Effect of super vacuum assisted high pressure die casting on the repeatability of mechanical properties of Al-Si-Mg-Mn die-cast alloys

doi:10.1016/j.jmatprotec.2018.10.030

Journal of Materials Processing Technology

Volume 266, April 2019, Pages 105-113

https://doi.org/10.1016/j.jmatprotec.2018.10.030 Get rights and content

Abstract

Two-stage super vacuum (19 mbar) assisted high pressure die casting (HPDC) was achieved by evacuating from the die cavity and the shot sleeve simultaneously. The effect of super vacuum assisted HPDC on the repeatability of the tensile properties of Al-Si-Mg-Mn die-cast alloy was investigated in comparison with conventional HPDC. The quantitative Weibull analysis confirmed that super vacuum assisted HPDC improved the repeatability of the tensile properties of the alloy. The data and deviation analysis verified that super vacuum assisted HPDC considerably decreased the fluctuations of the ductility of the alloy by 71% in as-cast state and 84% after solution and ageing treatment. The results also showed that super vacuum assisted HPDC improved the ultimate tensile strength and ductility of the as-cast alloy by 5.6% and 43%, respectively, and increased the ductility of the alloy by 21% after solution and ageing treatment. The significant improvements of ductility and the repeatability of tensile properties were originated from the decrease of porosity volume fraction and porosity size in the alloy processed by super vacuum assisted HPDC. The reduction of defect size can improve the stress distribution and retard the crack initiation in castings. Therefore, the tensile strength and ductility were enhanced in the die-cast alloy processed by super vacuum assisted HPDC.

Graphical abstract

Introduction

As a near-net shape manufacturing process, high pressure die casting (HPDC) is very attractive in producing thin-wall components with high dimensional accuracy, high production efficiency and considerable economic benefits for automotive and other industries. However, conventional HPDC castings usually contain randomly distributed gas porosities mainly due to the entrapment of gas under the high-speed shot. Wang et al. (2011) studied the content of gas in the castings made by HPDC. Li et al. (2016a,b) investigated the defect of porosity in the AM60B magnesium alloy processed by HPDC and its effect on the fracture. An electromagnetic transport (EMT) process was reported for HPDC (Dong et al., 2015a,b, 2016). The EMT process could decrease the porosity in A380 die-cast aluminium alloy, as reported by Dong et al. (2015a,b). The defect of porosity was detrimental to the tensile properties of HPDC components (Niu et al., 2000), resulting in the application of large safety factor in component design and thus reducing the potential of weight-saving of thin-wall components. Moreover, the formed gas porosities in die-castings were expanded in size at elevated temperatures, which resulted in the blisters on the surface of castings during solution treatment (Wan et al., 2013). Therefore, the component made by conventional HPDC was hard to be strengthened by the solution and ageing heat treatment.

Various techniques have been applied to decrease the level of porosity in castings made by HPDC. Ji et al. (2013) reported the decrease of porosity in the die-cast AM60 magnesium alloy under the treatment of melt conditioning. Wang et al. (2014) reported the improvement of porosity in the die-cast AZ91 magnesium alloy under vacuum assisted HPDC. The application of vacuum in HPDC process is promising for the significant reduction of the gas entrapment during die filling. Patel et al. (2012) reported the improvement of the deformation capability of castings under vacuum assisted HPDC. Wen et al. (2012) reported the enhanced property of fatigue of castings processed by vacuum assisted HPDC. Li et al. (2016a,b) reported the increase of tensile properties in the AZ91D alloy under vacuum assisted HPDC. Cao et al. (2017) reported the effect of vacuum levels on the castings made by HPDC. However, the benefits under vacuum assisted HPDC are still not well understood and developed. Limited information can be found for the quantitative relationship between the decrease of porosity and the improvement of mechanical properties under vacuum assisted HPDC. On the other hand, one of the main concerns in die castings is the large uncertainty and fluctuation of mechanical properties in individual casting, due to the random and irregular distribution of porosities in castings, as reported by Lee et al. (2006). Therefore, the reproducibility of castings is one of the serious concerns for the HPDC process. Limited researches have addressed the repeatability of die-castings, in particular for the effect of vacuum assisted HPDC on the repeatability. Weibull analysis was reported as a useful measure of the repeatability and reliability of manufacturing processes (Tiryakioğlu and Campbell, 2010; Tiryakioğlu, 2015). In addition, vacuum assisted HPDC can enable the strengthening of die castings through heat treatment, due to the low levels of gas porosities in the castings. However, more evidences are essentially needed to validate the effect of vacuum assisted HPDC on the heat-treated die castings because of the inconsistency of reported results. The generally achieved vacuum levels in literatures for vacuum assisted HPDC were above the critical value of 50 mbar, as reported by Wang and Xiong (2014). Shi et al. (2013) reported the super vacuum of ∼50 mbar during HPDC. Seldom did literatures achieve the super vacuum that is far below the critical value of 50 mbar.

In this investigation, the super vacuum assisted HPDC was achieved with a vacuum level that was far below the critical value of 50 mbar, and the Al-Si-Mg-Mn alloy was die-cast using the super vacuum assisted HPDC in comparison with conventional HPDC. The microstructural characteristics and the assessment of the tensile properties of the alloy were conducted. The repeatability of mechanical properties under as-cast and heat-treated conditions was analysed by Weibull models in order to obtain the quantitative results. The porosity levels were analysed and discussed in association with the tensile properties and repeatability of the alloy.

Section snippets

Materials preparation

The Al-Si-Mg-Mn die-cast alloys were melted in the electric resistance furnace, and the alloys for conventional HPDC and super vacuum assisted HPDC were melted in two different crucibles. The alloy was prepared by the addition of the master alloys of Al-50 wt.% Si, Al-20 wt.% Mn, Al-45 wt.% Fe and Al-10 wt.% Ti and the pure ingots of Al and Mg. The active element Mg was over added by 5% for the compensation of its burning during melting. After melting, 200 ppm Sr was added into the melt in the

Vacuum level during HPDC

Fig. 2 shows the evacuation process during the two-stage super vacuum assisted HPDC. At time T1, the head of the plunger just passed the feeding inlet of the shot sleeve, and the evacuation of the shot sleeve and the die cavity started due to the seal of the system. At time T2, the head of the plunger started passing the vacuum channel 1 in the shot sleeve. From time T1 to T2, the evacuation was conducted simultaneously from the vacuum channel 1 in the shot sleeve and the vacuum channel 2 on

Conclusions

(1)
Super vacuum level of 19 mbar was achieved for HPDC by the setting of two-stage evacuation from the shot sleeve and the die cavity simultaneously.
(2)
Super vacuum assisted HPDC improves the ductility of the Al-Si-Mg-Mn die-cast alloy comparing with conventional HPDC. The ductility is increased by 43% in as-cast state, and the ductility is enhanced by 21% after T6 heat treatment.
(3)
Super vacuum assisted HPDC reduces the fluctuation of the ductility of the Al-Si-Mg-Mn die-cast alloy comparing with

Acknowledgements

Innovate UK is acknowledged for the support of the work.

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Effect of super vacuum assisted high pressure die casting on the repeatability of mechanical properties of Al-Si-Mg-Mn die-cast alloys

Abstract

Graphical abstract

Introduction

Section snippets

Materials preparation

Vacuum level during HPDC

Conclusions

Acknowledgements

Acta Mater.

Vacuum

Mater. Sci. Eng. A

J. Mater. Process. Technol.

Int. J. Hydrogen Energy

J. Mater. Process. Technol.

J. Alloys Compd.

J. Mater. Process. Technol.

Mater. Sci. Eng. A

Mater. Sci. Eng. A

Mater. Sci. Eng. A

J. Mater. Sci. Technol.