Effect of holding pressure on the microstructure of vacuum counter-pressure casting aluminum alloy

doi:10.1016/j.jallcom.2010.04.103

Journal of Alloys and Compounds

Volume 501, Issue 2, 9 July 2010, Pages 352-357

https://doi.org/10.1016/j.jallcom.2010.04.103 Get rights and content

Abstract

The effect of holding pressure on the microstructure of vacuum counter-pressure casting aluminum alloy was analyzed by observing the changes of the Si phase size and alloy density. The results show that with the increasing of holding pressure of vacuum counter-pressure casting, the extrusion and infiltration ability among dendrites is enhanced and the microstructure of prepared alloy samples at the same location becomes finer, more uniform and denser. Under the same holding pressure, the smallest extrusion and infiltration ability takes place at the middle of sample. Accordingly, from the pouring gate to middle, the microstructure of vacuum counter-pressure casting aluminum alloy sample becomes coarser, more non-uniform and less dense, while the microstructure from the middle to top becomes finer, more uniform and denser.

Introduction

Vacuum counter-pressure casting technology is a kind of advanced counter-gravity method and has been employed widely in precision forming field. Adopting the advantages of vacuum filling mould under low pressure and melts crystallizing under high pressure, this fabrication technology has characteristics of vacuum suction casting, low-pressure casting and pressure kettle casting concurrently, which produce predominant filling hydrodynamics and notable solidification mechanism in comparison with the traditional casting process [1], [2]. So, it has enormous vitality in producing near net shape, large scale, thin wall and complicated nonferrous alloy castings in many fields, such as aviation, spaceflight, national defense and automobile industry.

Generally speaking, one of keys to obtain high quality products for vacuum counter-pressure casting is the control of crystallization and solidification, which ensures castings to possess good solidification feeding condition, dense microstructure and excellent mechanical property. Moreover, holding pressure is an important parameter for vacuum counter-pressure casting technology, and significantly affects castings quality [3]. At present, vacuum counter-pressure casting technology and equipment have been studied as a focal problem in the foundry industry. The advanced counter-pressure casting equipment and technology have been researched in many countries, such as Bulgaria, Japan, Germany, Italy and some other developed countries [4], [5]. In China, some university and institutes have also studied and explored on vacuum counter-pressure casting equipment and technology, such as control system, filling process and numerical simulation [6], [7], [8]. Accompanying the development of counter-pressure casting technology, a deep understanding of the relations between the processing conditions and resultant microstructure characteristic has important theoretical significance for fabrication of high quality and thin wall precision castings. Akad [4] studied crystallization process of counter-pressure casting, and discussed infiltration process through the growing crystal lattice during alloy solidification, so counter-pressure casting process had good feeding condition and could obtain dense castings. Meanwhile, Kovacheva et al. [9] investigated influence of the counter-pressure casting conditions on the microstructural characteristics of AlSi7Mg castings, and concluded aluminum alloy dendrites could take place microplastic deformation phenomenon under counter-pressure casting accompanied by infiltration through the growing dendrite, and obtained microstructure was fine and of no porosity. Moreover, Katzarov et al. [10] studied porosity formation in axi-symmetric castings produced by counter-pressure casting method, and concluded an important parameter that influences the porosity formation in a counter-pressure casting process is the solidification pressure, which showed bigger solidification pressure could obtain dense microstructure. These pioneering work offer essential theory basis for the future research of counter-pressure casting technique.

To give a further investigation of the effects of holding pressure on the microstructure of vacuum counter-pressure casting, the present paper focuses on the microstructure evolution of aluminum alloy A356, i.e., Si phase size and alloy density, under different holding pressure and different location. Appling the pressure dependent extrusion and infiltration ability, the observed experimental results can be well explained.

Section snippets

Experimental procedure

The vacuum counter-pressure casting equipment used in the present work is shown in Fig. 1(a), and it includes upper and down pressure kettle, Meanwhile, the mould was settled in the upper pressure kettle and the holding furnace was kept inside the down pressure kettle. The whole vacuum counter-pressure casting process can be divided into five stages, i.e., vacuumization, filling mould, rising pressure, holding pressure and releasing pressure, as shown in Fig. 1(b). It should be noted that the

Microstructure of the samples at the same location under different holding pressure

Under different holding pressure, the microstructures of vacuum counter-pressure casting aluminum alloy samples at the same location such as No. 1–1, No. 2–1, No. 3–1 and No. 4–1 are shown in Fig. 2. As can be seen clearly, with the increase of holding pressure, the morphology of Si phase at the same location changes from coarse to fine and its distribution becomes more uniform. From Fig. 3, it can be seen that the size of Si phase decreases gradually from 72 to 26 μm. Accordingly, the number of

Conclusions

With the increase of holding pressure, the extrusion and infiltration ability among dendrites gets strong, and the microstructure of vacuum counter-pressure casting aluminum alloy samples at the same location becomes finer, more uniform and denser. Under same holding pressure, the smallest extrusion and infiltration ability takes place at the middle of sample, and the microstructure of vacuum counter-pressure casting aluminum alloy samples from the pouring gate to middle location becomes

Acknowledgement

This research was financially supported by the State Key Laboratory of Materials Processing and Mould & Dies Technology Open Fund of China (grant number: 08-9), Aeronautical Science Foundation of China (grant number: 2009ZE56015), and Jiangxi Education Department Science and Technology Project of China (number: GJJ10508).

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Effect of holding pressure on the microstructure of vacuum counter-pressure casting aluminum alloy

Abstract

Introduction

Section snippets

Experimental procedure

Microstructure of the samples at the same location under different holding pressure

Conclusions

Acknowledgement

Int. J. Heat Mass Transfer.

Trans. Nonferrous Met. Soc. China

Foundry

J. Mater. Sci. Technol.

Foundry Trade J.

Mod. Cast.