Two problems encountered in the extrusion of some alloys are hot shortness, evidenced by circumferential cracking, and die pickup which causes longitudinal scoring on the surface of the extruded product. A major cause of hot-short cracking and pickup scoring is the excessive increase in temperature of the extruded product at its surface due to die and container friction. In the case of unlubricated extrusion, billet shearing along the dead-metal-zone surface also contributes to increasing temperature. The high temperature can result in seizing of small particles of the product to the die surface and subsequent scoring thereby of the extrusion. The high surface temperature (exacerbated by the friction of seized particles scoring the surface) may also exceed the solidus temperature of a low-melting phase (e.g., eutectic composition) in the alloy and cause local melting which results in circumferential cracks when acted upon by tensile stresses developed in the extrusion die.
Pressures developed within the billet can raise the solidus temperatures of the phases sufficiently to prevent melting at these high temperatures. However, when the pressure is relieved near the exit of a conventional die, the temperature may then exceed the solidus at the reduced (atmospheric) pressure and melting may occur. Together with the tensile stresses, the melting would then cause cracking.
In the past, extrusion speeds or ratios had to be minimized to prevent the increased friction and excessive billet temperature increases. Conversely, billet preheat temperatures could be reduced in order to allow a margin for higher extrusion speeds and concomitant larger temperature increases in the billet and extrusion within the die. Unfortunately, this often increases extrusion pressures excessively and extrusion ratios must then be reduced to permit extrusion at all.
In addition to the problems mentioned for hot-short-sensitive alloys, there are problems of die wear, product dimensional accuracy and product surface finish which are prevalent in metal extrusions, particularly the high-strength, high-melting-point metals and alloys. These problems may be reduced by lower temperature extrusion, but again, the extrusion pressure is increased.
Of course, prior references reveal the possibility of cooling a die to avoid higher temperatures therein. For example, U.S. Pat. No. 2,135,193 discusses the problem of pickup and proposes a water-cooled die.
U.S. Pat. No. 3,553,996 teaches a method for extruding brittle materials with a crack-free surface. One embodiment of the method includes the use of a double-reduction die similar to the die proposed herein. However, a relief portion is provided therein between reduction die faces. The material problems therein are different than for the hot-short sensitive materials herein and the disclosure does not address this problem.
German Patentschrift No. 429,376 teaches a method of reducing the tearing in extrusions by cooling the die land and by increasing friction in the die by lengthening the die land and by making the long die land slightly converging towards the exit. This German patent attempts to maximize friction in the die land whereas the present inventors have found the opposite conclusion; that friction should be minimized in order to produce a good product at fast rates and minimal extrusion pressures.