Thixoforming : Semi-solid Metal Processing

398j 10 Thixoforging and Rheoforging of Steel and Aluminium Alloys
Figure 10.27 Micrographs at different positions in a forged part (X210CrW12).
the microstructure is assumed to be a result of solid–liquid separation during the
forming. The average grain size is 70 mm and therefore larger than in other parts of
the workpiece, which can be traced back to the slower cooling in the middle of the
component.
For 100Cr6, homogeneous microstructures are observed all over the part
(Figure 10.28). Between the dark coloured martensite, an area of remaining austenite
(white part) occurs, which originates from the former liquid phase enriched with
chromium and carbon. Due to the transformation, the grain size of 100Cr6 cannot be
determined by conventional microstructure analysis. However, the final structure at
room temperature ( 50 mm) is much finer than that obtained via the thixo-route
( 500 mm).
In detailed observations of local positions of the parts, several defects, for example
incomplete filling and cold welded areas, were found for both steels, which mostly
appeared in the upper head, the flange and the bush (Figure 10.29). These defects are
assumed to be caused by freezing of semi-solid material during the complete filling.
The remaining gaps between the frozen material and die surface can be filled by
mostly liquid material at the end of the process. Both defects are found more
frequently in the parts from X210CrW12 than from 100Cr6.
Figure 10.28 Micrographs at different positions in a forged part (100Cr6).
Figure 10.29 Observed local defects.