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 | A P R I L 2 0 2 1 2 0 geometric features (NNS-3) is utilized in aeroengine applications. Dies used for the extrusion process are designed and manufactured in-house with modified tool steel. Parameters for the extrusion process are selected based on billet size, extrusion ratio, and the profile to be extruded. These profiles are designed for a thickness of 0.150 in. for a planned 30 ft of extruded length, although typical extrusions can be up to 40 ft long, depending on initial billet size. Extruded products are then subjected to hot straightening and annealing, in accordance with AMS 4935, to achieve the mechanical properties and key characteristics such as flatness across the width of the part, straightness (bow/camber), and twist along the full length of the extrusion. To enhance machinability, finished extrusions are chemically treated to remove the thin layer of surface alpha case. The sequence of operations is schematically illustrated in Fig. 3. RESULTS AND DISCUSSION Dimensions. The two selected profiles were successfully extruded to over 30 ft long (Fig. 4). Dimensional stability (uniform dimensions with minimal variation) and straightness of the extrusions play a major role in machining the finished product. Figure 5 shows the different dimensions measured for each profile and the location of samples for room temperature mechanical testing and microstructural evaluation. Results of dimensional measurements and variation to nominal planned dimensions for both profiles are shown in Figs. 6 and 7. While AMS 2245[6] allows for a deviation of ±0.060 in. on a given feature, Plymouth’s near net extrusions have outperformed the specification by presenting a variation less than 50% of the total permissible limit from the nominal. Straightness. Machining of titanium extrusions requires extreme control of tolerances on key characteristics such as transverse flatness, straightness, twist, and angularity along the full length of the extruded part. With reduction in the envelope due to near net shape profiles, holding these tolerances along the length of the extrusion is of paramount importance to successfully machine the final part. Due to space constraints, only the transverse flatness tolerances on the north and south ends of the near net extrusions are presented in Table 1. The maximum level of bow and twist observed over the full length of the extrusion are also shown in the table. It can be observed that the average values of critical characteristics are not only well within allowable limits, but the near net extrusions described here also offer much tighter tolerance limits than specified in AMS 2245, thus making them exceptionally favorable for machining. Mechanical Properties & Microstructure. Specimens for mechanical testing and microstructure were obtained from the location indicated in Fig. 5. Room temperature mechanical Fig. 2 — Geometric profiles of two selected near net titanium shapes. Fig. 3 — Titanium billets are extruded in the beta field using required dies, thermally straightened, surface alpha case chemically removed, and packed for shipment. Fig. 4 — Near net extrusions produced in Ti-64. A sample cut from the T-profile (top) and complex geometrical profile (bottom) are beta extruded to over 30 ft long. Fig. 5 — Location on the two extruded profiles where dimensions were measured and samples for mechanical testing and microstructure are excised.
RkJQdWJsaXNoZXIy MTYyMzk3NQ==