Thixoforming : Semi-solid Metal Processing

114j 4 Design of Al and Al–Li Alloys for Thixoforming
the intermediate group represent Class 2. Successful modification (Class 1) enhances
internal feeding, minimizes the porosity and the tendency for heat cracking of a
material, positively affects the flow and form-filling characteristics and improves the
mechanical properties of the component.
The formation of the eutectic grain structure in testing of thixoalloys can be
expected to be primarily affected by strontium. On the other hand, titanium and
manganese have very little effect. Due to the resulting interactions, alloying elements
such as titanium also have a significant worsening effect on the refinement and
uniformity of the eutectic if strontium is also present. If strontium is added, titanium
should be limited to between 0.15 and 0.25 mass% (Figure 4.4b). If 0.3 mass% of
manganese is added, the negative effect of titanium on modification of the eutectic
can be reduced to some extent.
4.2.5
Influencing the Semi-solid Temperature Interval
Determination of the freezing range for alloys is carried out by means of differential
thermal analysis (DTA). Both the grain refinement and modification should have a
favourable effect on the semi solid temperature interval. The liquidus temperature
becomes higher with increasing titanium content. In addition, it could be demonstrated
that the silicon and magnesium contents, and also the lead content in
particular, should also be taken into consideration because the liquidus temperature
reacts significantly to the alloying content.
The grain refinement element is primarily suitable in favourably affecting the
freezing range overall. All other alloying elements are of little significance in
comparison. Lead has a generally negative effect on the freezing range, so this
element should be avoided. The study confirmed the suitability of the thermal
analysis principle for assessment and control of the melt range. The hoped-for
evaluation of the modifying treatment and grain refinement by means of thermal
analysis was not achieved, however, due to the interfering influence of the other
alloying components (primarily Mg and Si). The grain-refined raw material differs
from commercial raw materials in respect of its smaller grain size and a more
uniform grain size distribution in the border and middle areas of the cast. At the same
time, it was noticeable that the grain-refined raw material had a somewhat more
dendritic grain structure composition than commercial raw material.
4.2.6
Characteristics of the Raw Material
Because the commercially available MHD grain structures and grain-refined raw
material can vary significantly from one another after reheating, both materials were
treated with the same reheating programme. The raw materials were heated to the
target temperature (580 C) horizontally in an induction unit. The chemically grainrefined
raw material was thereby transformed into a fine globulitic grain structure
similarly to the conventional MHD material. The average grain diameter and the