J MATER SCI 41 (2006) 3631–3644
Microstructure and mechanical properties
of rheo-diecast AZ91D magnesium alloy
Z. FAN
∗
,G.LIU,Y.WANG
BCAST (Brunel Centre for Advanced Solidification Technology), Brunel University, Uxbridge,
Middlesex, UB8 3PH, UK
E-mail: Zhongyun.Fan@brunel.ac.uk
Published online: 21 April 2006
A new semisolid metal processing technology, rheo-diecasting (RDC) has been developed for
production of Mg-alloy components with high integrity. The RDC process innovatively
combines the dispersive mixing power of the twin-screw mechanism for creation of high
quality semisolid slurry and the high efficiency, low cost nature of the high pressure die casting
(HPDC) process for component shaping. AZ91D Mg-alloy was used to optimise the RDC process
and to establish its advantages over both the HPDC process and other existing semisolid
processing techniques. In this paper we present the RDC process for processing Mg-alloys and
the resulting microstructure and mechanical properties of RDC AZ91D alloy. The solidification
behaviour of the Mg-alloys in the RDC process and the co-relationships between microstructure
and mechanical properties of the RDC AZ91D alloy are discussed. It was found that the RDC
process is capable of producing Mg-alloy samples with close-to-zero porosity and a fine,
uniform microstructure throughout the entire sample irrespective of the section thickness.
Compared with those obtained by other existing processing techniques, the RDC samples have
substantially improved or equivalent mechanical properties, with the tensile elongation
showing more than 100% improvement.
C
2006
Springer Science
+
Business Media, Inc.
1. Introduction
Due to increasing environmental concerns and tightening
government regulations on CO
2
emissions, reducing ve-
hicle weight and improving fuel economy are becoming
increasingly important in the automobile industry. For in-
stance, the European and North American car producers
are committed to reducing fuel consumption by 25% and
thereby achieving 30% reduction in CO
2
emission by the
year 2010 [1]. Magnesium alloys, as the lightest structural
materials, are very suitable for applications in the auto-
motive industry to assist the realisation of such goals. In
the past few years we have seen a 15% average annual
increase of Mg usage in the automobile industry, and it is
predicted that this growth trend will continue well into the
first decade of the 21st century and beyond. Although the
automobile industry will continue to be the dominant driv-
ing force for the future growth in magnesium application,
other areas, such as aerospace, electronics and health care,
will take a substantial share of the magnesium market in
the near future.
Currently, nearly all the current applications of Mg-
alloys are achieved by high pressure die-casting (HPDC)
∗
Author to whom all correspondence should be addressed.
and are limited to a few casting alloys, such as AZ91D
and AM60. The HPDC process is characterised by high
efficiency, high production volume and low production
cost. However, the HPDC components contain a substan-
tial amount of porosity due to gas entrapment during die
filling and hot tearing during the solidification in the die
cavity [2]. Such porosity not only deteriorates mechani-
cal properties and limits the applications to non-stress or
low-stress components, but also denies the opportunity for
property enhancement by subsequent heat treatment. Fur-
ther growth in Mg applications will largely depend on the
successful development of new processing technologies
capable of producing high quality, low cost components
and new alloys with higher operating temperatures [3].
One of the promising technologies capable of produc-
ing high integrity components is semisolid metal (SSM)
processing [4, 5]. In conventional die casting, liquid metal
is usually forced into a mould cavity at such a high speed
that the flow becomes turbulent or even atomised, lead-
ing to high porosity in the final components. In contrast,
SSM processing uses a SSM slurry with substantially in-
creased viscosity, resulting in a controlled die filling and
0022-2461
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2006 Springer Science + Business Media, Inc.
DOI: 10.1007/s10853-006-6248-x
3631