Advancements in Near Net Shape Extrusion for Aerospace Applications
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19 ADVANCED MATERIALS & PROCESSES | APRIL 2021 & PROCESSES MATERIALS ADVANCED ADVANCEMENTS IN NEAR NET SHAPE EXTRUSION FOR AEROSPACE APPLICATIONS The aerospace industry could benefit from recent progress in making near net shape titanium extrusions for applications beyond the long structural components of an aircraft. Phani P. Gudipati* and Michael B. Campbell Plymouth Engineered Shapes, Hopkinsville, Kentucky xtruded shapes are often ideal for presents different near net shapes ex- with an elongated grain structure that long aircraft components that re- truded at PES and provides the me- is often recrystallized by hot stretch quire consistent cross sections chanical properties, microstructures, straightening and annealing. The resul- E [1] throughout the length of the part . The and dimensional tolerances consis- tant structure consists of recrystallized most common aerospace extrusions in- tent throughout the full length of the prior beta grains with colony alpha, clude seat tracks produced using the extrusion. which offers an excellent combination workhorse titanium alloy, Ti-6Al-4V of strength, fracture toughness, and fa- (Ti-64). However, major advancements METAL EXTRUSION tigue life[4]. have occurred in the use of extruded The metal extrusion process can titanium shapes deployed in a variety be broadly classified into two main MATERIALS AND PROCESSING of other applications. Uses range from categories—direct and indirect[2]. In Ti-64 billets, with nominal compo- specialized parts in subsonic systems the process described here, the direct sition per AMS 4935[5] ranging from 6.0 to components in unmanned subma- or forward extrusion process was em- to 9.25 in. in diameter, were induction rines that take advantage of titanium’s ployed, where the die and ram are on heated to a temperature above the beta favorable strength-to-density relation- opposite ends and the billet travels in transus and extruded into two distinct ship and superior corrosion resistance. the same direction as the ram (Fig. 1). geometric profiles. These two shapes Billet temperatures used in the extru- Typically, the cross section of the work were selected for their uniqueness in sion of titanium alloy shapes are typi- billet is much larger than the cross sec- terms of geometry and application cally above the beta transus tempera- tion of the extruded part. To relate the (Fig. 2). While the “T profile” is often ture of the material, and the reduction cross section of the workpiece to that of used for aerospace structural compo- ratios are higher than those used in oth- the extruded product, a value common- nents, the shape with more complex er product types. Gross titanium extru- ly called the extrusion ratio was estab- sion, while producing radical savings in lished, which is defined as the ratio of materials due to closer shape approxi- the area of the original billet cross sec- mation, also requires less machining to tion (Ao) to that of the extruded product achieve the finished product. (Af). The extrusion ratio, or reduction In an effort to further reduce over- ratio, can be expressed as (Ao/Af). De- all manufacturing costs and improve pending on final part geometry, a wide the buy-to-fly ratio, the R&D division range of extrusion ratios for extrud- of Plymouth Engineered Shapes (PES) ing different titanium products is avail- has successfully developed an innova- able. Typically, most titanium extrusion tive process and manufactured near presses are water hydraulic systems net shape Ti-64 extrusions, roughly 30 ft with remarkably high strain rates in the Fig. 1 — Forward extrusion process where long, on a production scale that in- range of 10 s-1 or higher[3]. A beta extrud- the ram pushes the hot metal through the cludes various geometries. This article ed titanium billet will yield products die. *Member of ASM International 20 geometric features (NNS-3) is utilized over 30 ft long (Fig. 4). Dimensional sta- of tolerances on key characteristics in aeroengine applications. Dies used bility (uniform dimensions with mini- such as transverse flatness, straight- for the extrusion process are designed mal variation) and straightness of the ness, twist, and angularity along the and manufactured in-house with mod- extrusions play a major role in machin- full length of the extruded part. With re- ified tool steel. Parameters for the ex- ing the finished product. Figure 5 shows duction in the envelope due to near net trusion process are selected based the different dimensions measured for shape profiles, holding these toleranc- on billet size, extrusion ratio, and the each profile and the location of sam- es along the length of the extrusion is of profile to be extruded. These profiles ples for room temperature mechanical paramount importance to successful- are designed for a thickness of 0.150 in. testing and microstructural evaluation. ly machine the final part. Due to space for a planned 30 ft of extruded length, Results of dimensional measurements constraints, only the transverse flatness although typical extrusions can be and variation to nominal planned di- tolerances on the north and south ends up to 40 ft long, depending on initial mensions for both profiles are shown in of the near net extrusions are presented billet size. Figs. 6 and 7. While AMS 2245[6] allows in Table 1. The maximum level of bow Extruded products are then sub- for a deviation of ±0.060 in. on a given and twist observed over the full length jected to hot straightening and an- feature, Plymouth’s near net extrusions of the extrusion are also shown in the nealing, in accordance with AMS 4935, have outperformed the specification table. It can be observed that the aver- to achieve the mechanical properties by presenting a variation less than 50% age values of critical characteristics are and key characteristics such as flatness of the total permissible limit from the not only well within allowable limits, across the width of the part, straight- nominal. but the near net extrusions described ADVANCED MATERIALS & PROCESSES | APRIL 2021 | & PROCESSES MATERIALS ADVANCED ness (bow/camber), and twist along Straightness. Machining of titani- here also offer much tighter tolerance the full length of the extrusion. To en- um extrusions requires extreme control limits than specified in AMS 2245, thus hance machinability, finished extru- making them exceptionally favorable sions are chemically treated to remove for machining. the thin layer of surface alpha case. The Mechanical Properties & Micro- sequence of operations is schematically structure. Specimens for mechanical illustrated in Fig. 3. testing and microstructure were ob- tained from the location indicated in RESULTS AND DISCUSSION Fig. 5. Room temperature mechanical Dimensions. The two selected profiles were successfully extruded to Fig. 4 — Near net extrusions produced in Fig. 5 — Location on the two extruded Ti-64. A sample cut from the T-profile (top) profiles where dimensions were measured Fig. 2 — Geometric profiles of two selected and complex geometrical profile (bottom) and samples for mechanical testing and near net titanium shapes. are beta extruded to over 30 ft long. microstructure are excised. Fig. 3 — Titanium billets are extruded in the beta field using required dies, thermally straightened, surface alpha case chemically removed, and packed for shipment. 21 ADVANCED MATERIALS & PROCESSES | APRIL 2021 & PROCESSES MATERIALS ADVANCED testing on samples in the longitudi- microscope. As observed in Table 2, geometric profiles. In addition, optical nal direction was performed according mechanical testing results are in full micrographs present a microstructure to ASTM E8[7], and optical microscopy compliance with the strength re- resulting from beta processed Ti-64, was performed using a Keyence digital quirements of AMS 4935 for both with colony alpha along the recrystal- lized beta grain boundaries (Fig. 8). Av- erage grain size, measured according to ASTM E112[8], was found to be 100 µm. Longitudinal and transverse surface roughness was also measured using a standard profilometer that resulted in a range of 80-130 Ra for both profiles. Buy-To-Fly. Each preform process such as forging, casting, and plate stock incurs a considerable loss of material during machining due to the excessive material envelope built around the fin- ished product. Extrusions considerably reduce the buy-to-fly ratio and provide advantages not offered by alternative preform processes. With the develop- ment of near net shape extrusions, the buy-to-fly ratio is further reduced, thus Fig. 6 — Deviation from the nominal planned dimensions on the T-profile. Note that the near decreasing the costs associated with net extruded profile exhibits consistent dimensions with minimal variation along the full length machining. of the extrusion, much less than the permissible limits per AMS 2245. It must be stated that the original standard extrusion for a T-profile was already an improvement over its pre- vious preform (plate). Table 3 presents further reduction in the buy-to-fly ratio. Fig. 7 — Deviation from the nominal planned dimensions on NNS-3. Note that the near net extruded profile exhibits consistent dimensions with minimal variation along the full length of the extrusion, much less than the permissible limits per AMS 2245. TABLE 1 — AVERAGE TOLERANCES ON KEY CHARACTERISTICS OF NEAR NET EXTRUSIONS Product type Location Flatness, in. Max. bow, in. Max. twist, deg. T-profile North 0.013 0.055 0.3 Fig. 8 — Optical micrographs from the near South 0.008 net shape extrusions: T-profile (top) and NNS-3 (bottom) indicate colony alpha along NNS-3 North 0.000 0.010 0.7 the recrystallized beta grains. Samples were South 0.000 etched with Kroll’s reagent after polishing. 22 TABLE 2 ROOM TEMPERATURE MECHANICAL PROPERTIES OF For more information: Phani P. Gudi- pati, director of innovation and engi- NEAR NET EXTRUDED PRODUCTS neering, Plymouth Engineered Shapes, Product type YS, ksi UTS, ksi EL, % RA, % 201 Commerce Ct., Hopkinsville, KY, AMS 4935 120.0 130.0 10.0 20.0 42240, 270.839.2064, pgudipati@ T-profile 130.0 146.0 19.0 38.0 plymouth.com.