Thixoforming of a high performance HP9/4/30 steel

Abstract

Thixoforming is a forming process that shapes metal components in their semisolid state. Prior to forming, the microstructure of the alloy consists preferably of solid metal spheroids in a liquid matrix, which can be difficult to achieve with hot-worked, high alloy steels prone to strong microsegregation bands. A high performance HP9/4/30 steel has been assessed for thixoformability through a direct remelting route. Partial remelting was carried out between 1430 and 1470 o C. Liquation occurred initially at grain boundaries, then also along the segregation bands. With increasing time and hold temperature, these "columns" broke down into shorter, more equiaxed segments, offering more chance of being thixoformed. Successful thixoforming producing netshape demonstrator parts was achieved at processing temperatures in the range of 1470 -1480°C, which corresponds to approximately 50 to 80% liquid (based on Differential Thermal Analysis). A thin solid skin, formed on the surface of the slug as a result of heat loss, had prevented the slug from collapsing. The resulting thixoformed products are discussed in relation to their load-displacement signals.

Robustness of a Thixoforming Production Line

“…Different studies [4,5,6] have shown that the heating rate and the holding time have an impact on the liquid fraction and on the microstructure of the slug. Figure 2 shows the temperature records at the centre of the billet for two different heating strategies.…”

“…When using the slow heating, the material is kept in the semi-solid state for approximately 40 seconds, but this time is reduced to 10 seconds if we use the fast heating. Omar [5] and Seidl [6] have shown that holding the material at the same temperature in the semi-solid state for some time have an impact on its microstructure, mainly on the size and shape of the grains. The longer the holding time, the more spherical are the grains.…”

“…Tool (5). During this research we used the dies presented in [1] which are used to make a part of car engine.…”

A review of the semi-solid processing of steel

“…They found no significant difference and therefore suggest primary as-cast billet can be used. Omar and co-workers [23,24] have demonstrated that the high performance steel HP9/4/30, which is very difficult to form by other routes, can be successfully thixoformed in the assupplied state. Note that in the as-supplied state the material is already recrystallised and therefore this route does not fit with the 'classical' solid state routes of RAP and SIMA (Strain Induced Melt Activated).…”

Semi-solid Processing of Alloys: Principles, Thermodynamic Selection Criteria, Applicability

“…[11][12][13][14] Up to now, only a few attempts have been made to develop alloys specifically for the particular demands of SSP. 7,15) The influence of variations of the alloy composition on SSP has been studied by Kaufmann et al 16) Before the experimental development work on a particular system starts, thermodynamic calculations have proven useful to check whether the alloy may be expected to meet the requirements of SSP with respect to its phase constitution during solidification.…”