Magnesium alloy is promising structural light metal
because of its low densities, good recyclical potential and
abundant resources, which is expected to become a
next-generation material[1−4]. This material is received
special attention due to the renewed interest in energy
conservation. Its use is increasing in automobile
industries as a replacement for aluminum and steel.
Magnesium alloy road wheels have to be considered in
particular due to their beneficial effect such as safety,
comfort and energy saving. As a safety-related
component, the essential factors in wheel applications
are the fatigue strength and the impact strength. At the
same time, we should also take account of other
requirements such as corrosion resistance and cost.
In order to have a widespread use of magnesium
alloy wheels, casting and forging processes have been
adopted for wheel manufacturing. In the case of the
magnesium alloys, the manufacture of the entire wheel
only by the casting (low pressure die casting, squeeze
casting, gravity casting)[5−8] results in insufficient
strength of, for example, a rim portion which carries a
tire, as well as casting quality(porosity, inclusions) often
does not meet the requirements.
Forging is an alternative production technology for
Mg alloy parts with high strength requirements[9]. The
products manufactured by this process are characterized
by fine-grained microstructure without pores and
improved mechanical properties compared with casting
Mg parts[10−12]. For magnesium road wheel forging,
methods of flow-forming and spin forging had been
investigated[13−14]. But heavy-duty press equipment is
used and the fatigue strength cannot meet application
requirement[13, 15]. To enhance the fatigue strength and
reduce the forming load, a new extrusion process for
magnesium alloy wheels was introduced and investigated
especially in this work. The one-piece AZ80+ alloy
wheel was extruded successfully by means of the
developed technique.
2 Forging technique used for wheel
manufacturing
2.1 Forging and flow-forming process
The technique of forged Mg alloy wheels requires
the following two process steps: forging the wheel disc
blank and flow-forming the rim[13]. The billets are made
by continuous casting process, and are homogenized
before the forming process. Prior to the forging and
flow-forming process, the billets are heated up to suitable