The effect of plastic flow on the strengthening and toughening of alloy during liquid-metal forming under pressure

Liquid-metal forging and liquid extrusion are two typical technologies in the forming of liquid metal under pressure. The mechanical properties of the material processed by the two technologies can be enhanced greatly compared with those for traditional casting technology. In particular, the plasticity and the toughness increase by 10-20 times. The technologies open up a new way for the forming of brittle and difficult-to-deform alloys. However, there is a great difference between the degrees of improvement of the mechanical properties arising from the employment of the two different technologies, the improvement for liquid-metal forging being much less than that for liquid extrusion. The processes of the two technologies are studied in this paper. It is found that flowing solidification (occuring in the liquid-solid zone) and plastic flow (occuring in the solid zone) are the essential reasons which cause the above difference. The corresponding experiments show that the rate of nucleation is increased by flowing solidification and that casting defects are eliminated completely by plastic flow. For liquid-metal forging, the mechanism of strengthening and toughing of alloys is mainly dynamic solidification under pressure, the influence of plastic deformation being slight. For liquid extrusion, in addition to flowing solidification under pressure, plastic flow plays a very important role.