Cost effective manufacturing of tungsten heavy alloy foil and sheet material

Transcription

1 Manuscript refereed by Mr Dov Chaiat (Tungsten Powder Technology, Israel) Cost effective manufacturing of tungsten heavy alloy foil and sheet material D. Handtrack, B. Tabernig, H. Kestler, L.S. Sigl PLANSEE SE, A-6600 Reutte, Austria Corresponding author: Abstract The standard manufacturing route for tungsten heavy alloy sheets includes the hot rolling of sintered blocks. With decreasing sheet thickness, the production becomes progressively expensive due to extensive thermo-mechanical treatment and a low yield of prime material. In this paper the development of cost and material effective manufacturing of foil and sheet material by metal extrusion moulding (MEM) is described. The new technology uses proprietary mixtures of tungsten heavy alloy powder and organic binder, which are compounded, extruded and finally sintered to semi-finished products. Sheets up to 4mm thickness and foils down to 90µm can be produced in tight tolerances. The MEM material features a homogenous, isotropic microstructure and exhibits good strength and ductility, even at room temperature. Due to a tungsten content of 90 wt.% and its excellent X-ray absorption capability, the new tungsten heavy alloy material is qualified for radiation shielding applications. Introduction Tungsten heavy alloys (WHA) are metallic composites with a high tungsten content (>90%) bonded by a Ni-Fe-Cu binder phase. Due to their high density and mechanical properties, typical applications of such alloys are balance weights and kinetic energy penetrators. Furthermore, as tungsten heavy alloys exhibit excellent absorption behaviour against electromagnetic radiation, they are also used in shieldings for X-rays and -radiation [1, 2]. The latter applications require WHA material as sheet or foil. The conventional PM fabrication route to make such products is hot rolling of sintered blocks. The thickness is reduced by extensive thermomechanical treatment, i.e. consecutive rolling steps and heat treatments. Decreasing the sheet thickness is inevitably combined with a significant decrease in yield of prime material as a result of inevitable cutting losses. With respect to material and cost efficiency, near-net shape manufacturing offers an interesting cost savings potential for producing WHA sheet and foil material. The fabrication of thin sheets with a thickness below 1.5 mm (preferably below 0.4 mm) by tape casting was reported previously [3]. This technology, typically used for ceramics, was adapted to process tungsten heavy alloy powders. Another way of binder-based and near-net shape manufacturing of WHA sheets and foils is metal extrusion moulding (MEM) [4]. MEM is well known from the fabrication of thermoplastic plates, sheets and foils. It employs dedicated mixtures of metal powder and organic binder, which are extruded, debinded and finally sintered to semi-finished products. In the frame of this work, the metal extrusion moulding process was developed for the commercial tungsten heavy alloy Densimet D176. Foil and sheet materials in the thickness range of 0.09 to 3 mm were produced by the new manufacturing route. The material was fully characterized, i.e. its chemical composition and level of impurities, its microstructure and the resulting mechanical properties were determined. These properties were compared to reference material which was produced via the conventional rolling route. Finally, the applicability for foil and sheet material produced by the new fabrication route was demonstrated. Experimental Fig. 1 is a schematic of the metal extrusion moulding process investigated in this work. In the first step of compoundation, a tungsten heavy alloy powder mixture of Plansee s grade DENSIMET D176 was mixed with a binder to form a homogenous feedstock. DENSIMET D176 comprises 92.5 wt.% of a

2 highly pure W powder (FSSS = 4µm), with carbonyl Ni and carbonyl Fe as balance, with a theoretical density after sintering of th =17.6 g/cm 3. A suitable organic binder was identified by the evaluation and selection of various organic compounds which are typically used for metal injection moulding processes. Due to the special requirements for the processing of WHA by MEM, two proprietary compositions which differ in viscosity were developed for sheet and foil material respectively. The feedstock was pelletized to a granule size of < 1cm. Those granules are filled into a metal extrusion machine, worked and heated up to the process temperature and finally extruded through a die to form a green sheet. The thickness of the green sheet is easily controlled and monitored during production by an online laser thickness measurement device. In the following step, the green sheets and foils are debinded and sintered to nearly full density. Liquid phase sintering was performed in a batch furnace under hydrogen atmosphere at temperatures between C using standard conditions for DENSIMET D176. With this process, foils with thicknesses of 90 µm and sheets up to 3 mm with a length of 400 mm and a width of 120 mm were produced respectively. If required a single cold rolling step of the sintered full-metallic semi-finished products can be performed at room temperature to improve the surface quality or to slightly adjust the final sheet thickness. Fig. 1: Near-net shape fabrication process of WHA by metal extrusion moulding [*2, 4] The material produced via metal extrusion moulding was characterized with respect to chemical composition, microstructure and mechanical properties. The sheet thickness was measured with a micrometre calliper, density was assessed using Archimedes' principle. The residual impurities of C and O were determined in LECO standard testers by combustion analyses and carrier gas hot extraction respectively. The microstructure was characterized on metallographic sections by light microscopy. Vickers hardness was measured according DIN EN ISO , and HV1 and HV10 were used depending on the sheet thickness. Tensile testing was carried out at room temperature, 300 C and 500 C according to DIN EN part 1 and 5 respectively. The flat samples were taken in different orientations from 0.09, 0.8, 1.0 and 2.0mm thick sheets. For comparison, reference data were obtained from conventionally produced D mm sheet material, where a pressed and sintered ingot had been rolled in several steps. Table 1: Foil and sheet thickness of D176 produced by MEM in comparison to the specification for the conventional process (SD.. standard deviation) Results and Discussion Fig. 2 shows WHA sheet material at different stages of the manufacturing process. After mixing the metal powders and the binder components to a homogenous feedstock, granules are pelletized to facilitate handling (Fig. 2a). The granules are then extrusion moulded to green sheet material (Fig. 2b).

3 In this condition, the material is flexible, exhibits a fairly good green strength, and can easily be cut to the required dimensions. The subsequent debinding and sintering removes the organic binder completely, such that a fully metallic material is achieved. Typical examples of sintered WHA sheets in a thicknesses range from 0.09 to 3 mm are illustrated in Fig. 2c. Using the described near-net shape process it is possible to accurately meet the defined thickness target (Table 1). The determination of the thickness variation of the produced sheets gave evidence that the required specification data for WHA material can be fulfilled. The thickness variation inherent to the process is illustrated in Fig. 3 by the examples of a 0.09mm foil and a 0.8mm sheet material. (a) (b) (c) Fig. 2: Stages of WHA thin sheet production by metal extrusion moulding Fig. 3: Thickness distribution of a 0.09mm foil and a 0.8mm sheet D 176 material produced by MEM Independent of the sheet thickness, the D176 MEM material sinters to nearly 100% of theoretical density, with -values between and g/cm 3. The specified chemical composition of D176 is also met. The impurity levels are low, i.e. the residual C and O contents were determined to be less than 30 and 40 ppm respectively. In some batches the C content was below the detection limit of 5ppm. These results being comparable with conventionally produced 0.8mm sheet material (density g/cm 3 ; C=6 ppm; O=18 ppm), confirms the efficiency of the debinding process for the current binder system. (a) (b) (c) Fig. 4: Cross-sections of WHA D176 sheet material manufactured by MEM and the conventional process [6]

4 Figs. 4a,b show typical microstructures of foil and sheet material processed via MEM. The embedding of globular W grains in the NiFe matrix phase is characteristic for liquid phase sintered tungsten heavy alloys. As no rolling step is applied in the MEM route, the microstructure is equiaxed and isotropic. While the tungsten grain size of the 0.8mm sheet material is about 50µm, an adaption of sintering conditions for the 0.09 mm foil material resulted in the formation of finer W grains. In case of application as a shielding material (e.g. for X-ray collimators), the smaller W grains and the reduced intercept length of the NiFe binder are deemed beneficial for a homogeneous absorption behavior. In contrast to the isotropic microstructure of the D176 MEM material, the microstructure of the conventionally rolled D176 sheet exhibits W grains elongated in the rolling direction due to the thermo-mechanical treatment (Fig. 4c). Table 2: Mechanical properties of D176 and the impact of processing (a) Fig. 5: Stress-strain curves of 0.8mm D176 sheet material manufactured (a) via MEM and (b) by rolling [6]. The mechanical properties of D176 foil and sheet material produced via MEM are summarized in Table 2. The apparent hardness of 0.8mm to 2mm sheets ranged from HV10 and hence slightly below the values of conventionally rolled sheets. However, the 0.09mm foil material has a hardness of only HV1. The lower hardness may be due to the different sintering conditions for the foil material which may have caused less solution of W in the NiFe binder matrix. At room temperature the yield strength and ultimate tensile strength of 1 and 2 mm thick MEM material was found to be in the range of MPa and MPa respectively. Thus, the yield strength of D176 sheets produced via MEM is lower and the ultimate tensile strength slightly higher than in sheet material from standard production. With increasing test temperature a comparable decrease in strength was measured irrespective of the applied production route. Within the MEM materials, the foils generally exhibit lower strength and lower ductility than the sheets. We attribute this behavior to different loading conditions in testing (higher plain stress in foils) and to higher stress localization in the binder of foil material, as there are only a few tungsten grains within the cross-sectional area. The impact of the different production routes on the stress-strain curves is revealed in Fig. 5. While the conventional material exhibits a pronounced upper and lower yield strength (Fig. 5b), MEM-sheets (b)

5 display a smooth transition from elastic to plastic deformation. Furthermore, the strength level does not depend on the orientation with respect to the extrusion direction, i.e. there is hardly any strength anisotropy. With increasing test temperature a comparable decrease in strength was found in both MEM materials. The most significant difference between MEM and conventional materials is the ductility at room temperature. The MEM material is significantly more ductile with fracture strains of about 20%, while the conventional WHA-material fails essentially brittle. Inspection of the fracture surfaces of ruptured specimens reveals different failure modes depending on the processing route: the crack path in MEM sheets follows the NiFe matrix and is characterized by ductile dimples, while fracture in the rolled specimens is transgranular fracture through the tungsten grains. An exhausted deformation capability of the binder as result of the work hardening due the rolling process and the refinement of the NiFe matrix seem to be the major reasons for the brittle behaviour of the conventional D176 sheet material at room temperature [6]. With respect to X-ray shielding applications for medical imaging diagnostics, representative demonstrator parts were built from the new D176 foil and sheet material and their function was tested successfully (Fig. 6). In the case of collimators for high resolution X-ray detectors, the new cost efficient foil material is well suited to substitute conventionally employed pure tungsten foils, as the high tungsten content in the WHA in combination with a fine-grained microstructure generate a very homogeneous absorption behavior. On a long-term perspective, structural X-ray shielding parts manufactured from MEM sheet material, provide the possibility of replacing lead, because a potential thickness reduction and the possibility to implement new shielding concepts (e.g. self-supporting structures) seem to be feasible with the new material. Fig. 6: D176 demonstrator parts for 1D X-ray collimators and shielding in medical imaging diagnostics Conclusions Metal extrusion moulding (MEM) was developed as a cost and material efficient manufacturing process for tungsten heavy alloy foil and sheet material. The new technology uses proprietary mixtures of WHA powder and organic binder which are compounded, pelletized, debinded and sintered to fully metallic semi-finished products. In this paper the capabilities of the near-net shape technology was demonstrated for WHA Densimet D176 in the thickness range from 90 µm to 3 mm. The MEM material exhibits a homogeneous, isotropic microstructure which is typical for liquid phase sintering. The strength values determined at room and elevated temperatures are comparable to those of conventionally produced WHA sheet material, though the mechanical properties of 90 µm foils are slightly lower than of sheets. Neverteless, room temperature ductility of the MEM material is significantly better than in rolled material. The MEM materials are well suited for X-ray absorption applications due to their high tungsten content. D176 demonstrator parts were manufactured and qualified for radiation shielding applications in medical imaging diagnostics.

6 References [1] W. Schatt, K.-P. Wieters, B. Kieback, Powder Metallurgy - Processing and Materials, pp , EPMA, Shrewsbury, (1997) [2] R.M. German, in: A. Bose, R.J. Dowding (Eds.), Proceedings of the International Conference on Tungsten and Tungsten Alloys, MPIF, New Jersey, 1992, p. 3 [3] Patent WO A1 ( ). H.C. Starck GmbH. DE [4] Patent WO A1 ( ). PLANSEE SE. AT GM 529/ [5] R.M. German, A. Bose, Injection Molding of Metals and Ceramics, MPIF, Princeton, NJ, (1997) [6] D. Handtrack et. al, Tungsten heavy alloys for collimators and shieldings in the X-ray diagnostics, Proceedings of 18 th PLANSEE seminar on refractory metals and hard materials, (2013), in press