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Automotive cast parts.

Combining Toughness, Thermal Resistance in HPDC Die Steel

Nov. 15, 2019
New automotive casting objectives highlight the deficiencies in standard tooling materials, but an alternative promises quality, lower cost, longer tool life, and more

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 diecasting (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, and 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.

H13 die steel has a higher alloy content than H11, so H13 will have a slightly better temper- resistance and hot strength, due to precipitation of fine alloy carbides. H11 has a lower vanadium content, which lowers the risk of primary carbide formations, promoting higher toughness and ductility.

The Uddeholm experts’ alternative proposal is Dievar 25 Joules, which they describe as the perfect balance for providing toughness and heat-checking resistance for HPDC dies. Customer feedback and case studies indicate that Dievar delivers excellent results compared to H13 and H11 tool steels when heat checking is the main failure. Lab tests show Dievar has better heat-checking depth resistance than premium H13 grades.

The important material properties for heat-checking resistance are hot-yield strength, temper resistance, creep strength, ductility, and toughness — and Dievar reportedly outperforms premium H13 grades in each respect.

Ductility and toughness are especially of interest: for toughness, Dievar is rated at 25 Joules min./avg.; for ductility, recent testing of Dievar at 44-46 HRC in unnotched testing showed over 400 Joules impact value. High ductility and toughness also facilitate the possible use of a higher hardness level in the die, which contributes to improved heat-checking resistance.

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