AMP_06_September_2021

FEATURE 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 4 5 D uring forging operations, strain in the forged part can occur through three primary mechanisms. Strain due to load applied through dies is easily predicted from first principals and materials data captured at controlled temperature and strain rate conditions. A second source of strainduring forgingoperations is due to thermal contraction of the cooling part. Deformation due to thermal contraction is typically predicted from thermophysical materials data. A third source of strain during forging includes strain due to creep. Creep strain can occur in forgings while in the furnace, during forging, and during cooling. Such strains can be es- pecially difficult to predict during cooling because typically creep testing is not conducted at changing temperatures. There is value in determining the flow stress behavior of ductilemetals during cooling at a range of total strain values. This article examines the flow stress behavior of a common aerospace engineering alloy, Ti-6Al-4V, at low strain rates during continuous cooling. Ti-6Al-4V MATERIALS DATA The flow stress behavior of Ti-6Al-4V has been heavily studied. There are excellent sources of data that capture isothermal, constant-strain-rate testing including the Atlas of Formability [1] and others [2,3] . In addition, creep data for Ti-6Al-4V are plentiful [4,5] . The challenge lies in linking these data sets and predicting behavior in non-ideal conditions. GLEEBLE MEASUREMENT OPPORTUNITIES The Gleeble 3500 is designed for flow stress testing with precisely controlled heating, cooling, and deforma- tion conditions. Much of the research conducted using Gleeble instruments focuses on isothermal deformation testing. Here the Gleeble is used in a less traditional ap- proach to capture the flow stress behavior during cooling and at strain rates that are comparable to creep strain or- der of magnitude. NONTRADITIONAL FLOW STRESS TESTING OF Ti-6Al-4V: CAPTURING CRITICAL TRANSITIONS IN MATERIAL BEHAVIOR DURING COOLING Creep strain behavior during cooling was investigated by physical simulation, giving insight into the relationship of flow stress behavior and microstructure as a function of temperature and cooling rate. Kevin J. Zhang, Chase Sheeley, and Lesley D. Frame* Center for Materials Processing Data, Materials Science and Engineering, University of Connecticut, Storrs METHODS Ti-6Al-4V samples were supplied by Weber Metals from a single 9 in. diameter billet. Care was taken to select sample locations that were uniformly deformed during bil- let forging. Specimens with 10 mm diameter by 152 mm lengthwere used for tensile tests using a Gleeble 3500 ther- mal-mechanical simulator (Fig. 1). Type R thermocouples welded to the specimen center were used for temperature feedback control (Fig. 2), and all tests were conducted in *Member of ASM International Fig. 1 — Image of sample installed in the Gleeble 3500 with a Keyence optical micrometer assembled. Fig. 2 — Prepared sample with thermocouple wires and cement attached. 7