Development of the rheo-diecasting process for magnesium alloys

doi:10.1016/j.msea.2005.09.038

Materials Science and Engineering: A

Volumes 413–414, 15 December 2005, Pages 72-78

https://doi.org/10.1016/j.msea.2005.09.038 Get rights and content

Abstract

The rheo-diecasting (RDC) process is an innovative one-step semisolid processing technique for manufacturing near-net shape components of high integrity directly from liquid Mg-alloys. The process innovatively adapts the well-established high shear dispersive mixing action of the twin-screw mechanism to the task of in situ creation of semisolid metal (SSM) slurry with fine and spheroidal solid particles followed by direct shaping of the SSM slurry into a near-net shape component using the existing cold chamber high pressure diecasting (HPDC) process. In this paper, we present the RDC process, and the resulting microstructures and mechanical properties of various Mg-alloys. The experimental results show that the RDC samples have extremely low porosity (typically 0.1–0.3%), fine and uniform microstructure throughout entire casting, and consequently much improved strength and ductility, compared with those produced by the HPDC process and other semisolid processes. In addition, the RDC process is more tolerant to alloy composition in terms of processability.

Introduction

Currently, nearly all the applications of Mg-alloys are provided by high pressure diecasting (HPDC), due to its high efficiency, high production volume and low production cost. However, HPDC components contain a substantial amount of porosity due to gas entrapment during die filling, and hot tearing during the solidification in the die cavity [1]. Such porosity not only deteriorates mechanical properties, but also denies the opportunity for property enhancement by subsequent heat treatment. Further growth in Mg applications will largely depend on the successful development of new processing technologies capable of producing high quality, low cost components [2].

One of the promising technologies capable of producing high integrity components is semisolid metal (SSM) processing [3], [4]. Compared with conventional diecasting routes, SSM processing has a number of advantages, such as low porosity, heat treatability, consistency and soundness of mechanical properties, the ability to make complex component shapes and longer die life [3], [4]. One of the most popular SSM processes currently used is thixoforming [5], in which non-dendritic alloys pre-processed by electromagnetic stirring [6] are reheated to the semisolid region prior to the component shaping by either casting (thixocasting) or forging (thixoforging). It is therefore a two-stage process. The high cost of pre-processed non-dendritic raw materials is by far the greatest obstacle to the development of the full potential of this approach [7]. Due to the lack of reliable and quality feedstock supply, thixoforming of Mg-alloys has been very scarce. So far, only thixomoulding [8] of thin-walled components has achieved some success in engineering applications. In recent years, to overcome the technical and economical difficulties faced by the thixo-processing route, the rheo-route of SSM processing has become popular for research and development [9]. This includes the new rheocasting (NRC) process developed by UBE [10], the SSR process developed by MIT [11] and the continuous rheo-conversion (CRP) process developed by WPI [12]. A common feature of rheo-processing techniques is that they all use molten alloy as a starting material, therefore eliminating the need for specially prepared feedstock materials.

In this paper, we report a new semisolid processing technique, the rheo-diecasting (RDC) process. Microstructure and mechanical properties of the RDC Mg-alloys will be presented and compared with those produced by other processes.

Section snippets

The RDC process and experimental procedures

The RDC process, as schematically illustrated in Fig. 1, is an innovative one-step SSM processing technique for manufacturing near-net shape components of high integrity directly from liquid Mg-alloys. The RDC equipment consists of two basic functional units, a twin-screw slurry maker and a standard cold chamber HPDC machine. The twin-screw slurry maker has a pair of screws rotating inside a barrel. The screws have specially designed profiles to achieve co-rotating, fully intermeshing and

Microstructure of RDC Mg-alloy

The chemical compositions of the AZ91D alloy at different processing stages are presented in Table 1. There was very little change in the compositions of the alloying elements. Perhaps more importantly, there was little change in Fe content after both melting and rheo-diecasting, with the Fe content in the RDC sample being well within the specification of AZ91D alloy.

One of the most important features of SSM processing is the ability to achieve laminar die filling. To check this, the die cavity

Rheological consideration in the RDC process

The most important objective of SSM processing is to achieve laminar die filling, avoiding gas entrapment, by increasing the viscosity of the feed material, while maintaining adequate fluidity for complete die filling. Intensive experimental investigations [3], [4], [9], [18] have confirmed the effects of the particle morphology on the flow behaviour of semisolid slurries. The morphological effects have been modelled successfully through effective solid fraction by Chen and Fan [19]. Both

Concluding remarks

(i)
A new semisolid metal processing technology, rheo-diecasting, has been developed for the production of Mg-alloy components with high integrity. Rheo-diecasting can be easily achieved by adding a twin-screw slurry maker to the existing cold chamber diecasting machine.
(ii)
Solidification in the RDC process takes place in two distinctive stages; one is primary solidification in the twin-screw slurry maker under intensive forced convection, and the other one is secondary solidification in the die cavity

Acknowledgements

The author acknowledges the contribution to this work from Drs. S. Ji, G. Liu, Y. Wang, Z. Zhen, Y.Q. Liu and E. Zhang. The financial support from EPSRC (UK), Ford Motor Co. and Magnesium Elektron Ltd. (UK) is also acknowledged with gratitude.

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Development of the rheo-diecasting process for magnesium alloys

Abstract

Introduction

Section snippets

The RDC process and experimental procedures

Microstructure of RDC Mg-alloy

Rheological consideration in the RDC process

Concluding remarks

Acknowledgements

Acta Mater.

Acta Mater.

Acta Mater.

Metall. Trans.

Inter. Mater. Rev.

Inter. Mater. Rev.