Friction stir welding/processing of metals and alloys: A comprehensive review on microstructural evolution

Increasing demand of lightweight structures with exceptional properties elicits materials processing and manufacturing technologies to tailor blanks in order to achieve or enhance those pre...

Recent Development in Friction Stir Processing as a Solid-State Grain Refinement Technique: Microstructural Evolution and Property Enhancement

To develop ultrafine grains (UFG) in 6.35 mm thick magnesium alloy, stationary shoulder friction stir processing (SSFSP) with steel and copper backing plates was conducted. Steel backing plate produced uniform fine grains (FG) size of 4.98, 4.75, 4.12 μm in top, middle, bottom of the stir zone (SZ), respectively. In contrast, copper backing plate tailored microstructure from FG (4.1 μm) in the top to UFG (0.96 μm) in the bottom of SZ. SSFSP produced uniform and gradient microstructures, altering temperature gradient by placing steel and copper backing plates, respectively. It is worth to note that UFG microstructure achieved without usage of external cooling, owning to the copper backing plate. Most of the grains found under ~2 μm size in UFG microstructure. FG and UFG microstructures contributed to the enhancement in the ductility and strength, respectively. UFG resulted in significant improvement in hardness and tensile strength by ~80% and 24% of the base material, respectively. The intensity of strong basal texture throughout the thickness found independent of the backing plate type. Microstructure evolutions across the SZ thickness for both processing conditions are discussed using electron back scattered diffraction (EBSD).

Tailoring grain refinement through thickness in magnesium alloy via stationary shoulder friction stir processing and copper backing plate

Effects of grain size and texture on stress corrosion cracking of friction stir processed AZ80 magnesium alloy

Multi-pass submerged friction stir processing (M-SFSP) with an overlapping ratio of 50% was applied on as-cast AZ61 magnesium plate. The relationship between the microstructure and tensile behavior of the M-SFSP plate was investigated. With the cooling enhancement during processing, the grain size is significantly refined to be ~ 3.7 ± 1.7 μm in the stir zone. The initial network-like β-Mg17Al12 second phase in the cast alloy is effectively fragmented and dissolved into magnesium matrix, as revealed by a consistent right shift of magnesium peaks in the XRD patterns due to the lattice shortening caused by the smaller aluminum atoms getting into magnesium matrix to form substitutional solid solution. Texture is modified with a lower intensity in the center of stir zone than at the top surface. Compared with the base material, the strength and ductility of the M-SFSP Mg plate are significantly improved, mainly due to grain refinement along with dislocation strengthening and solid solution strengthening. Despite texture weakening, the M-SFSP plate still exhibits mechanical anisotropy to a certain extent, depending on the initial texture and loading direction. Cracking is observed to initiate from the transitional zone and propagate along the microstructural band due to the presence of high local residual stresses.

Multi-pass submerged friction stir processing of AZ61 magnesium alloy: strengthening mechanisms and fracture behavior

Friction stir processing (FSP) has been considered as a novel technique to refine the grain size and homogenize the microstructure of metallic materials. In this study, two-pass FSP was conducted under water to enhance the cooling rate during processing, and an AZ61 magnesium alloy with fine-grained and homogeneous microstructure was prepared through this method. Compared to the as-cast material, one-pass FSP resulted in grain refinement and the β-Mg17Al12 phase was broken into small particles. Using a smaller stirring tool and an overlapping ratio of 100%, a finer and more uniform microstructure with an average grain size of 4.6 μm was obtained through two-pass FSP. The two-pass FSP resulted in a significant improvement in elongation of 37.2% ± 4.3%, but a slight decrease in strength compared with one-pass FSP alloy. Besides the microstructure refinement, the texture evolution in the stir zone is also considered responsible for the ductility improvement.

Ductility Improvement of an AZ61 Magnesium Alloy through Two-Pass Submerged Friction Stir Processing

Microstructural changes and texture evolution during underwater friction stir welding of dissimilar 7003/6060 aluminum alloys were investigated via electron backscatter diffraction technique. In the stir zone, equiaxed fine grains were formed through continuous dynamic recrystallization. Due to the enhanced cooling rate, the average grain size of the stir zone was much finer than that of normal friction stir welds. In the heat affected zone, static recovery was the main mechanism for microstructure evolution, resulting in similar grain structure as compared with base materials, while the fraction of low angle grain boundary increased. In the thermo-mechanical affected zone, dynamic recovery and partial crystallization took place on the advancing side of 7003, and a mixed grain structure was formed; dynamic recovery was the dominant evolution mechanism on the retreating side of 6060, and the fraction of low angle grain boundary was increased. The texture in the heat affected zone was almost the same as the base material, and simple shear texture B/ B - {112} was formed in the stir zone. With the distance to the weld center decreasing, the texture density of relevant zone decreased.

EBSD study of underwater friction stir welded AA7003-T4 and AA6060-T4 dissimilar joint

In this study, three porous NiTi scaffolds with the same porosity but different pore sizes (900, 835, and 618 μm) were fabricated via selective laser melting (SLM). The microstructure, mechanical properties, shape memory properties, and in vitro biocompatibility of the different porous NiTi scaffolds were studied systematically by scanning electron microscopy, micro-computed tomography, differential scanning calorimetry, compressive testing, and in vitro cell culture experiments. The pore sizes of the SLM porous NiTi scaffolds were smaller than those of computer-aided design models owing to the existence of spatter powders, whereas the strut diameters showed the opposite tendency. Furthermore, the elastic modulus of the three NiTi scaffolds was close to that of human bones and lower than that of most previously reported porous NiTi and Ti alloys with similar porosities. Moreover, the scaffolds demonstrated better compressive strength than that of most of the reported porous NiTi alloys with similar porosities as well as better strength and fracture strain than those of human bones, particularly for scaffolds with larger pore sizes. Moreover, the best recoverable strain of 5.10% and recovery rate of 91.4% were obtained for porous NiTi scaffolds with a pore size of 900 μm owing to their minimal stress concentration during loading. The in vitro study further demonstrated that a relatively small pore size could promote the bridging growth of cells in pores, and the in vitro biocompatibility of the porous NiTi scaffolds was equivalent to that of bulk SLM NiTi. The results obtained in this study can provide guidance for the application of porous NiTi scaffolds in biological implants.

Microstructure, shape memory properties, and in vitro biocompatibility of porous NiTi scaffolds fabricated via selective laser melting

In this study, precipitation behavior of Mg–Y–Nd cast alloy during friction stir processing (FSP), and the effect of subsequent artificial aging on mechanical properties and fracture behavior of the FSP alloy were investigated It is found that the coarse α-Mg grains and large second phases are greatly refined after FSP Moreover, due to the heat input during processing and the natural cooling, β′ and β1 precipitates are also observed in the FSP alloy The FSP specimens were subjected to subsequent artificial aging treatment, and the peak hardness is obtained at 150 °C for 54 h and 180 °C for 30 h Strengths of the peak–aged specimens are further increased, which is attributed to the large quantity of β″ and β1 precipitates, respectively Meanwhile, elongations of the peak-aged specimens are both decreased Due to the comprehensive effects of banded structures and fine grains, failure mechanisms of FSP and peak-aged specimens are all mixed ductile–brittle fracture mode However, compared to the FSP specimens, different fracture paths are exhibited in peak–aged specimens

Effect of aging treatment on mechanical properties and fracture behavior of friction stir processed Mg–Y–Nd alloy

In this work, the effect of multi-pass submerged friction stir processing (MSFSP) on a casting AZ61 Mg plate was investigated. AZ61 plate was successfully prepared by repeating the processing with the subsequent pass FSP overlapped the retreating side with an overlapping ratio of 50%. The microstructure and resulting mechanical properties at room temperature were studied. The results revealed that the fine and equiaxed grains with an average grain size of 10.7 μm obtained in stirring zone (SZ) through MSFSP, and the second-phase Mg17Al12 was broken into small particles. Compared to BM, MSFSP specimens achieved an improvement in microhardness, tensile strength and elongation but exhibited obvious mechanical anisotropy, especially the ductility. It is considered that the enhancement in mechanical properties is benefit from grain refinement and the texture evolution may be responsible for the mechanical anisotropy.

Xicai Luo

Papers

Ductility Improvement of an AZ61 Magnesium Alloy through Two-Pass Submerged Friction Stir Processing

EBSD study of underwater friction stir welded AA7003-T4 and AA6060-T4 dissimilar joint

Microstructure, shape memory properties, and in vitro biocompatibility of porous NiTi scaffolds fabricated via selective laser melting

Effect of aging treatment on mechanical properties and fracture behavior of friction stir processed Mg–Y–Nd alloy

Effect of Multi-pass Submerged Friction Stir Processing on Microstructure Evolution and Mechanical Properties of AZ61 Magnesium Plate