24-06-2013, 04:25 PM
Failure analysis of dies for aluminium alloys die-casting
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Abstract:
Dies for aluminium alloys die-casting fail because of a great number of a different
and simultaneously operating factors. Die design, material selection, and thermal stress
fatigue due to the cyclic working process, as well as to low and inhomogeneous initial die
temperature contribute to the failures and cracks formation on/in dies.
In the frame of the presented work the intensity and homogeneity of the temperature fields on
the working surface of the testing die were checked through thermographic measurements,
and failures and cracks on the working surface of the die were analysed by the non-destructive
metallographic examination methods.
INTRODUCTION
Die-casting is the most economical and technical easy process of casting very sophisticated
and precise aluminium products of big-scale series [1]. Comparison of nine parameters of the
die-casting versus stamping, forging, sand casting, permanent mold casting and plastic
molding is presented in Tab. 1.
Aluminium die-castings are made for final installation, and need very little machining.
They are used in automotive industry, household appliences, electrical industry and
instalations, fittings, etc. [3].
THEORETICAL
Aluminium die-casting dies fail because of a number of different and simultaneously
operating stresses. The stresses are of two basic kinds [4]: the first which are created during
the manufacturing of the die, and the second which are produced during exploatation process.
For economical production of aluminium and its alloys die-castings it is important that the
dies have a long working life. The replacement of a die is expensive in both: money and
production time. The most frequent failures of aluminium die-casting dies are [1,2]: heat
checking, gross cracking or cleavage cracking, cracking in corners, sharp radii, or sharp
edges, and wear or erosion.
EXPERIMENTAL WORK
In the frame of our investigation work a complex analysis of a typical dies for die-casting
of aluminium alloys has been carried out. The fixed half of the testing die-casting die are
shown in Fig. 1.
The die was made from the well known BOEHLER W300 ISODISC [7] hot work tool
steel. This steel is mostly applied and considered material for all kinds of hot working dies.
Thermal and mechanical properties of BOEHLER W300 ISODISC steel are well known.
Liquidus temperature of aluminium alloy AlSi9Cu3 is approximately 593°C, therefore the
properties in the temperature interval from 20 up to the 700°C are important for the analysis of the discussed case. The density of BOEHLER W300 ISODISC steel at 20°C is
approximately equal of 7800 kg/m3, and it decreases with higher temperature. Up to the
temperature of 700°C it drops for about 200 kg/m3. It is very interesting that this steel has
relatively low and nearly linear increasing heat conductivity (19.2 to 26.3 W/m×K), and
proportionally constant thermal diffusivity (the whole time aproximately 5×10-6 m2/s). Specific
heat is increased with higher temperature to its values of 456 or 587 J/kg×K, respectively for
the boundary values of the chosen temperature range.
CONCLUSIONS
Cracking on/in die-casting dies for aluminium alloys is caused by a number of different and
simultaneously operating factors. Some of them that affect die failures may be controlled to
some extent by the die-casting experts.
In the experimental part of our work the failures on the working surface of the fixed half of
the testing die for die-casting of aluminium alloys were observed with the use of nondestructive
testing (NDT) methods: such as thermographic analysis, penetrants, and
metallographic examination of polymeric replicas.
The failures observed on the working surface of the discussed fixed die half for die-casting
of aluminium alloys belong to heat checking initiated at identification marks, and cracking in
corners, sharp edges and transitions.