Thixoforming : Semi-solid Metal Processing

390j 10 Thixoforging and Rheoforging of Steel and Aluminium Alloys
Figure 10.18 Sequence of the tool movement (a) and component
produced by thixojoining (b) with metallography of the contact
surfaces between semi-solid matrix and inserts.
Concerning the forming parameters, the following procedure gave the best results.
After the insertion of the semi-solid billet into the lower die, the forming operation
began by closing the upper die. This was followed by an upward movement of the
plunger with a velocity of 75 mm s 1 to perform the filling of the cavity. In the last
stage of the forming operation, the semi-solid material was solidified under a
compression force of 300 kN (which approximately equals 260 MPa) to avoid typical
material defects due to the solidification. A compaction time of 5 s was applied in the
trials. Finally, the part was slowly cooled to room temperature in air for the
subsequent metallographic inspections. The vertical cross-section of a produced
part (Figure 10.18b) shows good form filling in the area of the flange and the bush.
Also, the slot at the top of the shaft is completely filled with material. Complete filling
is achieved in all parts independent of the materials utilized for the functional
elements. Also, the metallographic analyses show that the functional elements are
completely embedded in a semi-solid matrix. At the contact surface the metallographic
analyses confirm a good form closure between the thixoformed basic
material and the inserts (Figure 10.18b). The two kinds of steel that were utilized
for the shaft do not show any significant influences on the joining quality with the
chosen parameters. The geometry of the bearing bush made of the copper–beryllium
alloy was kept in its original state, whereas the bearing bush made of sinter bronze
showed minimal fusion at the upper edge where the first contact with the semi-solid
X210CrW12 occurred.
A tight mechanical connection between the joining elements and the matrix was
achieved for most of the studied materials and parameter combinations. This shows
that various kinds of joining elements of different materials (higher melting steel,
nonferrous metals, etc.) can be integrated for different purposes such as a screw
thread or a nut as a junction for assembling, a gliding functional element or a