Thixoforming : Semi-solid Metal Processing

288j 8 Tool Technologies for Forming of Semi-solid Metals
Figure 8.42 Si3N4 thixoforging dies. (a) Segmented half-shell;
(b-d) plunger tops of different geometry.
Analysis of corrosion samples was carried out by careful preparation of crosssections
of the samples for SEM. Element distributions and phase compositions were
determined using EDS and XRD of the reaction products.
In the course of the collaborate research project, an extensive screening of
nearly 100 potential ceramic die materials regarding their chemical interaction
with aluminium, copper and steel alloys targeted for thixoforming was carried
out [5, 86]. With reaction mechanisms of the respective material pairings being
investigated, the most promising die materials regarding corrosion resistance for
the different applications were identified. Mechanical strength and thermal shock
resistance were further investigated in near-application crush tests. Steel billets
were partially remelted and positioned on plane circular samples of potential die
materials. Subsequently, a hydraulic press exerted a predefined mechanical
pressure on the billets. The sample surface was inspected macroscopically after
each test cycle. Fifty cycles without damage were set as the target value for
successful testing of the respective samples. Small-scale thixoforging series were
carried out using different types of ceramic dies and inserts in order to evaluate
the performance of silicon nitride in true steel thixoforming conditions. First
experiments focused on several Si 3N4 die segments of C-ring shape that were
mounted in a hot-working steel tool frame (Figure 8.42a). Preheated steel billets
were inserted in the fully assembled tool in the form of a half-shell, followed by a
plane punch pressing the semi-solid steel into the cavity. Additional forming
experiments were performed using Si3N4 plunger tops of different geometry
(Figure 8.42b-d) in the frame of the hybrid tool concept established within the
collaborate research centre. Plunger tops are necessarily made of ceramic materials
since these die parts inherently are subjected to the highest thermal process
loads during immersion into the semi-solid steel.
After testing, the ceramic dies and die segments were inspected visually. Optical
microscopy and SEM were used for fractographic determination of crack origins on
the fracture surface. Wear and corrosive attack on the die surface were examined
using SEM and EDS analysis of cross-sections.
8.6.3.3 Corrosion Mechanisms and Forming Performance
The model tests applied to investigate the chemical interaction of candidate die
materials with semi-solid steels revealed strong differences in applicability and test