Case Western Reserve University has a long record of die materials testing and evaluation done in close coordination with the NADCA Die Materials Committee. At the core of this activity is the Immersion Thermal Fatigue Tester that simulates the conditions encountered by the die material during die casting of aluminum alloys. The effect of composition and heat treating on the thermal fatigue resistance of die steels has been investigated. The results of these studies have been used to develop industry wide specifications for die steels such as the Ford Specification and have contributed to extending the life of die casting dies.
The thermal fatigue test has been employed successfully by Case Western Reserve University for over twenty five years to evaluate a large number of potential die materials for aluminum die casting dies. The sample is processed to the dimensions shown in Figure 1. It is a 2 x 2 x 7 inch rectangular parallepiped specimen with a 1.5 inch diameter hole in the center for internal water cooling. The four comers have a constant 0.010 inch radius that intensifies the predominately uniaxial stress at this location. The test produces considerable constraint and high thermal fluctuations during immersion and removal from the aluminum 380 alloy bath. The experimental set-up is illustrated in Figure 2 and the resulting thermal fluctuations at the specimen comers in Figure 3. The molten bath is maintained at 1350oF and the specimen is immersed for 12 seconds and then removed from the bath for 24 seconds to produce the thermal cycle shown above. The outer surface of the specimen is sprayed with a commercial water-base lubricant just before it enters the molten aluminum bath. Water flows through the central hole at a constant rate of four gallons per minute. The standard procedure is to operate the equipment for 5,000 immersion cycles, measure the cracking pattern and follow this method for 10,000 and 15,000 total cycles. A three inches long center section along the corners, equidistant from each end, is used to measure the cracks at 100X. The crack length is categorized and recorded in 50 micron intervals. The cracking pattern is reported as the average maximum crack length and the summation of the squares of the crack length for each of the four comers. The more severe the crack pattern, the lower the thermal fatigue resistance of the tested material. The results of this test have correlated closely with the behavior of dies in industry.