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Using prototypes for product assessment
Published in Fuewen Frank Liou, Rapid Prototyping and Engineering Applications, 2019
As the grain size of H13 becomes coarser or larger, the elongation, fatigue strength, impact transition temperature, etc. decrease. This is correlated to dislocation cracks that result from the coalescence of dislocations increasing with the grain size. Coarse-grained steels are also inferior when it comes to bending and fatigue testing. Moreover, coarse-grained materials are more prone to distortion and are more prone to crack during quenching or grinding. In a normalized condition, the coarse-grained steel is preferred during machining, but when finishing the part like grinding and polishing, fine grain is preferred. In this process, an equiaxed grain with the least possible grain size is desired as further heat treatment would be necessary to change the microstructure depending on the application.
Investigation of microstructure, mechanical properties, and corrosion resistance of AZ91-Ca alloy after friction stir back extrusion
Published in Canadian Metallurgical Quarterly, 2023
Maryam Mehdipour, Hamed Jamshidi Aval
Due to the temperature and severe plastic deformation during the process, the conditions for dynamic recrystallization during extrusion are provided. Measuring the grain size of different wires decreases from 26.1 ± 2.3 to 22.3 ± 1.9 μm as the extrusion speed increases from 20 to 60 mm/min. Two factors of temperature and plastic strain rate are influential in the formation of equiaxed microstructure. Based on the temperature measurement during the process, the wires’ wall temperature produced with the extrusion speed of 20, 40, and 60 mm/min was 421, 410, and 390 °C, respectively. On the other hand, according to the thermo-mechanical simulation performed according to the procedure reported by Motavallian et al. [26], the value of the plastic strain in the wires produced with the extrusion speed of 20, 40, and 60 mm/min was predicted at 18.1, 16.8, 15.3, respectively. According to Humphreys and Hatherly [27], temperature and plastic strain inversely influence dynamic recrystallized grain size. It can be seen that although the decrease in plastic strain and increase in temperature leads to the growth of recrystallized grains, the increase in grain size with the reduction of extrusion speed (despite the increase in plastic strain) means that the effect of temperature prevails in the formation of recrystallized grains.
Recent progress on microstructure manipulation of aluminium alloys manufactured via laser powder bed fusion
Published in Virtual and Physical Prototyping, 2023
Zhen Xiao, Wenhui Yu, Hongxun Fu, Yaoji Deng, Yongling Wu, Hongyu Zheng
In summary, growing attention has been drawn to the uptake of LPBF for the fabrication of aluminium alloys. The thermal dynamics of LPBF allows rapid melting and solidifying of powder materials and the subsequent intrinsic heat treatment. Typical microstructure includes columnar grains along the building direction and strong crystallographic texture, which results in anisotropy in the mechanical properties of the parts. Grain refinement resulting in more fraction of equiaxed grains favour the mechanical performance. For aluminium alloys with low strength, the major study on near eutectic Al–Si alloys with good printability was focused on processing parameters previously. Recently growing number of studies were performed on the columnar to equiaxed transition to achieve high UTS and isotropy. Grain refining by inoculants, the introduction of reinforced aluminium matrix composite, and alloying with Ni and Cu elements were explored. For moderate strength Al–Mg alloys, solidification crack was mainly eliminated by Sc and/or Zr alloying. A bimodal microstructure was reported and tailored by parameter manipulation. For high strength Al–Cu and Al–Zn–Mg alloys, three approaches were attempted to refine the grain and improve the feasibility of LPBF: (1) optimising parameters by extremely high or low energy input, substrate preheating and supportive structure construction; (2) narrowing down the freezing range; (3) inoculants treatments.
Mitigation of micro-cracks in dissimilar welding of Inconel 718 and austenitic stainless steel
Published in Philosophical Magazine Letters, 2020
The transverse cross-section of weld beads obtained at maximum (i.e. PD1) and minimum (i.e. PD4) welding speed are shown in Figure 1. Micro-fissures are observed in the SEM images of the PD1 weld near the fusion boundary of the Inconel 718 side (Figure 1b) and the SS316L side (Figure 1d), whereas the PD4 weld is free from any such types of solidification defects. The SEM images indicate a columnar dendritic mode of solidification near the fusion boundary for both welding conditions. The interior microstructure is of equiaxed type for the PD4 weld (Figure 1g), whereas columnar structure prevails in the case of PD1 (Figure 1c). Equiaxed type of microstructure results in the improvement of the mechanical properties, as it reduces the segregational effect of useful alloying elements in the interdendritic region and hence reduces the formation of intermetallic phases. Thavamani et al. [11] indicated that the generation of micro-fissures and cracks in the weld zone is mainly associated with the formation of low melting point intermetallic phases during the solidification.