Thermomechanical processing techniques consisting of a deformation process followed by an annealing treatment is a common approach to manipulate mechanical properties. In this research a combination of pre-straining and non-isothermal aging process is combined to study precipitation-hardening behavior of an AZ31B magnesium alloy. Effect of heating rate during non-isothermal aging and pre-straining on the kinetics of precipitation and evolution of hardness is investigated.

Pre-Straining Effect on Precipitation Behaviour of AZ31B

Hydrogen fuel cells have been used worldwide due to their high energy density and zero emissions. The metallic bipolar plate is the crucial component and has a significant effect on a cell's efficiency. However, the springback behavior of the metallic bipolar plate will greatly influence its forming accuracy in the micro-scale sheet metal forming process. Therefore, accurate calculation of the springback angle of the micro-scale metallic bipolar plate is urgent but difficult given the state of existing elastoplastic theory. In this paper, a constitutive model that simultaneously considers grain size effect and strain gradient is proposed to analyze micro-scale bending behavior and calculate springback angles. The specialized micro-scale four-point bending tool was designed to better calculate the springback angle and simplify the calculation step. A pure micro-bending experiment on a 316LN stainless steel sheet with a thickness of 0.1 mm was conducted to verify the constitutive model's accuracy. 

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Research on the Springback Behavior of 316LN Stainless Steel in Micro-Scale Bending Processes

Geometrical and microstructural size effects in progressive forming using wires

Incremental forming has gained a lot of importance in the recent past for sheet metal prototyping. The process is highly flexible and produces the required geometry by controlling the motions of a simple spherical-headed tool in three-dimensional space without any dedicated dies. Many variants were proposed in incremental forming to meet different functional requirements and applications in different sectors. Micro-incremental sheet forming is one variant that has gained importance recently due to its potential in the fabrication of micro components. However, industrial/commercial applications of this technology have not been wholly feasible and therefore appeal to the need for extensive research to improve process mechanics and performance matrices. Hence, this paper addresses this issue by discussing all the salient components involved in the micro-incremental sheet forming operation, namely machine tool set-up, forming tools, and numerical simulations. The range of the variable parameters has also been extensively reviewed to formulate a general interpretation of the essential specifications involved in the forming process. This review paper has undertaken a unique approach to analyse the keywords of numerous publications to perceive research trends through bibliometric analysis. 

A retrospective review on recent developments in the micro-incremental sheet forming process

Texture and Tensile Properties of Three-Layered Titanium Clad–AZ31 Magnesium Alloy Sheet by Single-Pass Hot Rolling

Micro-scaled progressive sheet metal forming is a promising process for producing bulk microparts, given its advantages of high efficiency and low cost. To enhance forming quality and efficiency, it is important to have an in-depth understanding of the forming mechanism and model the forming process and fracture behavior accurately. However, the prediction of fracture formation in sheet materials at the micro-scale has not yet been well explored, and thus current knowledge is not sufficient to support the continued development and application of microforming technology. This study investigated progressive sheet forming of magnesium-lithium alloy sheets of different grain sizes to produce bulk microparts directly from sheet metal via shearing, extruding, piercing, and blanking. Using the Gurson–Tvergaard–Needleman (GTN) damage model, the effects of the size factor on the formation and evolution of voids were considered, and the shear-modified GTN model was established by combining Thomason’s and Lemaitre’s damage mechanics models. The modified model could predict not only the ductile fracture behavior dominated by tension under high-stress triaxiality at the micro-scale, but also the damage behavior controlled by shear deformation under low-stress triaxiality. The progressive forming process was simulated using the modified model, which was verified by experimentation and simulation. Comparisons of the experiments and simulations revealed the size effects on the forming defects and fracture behaviors of microparts during progressive sheet forming. The results show that the stress during deformation is mainly concentrated at the edge of microparts, and irregular geometric defects including burr, rollover, incline, and bulge become deteriorated with the increase of the initial grain size. This study enhances the understanding and prediction of ductile fracture in the micro-scaled progressive forming of sheet metals. 

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Ductile fracture behavior in micro-scaled progressive forming of magnesium-lithium alloy sheet

The microstructure, texture and basic stamping properties of AZ31 magnesium sheet are studied in this paper with optical microscopy (OM), scan electron microscopy (SEM), X-ray diffraction (XRD) and MTS universal tester. The results show that the basal texture (0002) is enhanced and the grain size decreases as the thickness of sheet decreases. However the texture depresses in the 0.52mm sheet due to the complete recrystallization under deep rolling reduction. The strength and elongation decreases and the strain hardening index (n value) increases as the thickness increases. The precipitation decreases and the basal texture depresses after annealing treatment. The peak value ratio of crystal plane (0002) of 1.3mm sheet reduces from 67% to 53% when annealing treated at 300°C for 2h. In addition, the strain hardening index (n) value reduces from 0.2535 to 0.2053 and the anisotropic index (r) value increases from 1.966 to 2.108 when the sheet is treated at the same condition.

Xin Kui Wang

Papers

Ductile fracture behavior in micro-scaled progressive forming of magnesium-lithium alloy sheet

Effects of Annealing Treatment on Texture and Stamping Properties of AZ31 Magnesium Alloy