Characterization of Formability and Friction during Hot Stamping of Al-Si Coated Press Hardened Steel

Abstract

This thesis investigates the friction and formability characteristics of Al-Si coated press hardened steels (PHS) during direct hot stamping. The PHS grades examined were PHS1800 and PHS1500, with nominal ultimate tensile strengths after hot stamping of 1800 and 1500 MPa, respectively. Both grades were received with a coating weight of 150 g/m2, designated PHS1800-AS150 and PHS1500-AS150, while a coating weight of 80 g/m2 was also tested for the PHS1500, designated PHS1500-AS80. Testing was done under conditions representative of hot stamping. Friction characterization using the twist compression test (TCT) was performed considering sliding speeds in the range 10 to 38 mm/s and contact pressures of 5 to 30 MPa. Sliding speed did not have a significant impact on the coefficient of friction (CoF). The average CoF of PHS1800 increased from 0.3 to 0.4 over the range of contact pressure considered. At a constant sliding speed of 20 mm/s and a contact pressure of 30 MPa, the PHS variants ranked in increasing order of average CoF were PHS1800 at 0.41, PHS1500-AS150 at 0.46, and PHS1500-AS80 at 0.48. The effect of tooling wear was examined by repeating ten friction tests using the same friction cup with new PHS specimens. No significant change in the tooling surface roughness and CoF was recorded for the limited number of repeat tests considered. Formability characterization was performed using a hemispherical Nakazima punch, as well as flat Marciniak and Hybrid punches, which both employed a carrier blank. The Nakazima punch resulted in faster cooling at the center of the blanks (relative to the periphery) leading to failure iv near the die entry radius under plane strain conditions for all sample geometries and lubrication conditions. The Marciniak and Hybrid cup tests avoided direct contact between the blank center and punch, resulting in central failures with linear strain paths spanning from uniaxial to equibiaxial strain conditions. All of the formability tests achieved necking limit strains with major strain values above 0.35. In the Marciniak test, the PHS1500 exhibited higher limit strains than PHS1800 with both PHS1500 coating variants having similar limit strains. Among all PHS sample gage widths, the strain rate at the center increased while the cooling rate remained relatively constant and varied from 21.8 to 24.5°C/s. LS-DYNA, a commercial finite element solver used to model the hot stamping process, predicted the deformation behavior of PHS1800 sample geometries in Nakazima, Marciniak, and Hybrid formability tests. The coupled thermomechanical model featured an isothermal heating phase followed by a formability phase with deformation and quenching occurring simultaneously to accurately represent hot stamping conditions. The numerical results predicted the necking locations and strain paths for each formability test, including the effect of local cooling rate.