Page 25 - MetalForming Magazine June/July 2022 80th Anniversary Issue
P. 25

 Cutting Edge
By Eren Billur, Ph.D.
Forming of Heat Shields
for the Automotive Industry
  Internal combustion engines, albeit losing their once top position in the automotive industry, generate excessive heat—around the engine and its exhaust components. If not properly insulated, high temperatures may cause thermal damage to body com- ponents, electronics and internal trim. Typically, an embossed and formed heat shield (Fig. 1) is used for protecting the rest of the car from excessive heat.
Most heat shields are stamped from 0.3 to 0.5-mm, 1000- or 3000-series alu- minum; when double sheets are used, sheet thickness may be as low as 0.2 mm. 1050-O or 3004-O materials are delivered as coil, then roll-embossed to have hexagonal or hemispherical bulges. Embossing adds stiffness to the extremely thin sheet, while also increas- ing surface area and reflectivity to improve heat dissipation and reflectiv- ity. However, embossing also hardens the sheet, possibly detrimental to the second forming operation. Once an O- temper sheet is embossed, it should be renamed as H114.The ratio of bulge height to diameter affects the amount of thinning and hardening that occurs. Embossing can improve formability,
Eren Billur is the founder of Billur Metal Form, a consulting, engineering and training company in Ankara, Turkey. He stud- ied at Baskent University and Virginia Common- wealth University, received a Ph.D. in Mechanical Engineering from The Ohio State Uni-
versity, and worked as a researcher at the Center for Precision Forming. His areas of expertise include material characterization, sheet metal forming processes and finite element simulations. He has authored/co-authored more than 20 scien- tific papers (including proceedings) and con- tributed to four books, including “Hot Stamping of Ultra High Strength Steels,” published in 2018. Eren Billur
Billur Metal Form, Founder eren@billur.com.tr
Fig. 1—An embossed and formed heat- shield such as this protects vehicles from excessive heat.
but only if the geometry is optimized. During the second metal forming step, blanks are cut from the embossed coil. Here, cut-to-length rectangular blanks can be used to reduce costs; a blanking die may be required for some geometries. The blanks then transfer to a forming press. To reduce costs, crash forming—requiring only a punch and a die—is preferred, rather than draw forming, which requires a
blankholder.
Challenges During Heat-Shield Manufacturing
Wrinkles nearly always appear when crash forming thin sheet. However, because heat shields typically are unex- posed and their primary duty is not visual but thermal, automakers accept some wrinkles. The critical issue: The height of the wrinkles must not cause any problems during assembly.
During sheet metal forming, wrin- kles occur due to low stiffness of the sheet and compressive stresses during forming. Luckily, embossing increases sheet stiffness and, therefore, helps to reduce wrinkling (Fig. 2). If wrinkles cause problems during assembly, draw forming may be required rather than crash forming, using either a force- controlled blankholder or local pads, depending on part geometry.
Fig. 2—Embossing reduces wrinkles in addition to providing other thermal and mechanical benefits. Shown: a heat shield embossed and formed (right), and the same heat shield formed without embossing. Photo courtesy of TKG Automotive.
Edge cracks occur during forming of stretch or shrink flanges. If edge cracks occur only occasionally, the metal former may scrap any cracked parts. However, frequent cracking requires changes to the forming die or the embossing ratio, whichever is acceptable to the customer.
Local necks can create issues related to noise, vibration and harshness. In rare instances, they can cause splits after some time in service, due to vibra- tion. They commonly appear around sharp radii. Compared to body-in- white parts, necking may be easier to spot on a heat shield. Before necking initiates, it is common to see a flatten- ing of the embossment. The part may be acceptable if there is no risk of split.
Overlaps occur when wrinkles are flattened between the punch and die. After trimming, sharp edges may occur around the part, which can cause safety hazards as well as geometric problems during assembly. Metal formers can scrap parts with overlaps, or rework them.
How Simulation Helps
To investigate simulation of heat- shield production, we modeled the roll- embossing and crash-forming process-
  22 MetalForming/June/July 2022
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