Simultaneous Effect of Plunger Motion Profile, Pressure, and Temperature on the Quality of High-Pressure Die-Cast Alumin
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HIGH-PRESSURE die casting (HPDC) offers flexibility in design and manufacturing of different kinds of components. Despite the high productivity that makes it attractive (a typical cycle time lasts about a minute), the HPDC process often causes porosity and other kinds of defects, and hence care should be paid in controlling the entire process. Fiorese et al.[1] introduced a new classification of defects and imperfections for aluminium alloy castings, which was recently included in the CEN/TR 16749.[2] Such a technical report explains the main causes for porosity formation, i.e., shrinkage (macro or interdendritic), gas entrapment (such as air or, less frequently, hydrogen[3]), and lubricant entrapment. HPDC consists of three stages, which can be summarized as slow shot stage, fast shot stage, and upset pressure stage. In the slow shot stage, the molten metal fills up the chamber and the plunger moves at a low and usually constant speed to avoid air entrapment. In the fast shot stage, the filling of the die cavity is performed at higher and time-varying velocity to avoid premature solidification at the gate and incomplete castings. Finally, when the die cavity is filled and the plunger
ELENA FIORESE, Postdoctoral Fellow, and FRANCO BONOLLO, Full Professor, are with the Department of Management and Engineering, DTG University of Padova, Stradella S. Nicola 3, 36100 Vicenza, Italy. Contact email: fi[email protected] Manuscript submitted April 9, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
has almost reached the final position, upset pressure is applied on the molten metal to prevent the formation of shrinkage porosity and the expansion of previously entrapped gas. In their previous work,[4] the authors focused on the effect of the plunger motion profile on the quality of castings by providing a new and comprehensive approach. Indeed, it was proved that the suitable choice of the plunger motion profile can significantly improve the casting quality, by reducing porosity and by enhancing the static mechanical properties. Among the different mechanisms involved in porosity formation, the air entrapment during the slow shot stage also has been discussed in literature, by investigating the effect of the evolution of the melt wave profile during the shot sleeve filling on the porosity. For example, articles[5–9] focus on the influence of the plunger acceleration profile adopted to reach the critical velocity on the amount of entrapped air. These works are the first ones that paid attention to the issue of the rate of variation of the speed. Nonetheless, the focus on just the slow shot stage neglects the important contributions of the fast shot stage and the upset pressure in compensating for the previously entrapped air. Although the plunger motion often plays a prominent role in setting the casting quality, a more comprehensive study on the effect of process parameters should account also for pressure and temperature, whose relevance has been shown previously in the literature. In particular, two issues are not described through the app
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