Shitang Zhang

Friction and wear behavior of laser cladding Ni/hBN self-lubricating composite coating

NiAl/hBN coating was successfully prepared on a Ni-based superalloy substrate by means of laser cladding. The microhardness profile of the composite coating along the depth direction was measured, while its cross-sectional microstructures and phase compositions were analyzed by means of scanning electron microscopy and X-ray diffraction. Moreover, the friction and wear behavior of the composite coatings sliding against Si3N4 from ambient to 1000 °C was evaluated using a ball-on-disc friction and wear tester, and the worn surface morphologies of the composite coatings were observed using a scanning electron microscope. It was found that the laser cladding NiAl/hBN coating on the Ni-based superalloy substrate had high microhardness and good friction-reducing and antiwear abilities at elevated temperatures up to 1000 °C. The friction and wear behavior of the laser cladding NiAl/hBN coating was strongly dependent on test temperatures. The coating had a small friction coefficient and wear rate as it slid against the ceramic counterpart at elevated temperatures up to 1000 °C. The wear mechanism was characterized by mixed adhesion and abrasive wear as it slid against the ceramic ball below 300 °C and mild adhesion wear and plastic deformation up to 400 °C and above.

Approach of using a ductile fracture criterion in deep drawing of magnesium alloy cylindrical cups under non-isothermal condition

Abstract There have been many ductile fracture criteria published for predicting fractures in the isothermal metal-forming process, especially at room temperature. However, many materials, such as magnesium alloy and aluminium alloy, show an improved formability at elevated temperatures under non-isothermal conditions. It is significant to predict the occurrence of fracture in metal forming under these conditions since a forming limit diagram is almost powerless here. In the present work, an approach is proposed for extending the application of existent ductile fracture criteria in a non-isothermal metal-forming process. Using the new approach, combined with the finite element method (FEM), the deep drawing of magnesium alloy AZ31 cylindrical cups under non-isothermal condition is analysed. The fractures located in the punch radius region and in the cup wall close to the radius region of the female die resulting from the low and high punch temperatures, respectively, are predicted.

Warm deep drawing of magnesium alloy sheets - formability and process conditions

The influence of deformation conditions on the formability of magnesium alloy was systematically investigated using the experimental method and finite element analysis. The research activity aimed to gain further insight into the formability of magnesium alloy AZ31 (aluminium 3 wt per cent, zinc 1 wt per cent) and to have first-hand knowledge of the correlations between the materials properties and the processing parameters. In this work, the mechanical properties of magnesium alloy AZ31 sheet were analysed according to the results of tensile tests. A set of experimental equipment with temperature control was employed to perform warm deep drawing of magnesium alloy AZ31 sheets under various forming conditions. The most important process parameters, including the punch speed, the forming temperature, and the geometrical shape of the blank, were taken into account and optimal values were proposed. Finite element analyses were also performed to evaluate the effects of the process parameters on the formability of rectangular cup drawing and to predict the process defects during the process.

Warm Deep Drawing Of Rectangular Cups With Magnesium Alloy AZ31 Sheets

Recently, magnesium alloys have been widely applied in automotive and electronic industries as the lightest weight structural and functional materials. Warm forming of magnesium alloys has attracted much attention due to the very poor formability of Mg alloys at room temperature. The formability of magnesium alloy sheet at elevated temperature is significantly affected by the processing parameters. Among them the forming temperature, the punch speed, the geometrical shape of the blank, the blank holder force and the lubrication are probably the most relevant. In this research, the deep drawing of rectangular cups with AZ31 sheets was conducted at elevated temperatures with different process parameters. The finite element analyses were performed to investigate the effects of the process parameters on the formability of rectangular cup drawing and to predict the process defects during the process. The material yield condition was modeled using the isotropic Von Mises criterion. The flow stress data were obtained from tensile tests.

Applications of Computer Simulation Methods in Plastic Forming Technologies for Magnesium Alloys

Applications of computer simulation methods in plastic forming of magnesium alloy parts are discussed. As magnesium alloys possess very poor plastic formability at room temperature, various methods have been tried to improve the formability, for example, suitable rolling process and annealing procedures should be found to produce qualified magnesium alloy sheets, which have the reduced anisotropy and improved formability. The blank can be heated to a warm temperature or a hot temperature; a suitable temperature field is designed, tools should be heated or the punch should be cooled; suitable deformation speed should be found to ensure suitable strain rate range. Damage theory considering non‐isothermal forming is established. Various modeling methods have been tried to consider above situations. The following situations for modeling the forming process of magnesium alloy sheets and tubes are dealt with: (1) modeling for predicting wrinkling and anisotropy of sheet warm forming; (2) damage theory used for ...