Characterization of heat transfer coefficient for non-isothermal elevated temperature forming of metal alloys

To accurately model warm and hot forming operations, correct values for heat transfer coefficient (HTC) are required, among other parameters. In this work, an experimental methodology is proposed to obtain HTC data for a high strength steel alloys (Usibor® 1500 A-S), seven aluminum alloys (AA5182-O, AA5754-O, AA6013-T6, AA6063-T6, AA6061-T6, AA7075-T6 and a developmental AA7xxx-T76) and two magnesium (AZ31B-O and ZEK100-O) alloys. The HTC was characterized as a function of contact pressure from 2 to 80 MPa. Overall, the HTC values exhibited significant variation (ranging from 355 to 1887 W/m2-K), but tended to correlate primarily with contact pressure and material strength/alloy system. Two lubricants were also considered (Fuchs Forge Ease 278 and PTFE spray) for some alloys, but had only a minor effect on HTC relative to “dry” tooling. Neither the initial temperature of the tooling nor the ramping up of the press tonnage had a noticeable effect on the HTC since the HTC values were identified based only upon the relative temperature difference and thermal properties of the materials. The experimentally determined HTC values were then applied in a series of non-isothermal cylindrical cup warm deep drawing experiments. The deep draws were modelled using finite element (FE) techniques, which utilized the measured HTC values. The temperature-time profiles from the deep draws were found to agree well with the predicted profiles, indicating that the methodology used to determine HTC and the resulting HTC values, are applicable for use in a metal forming simulation involving heat transfer.