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 7 RESULTS The method employed for characterizing the flow stress behavior of the samples in this study departs from the tradition- al approach, and an example of the sequence for the tests is pro- vided in Fig. 3. The changes in strain, temperature, and stress as a function of the test time for the 1000°F/min cooling rate samples are shown for both the 3% total strain test (sample 9A-1) and the 6% total strain test (sample 8B-1). Due to cooling limitations within the Gleeble, the 1000°F/min rate was not achieved. Actual cool- ing rate for the tests ranged from 875 to 885°F/min. Figure 3 illustrates the con- stant cooling rate and strain rates that were achieved for the tests. The arrow on the time axis indi- cates the test start for the addi- tional analyses. When plotting the Gleeble test results as true stress-strain, one can more easily see the im- pact of temperature on the strength behavior of the material. In Figs. 4 and 5, the stress and tem- perature for each test as a function of strain are shown. Most interest- ing from these charts are the in- flection points in the stress-strain curves. Notice that the sample in the 3% total strain test illustrates strength increases at just below 800°C (Fig. 3). This corresponds to a drastic change in the CTE of Ti-6Al-4V at that temperature (see Heat Treat 2021 presentation). In addition, these curves illustrate a second inflection point at just be- low 700°C where the strength of the material increases more rapid- ly. This shows the combined effects of strain hardening and loss of duc- tility due to temperature decrease in these curves. Such combinatorial effects are representa- tive of forging conditions where a material is deformed as it continues to cool. In the 6% total strain test (Fig. 5), the higher tempera- ture inflection point (at around 800°C) is not present. This further supports that the 800°C inflection point in Fig. 4 is 8 Fig. 4 — Sample 9A-1. Flow stress curve for 1000°F/min sample subjected to a total strain of 3%. Temperature is shown along with stress values, and the inflection points in the stress curve are indicated by arrows. Fig. 5 — Sample 8B-1. Flow stress curve for 1000°F/min sample subjected to a total strain of 6%. Temperature is shown along with stress values, and the inflection points in the stress curve are indicated by arrows. 9