processing

Friction stir processing, a lately devised grain refining and also microstructure homogenizing technique, has extensively been used on aluminum alloys. Significance of limiting the grain growth during the process, has made lots of researchers make endeavor to keep, as one of the ways of controlling grain growth, the process temperature low; one way of doing so, is performing the friction stir process under water, which keeps the peak temperature low and increases the cooling rate as well. In the present work, research has been done to make known the effects of doing submerged friction stir processing on mechanical properties of AA5083/Al2O3p composite. The process was completed on each sample without changing the rotation and traverse speed of the tool. The pin of the tool was a threaded cylindrical one. Tensile and micro-hardness tests were used to evaluate the effect of the process on these properties of the samples. Also to study the microstructure of the samples, optical microscopy (OM) and scanning electron microscopy (SEM) micrographs were used. The results show that, underwater friction stir process is capable of producing defect-free AA5083/Al2O3p nanocomposite. Analyzing the properties of the processed samples showed that, by significantly reducing the grain size, water environment has positive impact on the mechanical properties of the alloy; And that, Hall–Petch effect is more powerful than Orowan mechanism in enhancing the mechanical properties of the samples.

Superplastic behavior and microstructure evolution of a fine-grained Mg–Y–Nd alloy processed by submerged friction stir processing

In this study, a fine-grained Mg–Y–Nd alloy with an average grain size of ~1.3μm and small second phases of ~280nm was prepared by submerged friction stir processing (SFSP). Microstructure evolution and superplastic behavior of Mg–Y–Nd alloy processed by SFSP was investigated in the temperature ranges of 683–758K and the strain rate ranges from 2×10−2 to 4×10−4s−1. Due to the fine-grained and stable microstructure, excellent high strain rate superplasticity (HSRS) of 900% was achieved at 758K and 2×10−2s−1, and the maximum elongation of 967% was obtained at 733K and 3×10−3s−1. Because of the good deformation compatibility, cavities were easily formed at the grain boundaries instead of the interface between particles and matrix. Grain boundary sliding accommodated by lattice diffusion was the dominated deformation mechanism during superplastic deformation.

Microstructure and Mechanical Properties of Submerged Friction Stir Processing Mg-Y-Nd Alloy

Mg-2.5wt%Y-4wt%Nd-0.5wt%Zr casting alloy was subjected to submerged friction stir processing (SFSP) with different rotation rates (ω) and travel speeds (υ). The influence of the ratio of ω/υ on the microstructure and mechanical properties of Mg-Y-Nd alloy was investigated in the present work by optical microscopy, scanning electron microscopy, transmission electron microscopy, tensile test and hardness measurement. The results showed that the average grain sizes of SFSP samples were significantly refined compared with as-cast sample, and the coarse net-shaped Mg12Nd phases which located at grain boundaries in as-cast sample were changed into small particles. The combined effect of grain refinement and uniform particles distribution was responsible for the enhancement of mechanical properties. The relative optimal parameter of 600 rpm/60 mm·min-1in this research obtained the finest grain size and the best mechanical properties, which were 1.1 μm for average grain size, 305 MPa for ultimate tensile strength and 22% for elongation, respectively.

Microstructure evolution and mechanical properties of Mg–Nd–Y alloy in different friction stir processing conditions

Microstructure evolution and its effect on mechanical properties of Mg–Nd–Y alloy during normal friction stir processing (NFSP) and submerged friction stir processing (SFSP) were investigated. It was found that microstructure of the as-cast Mg–Nd–Y alloy was mainly composed of coarse α-Mg dendrites and net-shaped Mg12Nd phases. After NFSP and SFSP, α-Mg dendrites were greatly refined with the average grain size of 2.7μm and 1.9μm, respectively. The finer grain size obtained by SFSP was attributed to its rapid cooling rate. Moreover, the Mg12Nd networks were changed into small particles. The microstructure of SFSP specimen was more homogeneous than that of the NSFP specimen, in which coarse grain band could be found. Tensile tests indicated that the as-cast alloy exhibited the lowest ultimate tensile strength (UTS) of 178±3.5MPa, the lowest yield strength (YS) of 172±3MPa and the lowest elongation of 3.4±0.5% because of the coarse grains and large eutectic phases. Due to microstructure refinement, the UTS, YS and elongation of NFSP and SFSP specimens were increased to 291±4.6MPa, 270±5MPa, 7.5±0.5% and 297±6.9MPa, 281±3.9MPa, 20.3±1.8%, respectively.

Effect of rotation speeds on microstructures and tensile properties of submerged friction stir processed AZ31 magnesium alloy

Submerged friction stir processing (SFSP) is a novel severe plastic deformation technique conducted underwater, which has great potential in preparation of fine-grained materials. In this study, AZ31 magnesium alloy is subjected to FSP in air (NFSP) and underwater at a fixed processing speed (60 mm min−1) and various rotation speeds (1100, 1300 and 1500 rev min−1), and the effect of rotation speeds on microstructures and tensile properties of the experimental materials is investigated. As the rotation speed increases, the grain size in the stir zone (SZ) increases. SFSP results in remarkable grain refinement compared with NFSP. The tensile properties of the NFSP and SFSP specimens decrease with the rotation speed increasing. Owing to much finer microstructure, SFSP results in improvement of tensile properties in comparison with NFSP, especially for the ductility. The fracture surfaces of the SFSP specimens exhibit much more uniform dimples and the dimples are much deeper.

Effect of Thermal History on Microstructures and Mechanical Properties of AZ31 Magnesium Alloy Prepared by Friction Stir Processing.

Hot-rolled AZ31 (Mg-2.57Al-0.84Zn-0.32Mn, in mass percentage) magnesium alloy is subjected to friction stir processing in air (normal friction stir processing, NFSP) and under water (submerged friction stir processing, SFSP). Thermal history of the two FSP procedures is measured, and its effect on microstructures and mechanical properties of the experimental materials is investigated. Compared with NFSP, the peak temperature during SFSP is lower and the duration time at a high temperature is shorter due to the enhanced cooling effect of water. Consequently, SFSP results in further grain refinement, and the average grain size of the NFSP and SFSP specimens in the stir zone (SZ) are 2.9 μm and 1.3 μm, respectively. Transmission electron microscopy (TEM) examinations confirm that grain refinement is attributed to continuous dynamic recrystallization both for NFSP and SFSP. The average Vickers hardness in the SZ of the NFSP and SFSP AZ31 magnesium alloy are 76 HV and 87 HV. Furthermore, the ultimate tensile strength and the elongation of the SFSP specimen increase from 191 MPa and 31.3% in the NFSP specimen to 210 MPa and 50.5%, respectively. Both the NFSP and SFSP alloys fail through ductile fracture, but the dimples are much more obvious in the SFSP alloy.

Microstructure evolution during high strain rate tensile deformation of a fine-grained AZ91 magnesium alloy

A fine-grained AZ91 magnesium alloy prepared by submerged friction stir processing is subjected to high temperature tensile test at 623K and 2×10−2s−1 to intermediate strains of 270%, 510%, 750% and failure strain of 990%, and microstructure evolution of the experimental material during tensile test is investigated. The initial grain size is about 1.2μm. Microstructures within the gauge region are much finer than those of grip region, and the grain aspect ratios remain approximately 1.0 in the whole superplastic deformation. With the tensile strains increasing, the average size of β-Mg17Al12 particles increases, and the density of the β-Mg17Al12 particles decreases. Due to the pinning effect of β-Mg17Al12 particles and the occurrence of DRX, the fine microstructures are maintained in the whole superplastic deformation process. Grain boundary sliding is the main deformation mechanism, and cavities are formed in the triple junctions of grains and around the second phase particles during deformation. The excellent high strain rate superplasticity of the AZ91 magnesium alloy is mainly attributed to its initial fine microstructure and good thermal stability.

Effect of Processing Speed on Microstructures and Mechanical Properties of Submerged Friction Stir Processed AZ91 Alloy

Normal and submerged friction stir processing (SFSP) were conducted to AZ91 magnesium alloy plates with 6mm in thickness, and influence of processing speed (ν) on microstructures and mechanical properties of the experimental materials was investigated. The results revealed that fine and equiaxed grains were observed in the stirred zone (SZ). As the processing speed increased from 60mm/min to 150mm/min, the average grain size in the SZ of normal FSP material decreased. However, the grain size of the SFSP specimens first increased with the processing rate increasing from 60mm/min to 120mm/min, and then decreased when the processing rate increased to 150mm/min. Microstructure of the SFSP specimen was much finer compared with the normal FSP one, and the grain size of α-Mg was about 1.2µm when the processing speed was 60mm/min during SFSP. Because of much finer microstructure of SFSP, the microhardness, tensile strength and elongation were all improved. SEM fracture observation showed that fine dimples and tearing edges could be observed on SFSP specimen which showing good ductility. In addition, high temperature tensile tests showed that SFSP AZ91 alloys exhibited excellent superplasticity at high strain rate, with an elongation of 1202% at 623 K with a strain rate of 3x10-3s-1. The present study demonstrated that SFSP possesses great potential in preparing fine-grained materials.

High strain rate superplasticity of a fine-grained AZ91 magnesium alloy prepared by submerged friction stir processing

The as-cast AZ91 plate was subjected to normal friction stir processing (processed in air) and submerged friction stir processing (processed in water, SFSP), and microstructure and superplastic tensile behavior of the experimental alloys were investigated. SFSP results in remarkable grain refinement due to the enhanced cooling rate compared with normal FSP, with an average grain size of 1.2μm and 7.8μm. The SFSP AZ91 specimen exhibits considerably enhanced superplastic ductility with reduced flow stress and higher optimum strain rate, as compared to the normal FSP specimen. The optimum superplastic deformation temperature is found to be 623K for both the normal FSP and SFSP AZ91 specimens. An elongation of 990% is obtained at 2×10−2s−1 and 623K for the SFSP specimen, indicating that excellent high strain rate superplasticity could be achieved. By comparison, maximum ductility of the normal FSP specimen strained at high strain rate is 158%. Grain boundary sliding is the main mechanism for the superplastic deformation of the normal FSP and SFSP specimens. The excellent high strain rate superplasticity of the SFSP alloy is attributed to its finer grain structure and higher fraction of grain boundary.

Thermal history analysis of friction stir processed and submerged friction stir processed aluminum

In this research, equations are developed to model the rate of heat input from different geometries of friction stir processing (FSP) tools. The model is then compared with actual heat input obtained from embedded thermocouples within the stirred region. The cooling curves obtained from the thermocouple data are then applied to the Derby–Ashby model for high angle grain boundary migration to predict the final grain size of a bulk sample produced by the friction stirring method. Submerged friction stir processing (SFSP) is introduced as a way of increasing the cooling rate of the bulk samples in an attempt to decrease the grain size. Microstructures obtained from both FSP and SFSP are characterized using transmission electron microscopy.

Submerged friction stir processing (SFSP): An improved method for creating ultra-fine-grained bulk materials

This research demonstrates the use of submerged friction stir processing (SFSP) as an alternative and improved method for creating ultra-fine-grained bulk materials through severe plastic deformation. SFSP is compared with friction stir processing (FSP) done in air, through the use of thermocouples and transmission electron microscopy. Samples of Al-6061–T6 were created by SFSP that exhibit grain sizes less than 200 nm, while still obtaining a suitably stirred region.