New automotive casting objectives highlight the deficiencies in standard tooling materials, but an alternative promises quality, lower cost, longer tool life, and more
Excerpt from the Foundry Management & Technology November 2019 article
The difficulties of casting high-volume automotive parts are well established, and those issues become more critical in production of critically engineered parts, such as the increasing volume of parts produced by high-pressure die casting (HPDC.) Three experts in hot-work production addressed the issues recently in a white paper for Uddeholm AB's newly developed Uddeholm Dievar 25 Joules tool steel for die manufacturing. It gives "the perfect balance between toughness and heat-checking" for HPDC, and other applications, they claim.
Most dies used by foundries, diecasters, and OEMs are formed in AISI H13 or H11, but the Uddeholm experts raise the concern that the die-related problems of HPDC powertrain and transmission castings have not changed in decades, and may be more acute with the advent of production for hybrid and electric vehicle castings.
Four main failures in HPDC dies are identified: erosion, soldering, heat checking and gross cracking, and the most common of these is "heat checking" — thermal fatigue that leads to material cracking. It is just as common for HPDC of structural and e-mobility castings as for more standard automotive parts, but heat checking may occur sooner and more severely in automotive structural casting than for more traditional castings. "Often a die made in H13 for powertrain parts has to last around 80,000-150,000 shots (depending on design and press), but for structural parts this can be under 75,000 shots," they noted.
For example, some shock-tower dies may last fewer than 30,000 shots due to their design, complex geometry, and how they are used in operation. Some will crack prematurely if the die is used improperly, in addition to the die material being insufficient to the task. Poor die performance is a concern for all structural parts, which show increasingly complex designs and push die materials to their performance limit.
In an example demonstrating their point, the Uddeholm experts described a longitudinal bridge part and die involving a very large surface area with many thin and thick sections. Quality and performance of the finished castings underscore the importance of the injection process to avoid porosity and other internal defects. So, gate speeds often are high to fill the die as fast as possible and a typical structural part die may have many more gates than the traditional powertrain die. And so, extra heat is generated in the gates. This combined with the general heating and cooling of the casting cycle along with spraying of the die, induces high levels of thermal fatigue (i.e., heat checking). Cracks in the die may be a cause of reduced casting reliability.
Another trend influencing the performance of die materials is larger casting designs, which involve larger presses and larger tooling inserts. Larger dies increase the risk of cracking. The die steel material must address the main problem of heat checking but also must be very durable and ductile in operation.
Greater temperature differences coupled with full production will increase thermal fatigue and reduce die life. The heat-checking pattern on a die surface will mark the castings, reducing casting appearance and performance.