材料成型及控制工程外文文献翻译--az31镁合金在高温下的吹塑成型

材料成型及控制工程外文文献翻译--az31镁合金在高温下的吹塑成型内容摘要：

chnique in industrial manufacturing processes. These alloys have already demonstrated to have a superplastic behaviour at elevated temperatures in different conditions [4–7]. The ultimate goal is to look at this forming technique towards its mercially exploiting, once significant results in the optimization of process parameters and, above all, in the cycle time reduction could be achieved. Experimental setup Both the material characterization and the closed die forming tests have been performed on a laboratory scale equipment embedded in the cylindrical split furnace of an INSTRON universal testing machine. The equipment consists in: (i) a blankholder, (ii) a female die with different cavity shapes for generating on the blank different forming conditions, (iii) a pneumatic circuit for gas supply with an Argon cylinder, proportional electronic valves, steel tubes in proximity of the forming chamber and flexible polyurethane tubes in colder zones, (iv) an electric furnace with its electronic controller for upper, central and lower zones which can be set with three different temperatures for pensating thermal dispersion, (v) thermocouples to monitor thermal condition on the sheet and on the tools, (vi) a transducer for measuring, during bulging test, the dome height on the specimen and (vii) a PC with a data acquisition I/O device by which pressure, temperature, blank holder force can be monitored and managed. For material characterization, bulge tests are performed with a cylindrical die cavity (diameter 45 mm) in which the sheet can freely expand。 the dome height of the specimen is monitored during the whole test by the digital acquisition of a position transducer signal. Further details on the equipment can be found in [8]. In closed die forming tests, a die with a mm deep prismatic cavity has been used. The cavity has a squared section with a side length of 40 mm and a fillet radius between sides of 5 mm. A schematic representation of the equipment is shown in Fig. 1 Fig. 1 Experimental setup for closed die forming test Material characterization Commercial AZ31B Mg Sheets have been tested in the asreceived conditions. No mechanical or thermal treatment has been carried out on the material。 sheet has been purchased in the annealed conditions with an average grain size of 15177。 3 μm and a thickness of mm. In superplastic material characterization, usually tensile tests at different temperatures and strain rates are performed in order to get optimal conditions in which material has the best performances with the highest elongation to failure. This can be done by measuring the elongation to failure in standard tensile tests and the strain rate sensitivity index in jump strain rate tests [9]. Some authors have demonstrated that, when grain boundary sliding (GBS) is the predominant deformation mechanism, the stress and strain condition has a marginal role in the material characterization [10]. Some other authors have demonstrated also that uniaxial tensile stress and strain conditions are not effective for obtaining material parameters due to the fact that during a forming process the sheet, interacting with the die, undergoes to a stress and strain condition that is pletely different. Moreover, Mg alloys have a great tendency to grain coarsening and in several cases GBS cannot be considered as the predominant deformation mechanism [11]. Furthermore, testing setup and specimen geometry for uniaxial tensile tests in superplastic conditions have to be properly designed. Some standards exists, such as ISO 20202 and ASTM E2448, giving indications on the best test procedures and equipments. In superplastic conditions, the great advantage of tensile tests is the possibility of controlling in a sufficiently accurate way the strain rate during the test, but on the other hand it can be said that: – cutting accuracy in the specimen preparation must be very high, since also the cutting technique can influence test results。 mechanical cutting processes have to be preferred to thermal cutting processes that can modify the material microstructure near the cutting edge。 – specimen dimensions and shape (gauge length and width, fillet radius between the parallel portion and the clamp section) can affect test results – furnace must be sufficiently large to acmodate the large strain that the specimen undergoes during the test In order to overe these problems and to test the sheet in a strain condition more similar to the real process one, several tests alternative to uniaxial tensile tests have been proposed and reported。 one of most mon is based on bulge tests by means of the BF technique [12–14]. In this work, the material has been characterized with blow forming tests: constant pressure bulge tests at different temperatures and different pressure levels have been performed using the aforementioned laboratory equipment. Tests have been performed ranging pressure from MPa to MPa (with 7 different levels) and temperature from 360176。 C and 520176。 C (with 4 different levels), according to material physical properties and to the equipment capabilities. Pressure has been kept constant during the whole test until rupture occurred. Tests with an expected forming time to failure greater than 3000 s have been excluded from the experimental plan. For each test, the dome height has been acquired during the whole test using the position transducer that described before. Final height of the specimen has also been measured after the test. In Fig. 2, tested specimens are shown and their height to failure is plotted.. Fig. 2 Dome height at failure as a function of the forming pressure applied on the sheet, plotted for four different temperatures The highest value of the height is reported for 520176。 C and MPa. In spite of the use of an in。