The Science of Forming


 

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Material Selection: The Rest of the Story

By: Daniel J. Schaeffler, Ph.D.

Danny Schaeffler, with 30 years of materials and applications experience, is co-founder of 4M Partners, LLC and founder and president of Engineering Quality Solutions (EQS). EQS provides product-applications assistance to materials and manufacturing companies; 4M teaches fundamentals and practical details of material properties, forming technologies, processes and troubleshooting needed to form high-quality components. Schaeffler, who also spent 10 years at LTV Steel Co., received his Bachelor of Science degree in Materials Science and Engineering from the Johns Hopkins University in Baltimore, MD, and Master of Science and Doctor of Philosophy degrees in Materials Engineering from Drexel University in Philadelphia, PA. Danny Schaeffler Tel. 248/66-STEEL E-mail ds@eqsgroup.com: or Danny@learning4m.com

Friday, December 27, 2019
 


Fig. 1—The Jaws of Life evolved to tackle advanced automotive body structures in extraction of vehicle-crash victims.
Automotive bodies include increasing amounts of higher-strength steels and aluminum alloys to address the challenges of improving fuel economy and safety while reducing tailpipe emissions. Stamping and assembly hurdles are tackled regularly, with the impact of these materials choices reaching beyond the metal forming community and affecting the entire supply chain.

Producing higher-strength steels requires upgrades at the steel mill. The molten steel requires tighter control of alloying chemistry. Hot and cold rolling demand greater rolling mill forces, placing greater strain on mill stands. Achieving improved thickness tolerances requires upgrades in roll profiling and maintenance schedules. Producing advanced high-strength steels (AHSS) often necessitates significant capital expenditures due to the thermal cycle needed to achieve the relevant microstructures.

These grades, once produced, often must undergo additional processing. Shape issues such as flatness, waves and coil buckles require upgraded leveling equipment, especially when working the highest-strength materials. Slitting or blanking may be particularly challenging. In addition to the issues associated with increased material strength, cut edges of advanced grades, with their engineered microstructures, behave differently than those of other grades. Optimal clearances and cutting steels likely will need upgrades or else premature failure will occur.

Cutting-knife sharpness, alignment, maintenance and capabilities demand careful consideration even outside of coil processing shops. After the first form operation, metal stamping companies trim away the binder and addendum, with the scrap collected, bundled and shipped to a scrap processor. These processors further cut, shred and bale these steels. By mid-decade, automotive usage of steel grades with tensile strength of at least 100,000 psi is expected to grow five-fold.

Crash Access

The high strength of alloys used for passenger-cage parts promotes greater occupant safety but doesn’t eliminate the risk of a crash. When crashes do occur with passengers trapped inside, rescue personnel use power tools, including the Jaws of Life (Fig. 1), to cut through sections of the passenger cage such as the B-pillar or roof rail.


Fig. 2—This image of a B-pillar inner reinforcement, formed from a tailor rolled blank, shows regions of differing thicknesses. When extricating vehicle-crash victims, rescue crews must know which areas are easiest to cut. These areas differ with various vehicle models.
When the first Jaws of Life patent was filed, the highest-strength steel available had a tensile strength of about 50,000 psi. Today, passenger-cage parts typically feature steel with 200,000-psi tensile strength, with 300,000-psi tensile grades becoming commercialized. Over the past decade, rescue teams had to purchase new tools capable of attacking these ultra-high-strength sections, while learning vehicle-specific best practices of how and where they should target their efforts.

Early challenges faced by rescue teams encountering ultra-high-strength steels: teeth burning off of reciprocating-saw blades; axes bouncing off of some components; and sparks produced when cutting. Regarding the last, sparks can start a fire, which now brings higher risk given the use of high-voltage batteries in electric vehicles.

In addition to producing B-pillars from ultra-high-strength steels, some automakers save even more weight by using tailor rolled blanks. These variable-thickness blanks allow OEMs to control thickness profiles in the formed components to optimize crash response (Fig. 2). First responders must know vehicle-specific designs to ensure targeting of the thinnest areas when cutting.

The instrument-panel structure in an increasing number of vehicles is made from a magnesium alloy, which allows for significant weight reduction. Magnesium may fracture into pieces when stressed by direct contact from rescue tools, with shards flying near the pinned occupants. Different practices are needed to extricate occupants pinned by the structure.

Accident Repair

Fortunately, most accidents do not require first responders. But different techniques may be needed to repair parts made from newer steel grades. AHSS get their properties from a microstructure produced after a tightly specified thermal cycle. In moderately severe accidents, standard practice had been to flame-straighten certain parts such as rails. Automakers publish manuals for each vehicle stating required repair procedures, and in some cases forbid the use of previously common practices. To avoid using heat, large vehicle sections now may require replacement at factory-engineered locations, rather than repair of a smaller section surrounding the local damage. Remember that welding changes the local microstructure and, therefore, the local properties.

The increasing use of aluminum alloys also promotes downstream changes. Repair shops have incurred significant costs to become certified locations. The certification process typically required them to separate steel-panel repairs from repairs performed on aluminum due to the cross-contamination risk of corrosion (aluminum does not rust, but it can corrode). In addition to performing repairs in separate areas, shops needed to purchase duplicate sets of tools to ensure that no one tool would be used on both types of metal. Fortunately, recent studies have shown that repair costs are comparable, at least partially due to components that have been designed with repairability in mind.

The presence of aluminum also affects scrap handling, as magnets no longer are useful. Aluminum has a much higher scrap value than steel, though only realized only after segregating the different alloys. The alloy used for beverage-can bodies (3XXX series) is not used to produce automotive stampings. Automotive aluminum parts stamped at room temperature are made from 5XXX- and 6XXX-series alloys, with segregation of their scrap streams critical in maintaining their scrap value.

Hear Danny Schaeffler’s presentation, The Role of Engineered Lubricants in Advanced Automotive Body Structure Construction, at the Lubrication Technology for Metal Formers and Die Shops event, sponsored by MetalForming magazine and held in Novi, MI, February 12-13. For more, visit www.metalformingmagazine.com/lubetech. MF

 

See also: 4M Partners, LLC, Engineering Quality Solutions, Inc.

Related Enterprise Zones: Lubrication, Materials/Coatings

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