Countries outside of the United States place restrictions on gross vehicle weight (GVW). In the European Union, for example, driving an ICE vehicle with more than a 7700-lb. GVW requires possession of a commercial driver’s license. However, the limit increases to 9400 lb. if the vehicle has an alternate powertrain. These mandates bring a tradeoff between the range and the payload—especially important for last-mile-delivery commercial vans.

Fig. 1 Governments worldwide periodically modify crash-test requirements in their quests to increase passenger safety. Energy-absorption requirements to pass European New Car Assessment Programme side-impact testing increased 55% in 2020, while 2021 saw an increase of 82% to achieve a good rating in the U.S. Insurance Institute for Highway Safety (IIHS) testing. Within the United States, IIHS roof-strength tests are based on dividing the peak force that a vehicle can absorb by total vehicle weight, with the resultant value required to be greater than 4 for a good rating. As the total vehicle weight increases in the fraction denominator, achieving a passing value requires an even-greater ability to absorb roof-crush energy.

Attacking these challenges requires body structures made from higher-strength metal alloys with sufficient ductility that provides flexibility to form complex shapes and promote part consolidation.

The Steel-Based Approach

Fig. 2 Press-hardening steels (PHS) have been available for several decades, with by far the most common grade known as CR1500T-MB, PHS1500 or 22MnB5. Produced at the steel mill, the grade features a composition of approximately 0.22% carbon, 1.25% manganese and 50 ppm of boron, with a tensile strength of about 600 MPa and a low-enough ductility that limits part-shape complexity. After heating to 900 C and concurrently forming and quenching, the part after processing has a tensile strength of about 1500 MPa. Forming PHS at elevated temperatures allows for production of complex shapes while achieving dimensionally precise parts, as the quench occurs under full press tonnage.

Multiple factors limit applications of CR1500T-MB, with many stemming from the furnace. When heated in air, uncoated steel develops a thick oxide layer known as scale. Scale prevents the underlying steel from reaching the critical cooling rate during quenching, preventing formation of the high-strength metallurgical phase of martensite. Furthermore, post-quench scale removal requires an additional operation such as shotblasting. Even when successful, this leaves the part without any corrosion protection.

Over the past 20 yr., the most common way to avoid scale involved using a 22MnB5 chemistry coated at the steel mill with an aluminum-silicon alloy. The Al-Si coating prevents scale formation and provides a level of barrier corrosion protection. However, optimal use of this coating delivers constraints in furnace-heating rates and laser-welding strategies, with some of these strategies having intellectual-property protections.

Fig. 3 Faster heating occurs when the coating surface absorbs more energy instead of reflecting a portion. Thermoboost, a black primer-paint layer on top of the Al-Si layer, allows for dwell-time reduction to 45% while enlarging the process window. The technology helps provide uniform heating for variable-thickness parts. Another approach uses a primer coating on top of bare steel to achieve faster heating and avoid oxidation within the furnace.

Substituting a zinc coating with no other process changes does not work due to the risk of liquid metal embrittlement (LME), where the low-melting-temperature zinc flows into microcracks produced during forming. This results in very brittle stampings. Process modifications where the majority of the sample is formed at room temperature followed by a thermal cycle reduces the LME risk, but comes with other constraints.

Newer Strategies

…facilitating the use of zinc-coated steels require a slight change in the base-material composition and use of a precooling station. These have the benefit of a reduced critical cooling rate, possibly to the point of the alloy being air-hardened and formable in a servo-driven transfer press.

The ideal solution might be to avoid using a coating altogether. Coating-free PHS grades use a chemistry modification to produce a stainless-steel-like effect that prevents furnace oxidation and provides adequate lifetime oxidation resistance. The result: improved strength and ductility.

Recent years have seen the expansion of grade options with targeted properties (Fig. 2). PHS grades now can reach 2000 MPa using a different composition than CR1500T-MB, with these new grades capable of providing better resistance to passenger-compartment intrusion. Other PHS grades with tensile strength of approximately 1000 to 1200 MPa have greater ductility and allow for improved crash-energy management. Still other grades have even better ductility at a strength of 500 to 600 MPa—these are referred to as press-quenched steels (PQS) as they do not harden during the hot stamping process.

Tailoring properties in a single stamping efficiently place strength and stiffness only where needed, driving mass and cost down in other areas of the stamping. A properties-tailoring strategy in use for several decades: laser-welded blanks, comprised of sub-blanks of possibly different thicknesses, grades and coatings.

Since 2013, laser-welded or monolithic door rings have been used in several cars, trucks and minivans, typically only in front doors. Double-door ring designs now see deployment, produced as extreme-sized stampings. This approach, targeting multi-part integration (Fig. 3), results in part consolidation that changes the economics when attempting to justify its use. For example, around a battery frame, two laser-welded blanks replaced 10 individual stampings.

Part consolidation via patchwork blanks also has been commercially used for a long time. Here, a smaller blank of one set of thickness and grade is spot welded onto a master blank having different properties. Spot welding performed while the blanks lie flat proves substantially easier than attempting to spot weld a formed reinforcement onto a formed master stamping. In addition, spot welding occurs before hot stamping, so the newly formed welds go through the thermal cycle and results in a stronger welded product.

The Aluminum-Based Approach

Some aluminum alloys lend themselves to hot forming, but owing to the vastly different melting temperatures, hot forming for aluminum occurs at much lower temperatures than for steel.

While limited at room temperature, ductility of 7000-series aluminum grades improves dramatically when formed while warm or hot. Stamping AA7075-T6 from the aluminum mill, for example, at a warm-forming temperature of 200 C results in parts with strength below that associated with T6.
Several OEMs commonly accept shock heat treatment or W-temper forming, another aluminum-forming method. After solution heat treatment by heating to more than 430 C and water quenching, some grades exhibit very good room-temperature ductility, but this lasts only for minutes after water quenching. Because W-temper forming occurs at room temperature, expect reduced tool maintenance.

Also, patented Hot Form Quench (HFQ) technology creates aluminum stampings with process steps similar to those used in the press hardening of steel. HFQ uses F-temper 6000- and 7000-series aluminum grades as feedstock—a lower-cost option than other processes because F-temper alloys are not yet heat treated. HFQ enables the forming of complex shapes while improving dimensional precision at T6 strength levels.

All of these parts strengthen after the paint-baking process.

Alternative Cold Stamping Advances

Advanced high-strength steels (AHSS) continue making inroads, with many steelmakers providing options with strength levels of 1200 to 1500 MPa in their cold-stamping offerings. While controlling dimensional precision in these products proves more challenging as compared to press-hardening options, cold stamping offers advantages such as ease of mechanical trimming, avoidance of heating and associated energy requirements, and potentially shorter cycle times. A potential hurdle: press tonnage and energy limitations associated with cold forming of these high-strength grades.

Some 6000-series aluminum alloys can be cold formed and later post-form heat treated, with resulting parts offering strength to 300 MPa after heat treatment. These grades may strengthen further in the paint-baking process, making them comparable to AHSS in terms of lightweighting. MF