AMP_06_September_2021

A D V A N C E D M A T E R I A L S & P R O C E S S E S | S E P T E M B E R 2 0 2 1 2 6 native to high-strength aluminum al- loys while improving strength, safety, durability, and longevity. Table 1 provides a comparison of the density (ρ), modulus of elas- ticity (E), specific stiffness (E/ρ), yield strength (YS), specific yield strength (YS/ρ), ultimate tensile strength (UTS), specific ultimate tensile strength (UTS/ρ), and total elongation (El) of the Cold-Stamp-Steel detailed in Fig. 1 with high-strength 6082-T6 and 7075-T6 aluminum alloys. Battery enclosures made of the elec- tro-galvanized ColdStamp-Steel sheets of 0.04-in. (~1.0-mm) thickness can sub- stitute for the same or higher weight battery enclosures made of 6082-T6 alu- minum alloy sheets of 0.2-in. (~5.0-mm) thickness, while stiffness, strength, and durability of the steel enclosures are significantly higher. Battery enclosures made of the hot- dip galvanized or galvannealed Cold- Stamp-Steel sheetsof0.04-in. (~1.0-mm) thickness can substitute for the same or higher weight enclosures made of 6082- T6 aluminum alloy sheets of 0.12-in. (~3.0-mm) thickness. The strength and durability of steel enclosures are higher. ColdStamp-Steel, coated by elec- troplating or hot-dip processes, offers certain benefits for use in battery en- closure applications. The hot-dip gal- vanized and galvannealed Cold- Stamp-Steel can prevent failure of bat- tery enclosures at temperatures up to 1000°F (538°C), as well as withstand fire up to 2200°F (1205°C), allowing more time for EV passengers to evacuate in case of an emergency. The durability of battery enclosures made of aluminum alloys cannot compete with the dura- bility of enclosures of the same weight made of coated ColdStamp-Steel at ambient and elevated temperatures. BATTERY ENCLOSURE DESIGN The proposed design is based on a concept wherein the battery enclosure is an integrated part of the car body-in- white (BIW) and the battery is placed di- rectly in the BIW. This design allows for increased stiffness, strength, and dura- bility of the entire BIW, as well as bat- tery safety. ColdStamp-Steel can be used as the primary material for the new en- closure design. The enclosures made of this steel have the following advantages compared to high-strength aluminum alloys, including the strongest 7075-T6 aluminum alloy: the same ambient temperature and significantly higher elevated temperature corrosion resis- tance; better weldability; the same or less weight; the same or higher specific stiffness and strength; safety of EV pas- sengers in case of overheating or fire; and 40-50%material cost savings. Concurrently, a comparison of ColdStamp-Steel with the commercial high-strength steels demonstrates its combinationof higher strengthandduc- tility, weldability by spot welding, and galvanizing (including electro, hot- dip) and galvannealing. Several enclosure designs made of the hot-dip galvanized and galvannealed ColdStamp-Steel sheets with different thicknesses have been computer modeled. An example of the optimum design with low weight and high stiffness and durability is de- scribed below. The enclosure’s dimensions are 5 x 50 x 65 in. (127 x 1270 x 1651 mm). It consists of a top cover made by cold stamping from the 0.025-in. (0.635-mm) thickness sheet of approximately 23 lb (10.5 kg) weight; a crash structure made by welding from the 0.04-in. (~1.0-mm) thickness sheet of approximately 36 lb (16.4 kg) weight; a housing tray made by cold stamping from the 0.025-in. (0.64-mm) thickness sheets of approxi- mately 30 lb (13.6 kg) weight; a battery frame made by stamping and welding from the 0.04-in. (~1.0-mm) thickness sheet of approximately 34 lb (15.5 kg) weight; a cooling system made of the 6082 aluminum alloy of approximate- ly 12 lb (5.5 kg) weight; and a bottom cover made by cold stamping from the 0.025-in. (0.64-mm) thickness sheet of approximately 23 lb (10.5 kg). The enclosure has a total weight of approximately 158 lb (72 kg), which is similar to enclosures made of alumi- num alloys. At the same time, the en- closure features strength, stiffness, durability, and lifetime that are signifi- cantly higher than any high-strength aluminum alloy enclosure. The cost of 146 lb of galvanized ColdStamp-Steel and 12 lb of 6082 alu- minumalloy used for the steel enclosure TABLE 1 — PROPERTIES OF COLDSTAMP-STEEL COMPARED TO HIGH-STRENGTH 6082-T6 AND 7075-T6 ALUMINUM ALLOYS Materials ColdStamp-Steel 6082 7075 Heat treatment Q+LT Q+HT T6 T6 ρ lb/in 3 g/cm 3 0.282 0.282 0.098 0.102 7.800 7.800 2.710 2.810 E ksi MPa 28100.0 26400.0 10152.7 10442.7 193742.5 182021.4 70000.0 72000.0 E/ ρ ksi/(lb/in 3 ) MPa/(g/cm 3 ) 99718.6 93685.8 103699.0 102866.1 24838.8 23336.1 25830.3 25622.8 YS ksi MPa 217.0 170.0 37.7 73.0 1496.2 1172.1 260.0 503.0 YS/ ρ ksi/(lb/in 3 ) MPa/(g/cm 3 ) 770.1 603.3 385.2 718.6 191.8 150.3 95.9 179.0 UTS ksi MPa 246.0 182.4 45.0 83.0 1696.1 1257.6 310.0 572.0 UTS/ ρ ksi/(lb/in 3 ) MPa/(g/cm 3 ) 873.0 647.3 459.2 817.2 217.4 161.2 114.4 203.6 El % 5.1 9.7 8.0 7.0