Tungsten Heavy Alloys for Collimators and Shieldings in the XRay Diagnostics D. Handtrack*, B. Tabernig*, H. Kestler*, P. Pohl*, W. Glatz*, L.S. Sigl* * PLANSEE SE, 6600 Reutte, Austria Abstract Due to the excellent shielding against electromagnetic radiation tungsten based materials are typically used for collimators and shielding components in medical imaging diagnostics. However, the conventional manufacturing of pure tungsten sheet and foil material is quite expensive because of high temperature sintering and thermomechanical treatment. A cost and material efficient manufacturing for tungsten based material has been developed. The new technology uses dedicated mixtures of tungsten heavy alloy powder and organic binder, which are formed and sintered to fullmetallic semifinished products. In this work material properties are discussed and the shaping of structural parts is demonstrated. Due to the tungsten content of 90 wt.% the material exhibits excellent Xray absorption. Furthermore the fullmetallic material overcomes possible drawbacks of tungsten filled polymers with respect to aging and degradation under Xray radiation. Sheets up to 4mm thickness and foils down to below 100µm can be produced in tight tolerances. The isotropic microstructure allows the forming of structural parts by roll bending and deepdrawing technologies. Keywords Tungsten heavy alloy, metal extrusion moulding, mechanical properties, Xray, shielding, collimator Introduction Due to its high density and atomic mass, tungsten exhibits excellent absorption behaviour against electromagnetic radiation such as Xrays and radiation [1]. For this reason tungsten based materials are used for shielding applications in medical imaging diagnostics and therapy. In the case of Xray diagnostics, typical applications are collimators in the detector system and shielding parts in computer tomography. Most of these components are sheet based applications with a thickness range of about 0.1mm to 2mm. However, the conventional manufacturing of pure tungsten sheet and foil material is quite expensive because of high temperature sintering and several process steps of thermomechanical treatment. Given by the high competition between suppliers and increased costs of tungsten powder there is a strong drive for a cost and material efficient production of tungsten based material. Another aspect is coming by the possible tightening of regularities (RoHS, REACH and other bans of substances) in medical applications. The discussion about the replacement of the most widely used shielding material lead (Pb) was intensified and activities on the search for substitutes have been significantly increased for
the last 5 years. For example, polymers with Xray absorbing fillings are a considered as one alternative [2, 3]. However, they may have a risk of degradation under long term radiation due to aging. Especially for applications at elevated service temperaturet es or at loading by highh acceleration forces fullmetallic parts made from tungsten or its alloys are still of primary interest. With this respect nearnet shape manufacturing offers an interesting potential for a material and cost efficient e production of tungsten based materials. The fabrication of tungsten t heavy alloy thin sheets with w a thickness of below 1.5 mm (preferably below 0.4mm) by tape casting technology was already reported [4]. In the frame of this work another innovative and binderbased technology, the metal extrusion moulding, was investigated for tungsten heavy alloys. The near nett shape production of sheets and foils for a thickness range of below 0.1 until 4mmm was developed. In comparison too the conventional powder metallurgical fabrication of tungsten and tungsten heavy alloys sheets, produced by pressing, sintering, several rolling steps and thermal treatments, the new technology enables a nearly 100% materials yield and further the t saving of expensive thermomechanical processing steps. In this work the neww tungsten heavy alloy foil and sheet material is introduced with respect to microstructure and mechanical properties, as well as shielding capability against Xrays. Experimental According to a proprietary process [5][ the technique of metal extrusion mouldingg was used to produce fullmetallic tungsten heavy metal alloy foil andd sheet material. The process comprised the mixing of a tungsten heavy alloy powder (PLANSEE gradee DENSIMET D176 with w a content of 92.5 wt.% W and the balance of Ni, Fe) with an organic binder, thermoplastic shaping of o the mixture to a green sheet material by an extrusion process, followed by debinding and sintering. The sintering step was performed in hydrogen atmosphere under liquid phase sintering conditions at a temperaturet e range of 1450 C to 1500 C. Foil material of thicknessess below 0.1 and sheet material of thickness up to 3 mm has been produced respectively (Fig. 1). Figure 1: Tungsten heavy alloy foil and sheet material D176 of thicknesses 0.09 to 3 mm The new foil and sheet material was investigated with respect to microstructure and the resulting mechanical and physical properties. For purpose of comparison, dataa was used from conventionally produced D176 material, where a pressed and sintered ingot is rolledd in several steps to sheet material.
Characterization of the microstructure was performed on metallograp phic sectionss by light microscopy. Tensile testing was carried out at RT, 300 and 500 C according to DIN EN 10002 part 1 and 5 respectively. The flat samples were taken in different orientations fromm 0.8 mm thick sheets. Investigationss of the fracture surface of the ruptured samples by SEMM gave information about the deformation mechanism as a function of processing route and test temperature. The radiation shielding capability of the new material was ested in comparison to conventionally produced tungsten and tungsten heavy alloys at the PhysikalischTechnische Bundesanstalt (Braunschweig, Germany). Using sheet material of different thicknesses the change in Xray beam intensity was measured in a test setup of narrow beam (as shown schematically in Fig. 2) and the linear attenuation coefficient was determined for tube voltages of 50 to t 150 kv. Based on the airkerma, which is the absorbed Xray dose in air, the equivalent lead thickness of the different materials was calculated as a function of the tube voltage [6]. Figure 2: Schematic illustration of the test setup for the measurement of radiation shielding capability In order to evaluate the workability of the new material to final components, fabrication trials comprising bending, roll bending and deep drawing were carried out on D176 sheet material. Demonstrator parts which are representativee for shielding applications in medical diagnostics were successfully realised. Results and Discussion Metallographic investigations on D176 sheet material gave evidence that for all produced thicknesses a nearly full dense material quality (density of 17.6 g/m 3 ) was achieved by the neww production route. The micrograph in Fig. 3a shows the typical microstructure of such a sheet. The W grains have a globular shape and are embedded in the NiFe phase. The microstructure is isotropic andd characteristic for the condition as liquid phase sintered. In contrast to that, the microstructu ure of a conventionally produced, rolled D176 sheet exhibits elongated W grains in the rolling direction due to the applied thermo decreasing thickness lesss W grains are found in longitudinal sections. In order to ensure a homogeneous Xray mechanical treatment (Fig. 3b). Similar resultss were obtained for foil material with thickness of about 0.09mm. However, with absorption the sintering conditions were optimised with respect to smaller W grains and a reduced size of NiFe intersections (Fig. 4). This fact f might be significant for the use of such thin tungsten heavy alloy material for radiation shielding applications, such as Xray collimator foils.
(b) Figure 3: Longitudinal crosssections of 0.8mm thick D176 sheet material manufactured by new and (b) conventional production route Figure 4: Longitudinal crosssections of 0.09 mm thick D176 foil material manufactured by the neww production route The different microstructure of sheet material produced by the conventional and new processing route affected the stressstrain curve of tensile testedd specimens (Fig. 5). While the rolled material exhibited pronounced upper and lower yield strength, sheets of the new processing route were characterized by a smooth transition from elastic to plastic deformation. The strength level was found to be rather irrespective of the orientation to fabrication direction. With increasing test temperature a comparable decrease in strength was found for both materials. The mechanical properties off 0.8 mm thick D176 sheet material are summarized for the t investigated manufacturing routes in Table I. The yield strength of the new material was somewhat lower than thee values of the conventionally rolled material. However, due to higher work hardening of the binder matrix the new material had a higher r ultimate tensile strength. The most significant difference between both materials was the high ductility of the new material at room temperature. Values of elongation to fraction were determined at about 20%, while the rolled material showed a nearly brittle behaviour.
(b) Figure 5: Stressstraincurveroute of 0.8mm thick D176 sheett material manufactured by new and (b) conventional production Table I: Mechanical properties of D176 sheet material with a thickness of 0.8mm: The influence off processing production route hardness YS [MPa] UTS [MPa] f [%] HV10 RT 300 C 500 C RT 300 C 500 CC RT 300 C 500 C conv. rolling (ref.) 315350 850865 465480 420445 845870 680700 595615 0.24 4045 3541 new process 3003300 710750 420435 360380 930950 705730 640645 1722 3542 3032 The investigation of the fracture surface of the ruptured specimens revealed different failure modes dependant on the test temperature and the processing route (Fig. 6). At RT testing specimens from sheets of the new production route failed f predominantly in the NiFe matrix. Thiss is illustrated by a highh proportion of the NiFe matrix (dark coloured inn Fig. 6) in relation to the total fracture area, while only some fractured W grains (bright coloured) are given. The high value of o elongation to fracture is therefore related to the good ductility of the NiFe matrix.. In contrastt the fracture surface of specimens from rolled material is characterized by the transcrystallinee fracture of tungsten grains. g The exhausted deformation behaviour as result of work hardening from thee rolling process and the refinement of the NiFally produced D176 sheet material at RT. At elevated temperature (300 C) the fracture mode for the new material changed significantly. Failure is referred to the transcrystalline fracture of the large W grains. matrix seem to be the main reasons for the brittle fracture behaviour of the conventionac
(b) (c) (d) Figure 6: SEM (BSE) images of the fracture surfaces of tensile test samples (a, c) from new and (b, d) conventional production route, ested at RT and 300 C respectively The results regarding Xray radiation shielding capability are summarized in Table II. Using sheet material with different thicknesses the equivalent lead thickness of Densimet 1766 and tungsten was determined for a tube voltage spectrum of 60150 kv. For sheets withh thickness larger than 0.7mm the leakage radiation at 60 to 80kV was too low forr an exact measureme nt. In case of Densimet 176 specific attention was paid to the impact of the t processing route (conventional rolled vs. new production) on the shielding performance. Under the test configuration, the equivalent lead thickness of tungsten and Densimet is higher than the actual sheet thickness. Table II: Equivalent lead thickness of D176 and tungsten material production process thicknesss [µm] 60kV equivalent lead thickness [µm] for given tube voltage 80kV 100kVV 120kV 150kV new material 430 472 541 680 6933 631 D176 rolled (conv.) new material 540 780 597 690 883 1313 8900 798 1333 1144 rolled (conv.) 1010 1708 1736 1467 W rolled (conv.) rolled (conv.) 400 870 525 594 764 1684 7800 707 1720 1462
Based on these data one can calculate the relative thickness of tungsten and Densimet sheet material to achieve the same shielding efficiency as lead (Fig. 7a). The pure W sheet s material exhibits the best shielding performance. Depending on tube voltage the thickness of W sheets can be reduced up to 50% compared to a shielding made of lead. The decrease of tungsten content to 92.55 wt.% by using a D176 material results in a slight decrease of shieldingg capability in comparison to puree tungsten. However, the heavy metal alloy is still superior to lead, with a potential for thicknesss reduction of about 40% for tube voltages between 100kV and 120kV. (b) Figure 7: Relative thickness and (b) relative area density of tungsten and tungsten heavy alloy D176 in comparison to lead. Testing the D176 sheet material from the conventional and new processing route, no influence on the shielding behaviour was found for the investigated range of tube voltage. The relative thickness and its dependence of the tube voltage were comparable for both D176 materials. The nominal tungsten content in the chemical composition seems to be the key factor for controllingg the shielding capability. The microstructure may only have an impact in the case of thin foil material, where the tungstenn grain size and distribution may affect the homogeneous Xray absorption behaviour. However, testing of the new D176 foil material with a thickness of 0.09mm and an optimized microstructure gave evidence that the level and homogeneity of absorption was comparable to a 0,085mm thick t W foil material. For dynamic shielding applications the absorption behaviourr described by the relative area density is an interesting parameter (Fig. 7b). For this comparison the equivalent lead thickness is normalised by the density of the different materials (W: 19.3 g/cm 3, D176: 17.6 g/cm 3, Pb: 11.34 g/cm 3 ). For a direct substitution of lead a benefit in weight saving can only be expected for tube voltagess between 100 to 130 kv. However, as tungsten based materials offer high stiffnesss and strength, new design concepts can be realized. (b)( (c) Figure 8: Basic trials for shaping of the new D176 sheet material; bending, (b) roll bending andd (c) deep drawing.
In order to evaluate the feasibility off designs, basic experiments for shaping the new material have been carried out. Samples from these studies are presented in Fig. 8. Bending and roll bending was successfully demonstrated on sheets with a thickness in the range off 0.5 to 2 mm. In comparison to conventionally produced material the higher ductility of the new material, especially at RT, proved to be beneficial for shaping geometries. Even deep drawing is possible withh the new material as shown in Fig. 8 c. Figure 9: D176 foils for 1D collimators in high resolution Xray detectors (copyright Siemens S AG) for medical imaging diagnostics. Figure 10: D176 sheet material as Xray shielding components in medical diagnostics. Foil and sheet based prototypes have been produced and passed qualification tests successfully. Typical applications of D176 material as 1Dcollimators in Xray detectors and shielding components in medical diagnostics are shown in the Figs. 9 and 10. Conclusion In this work an innovative, material and cost efficient manufacturing route for tungsten heavy alloy foil and sheet material was developed. The technology established is thee metal extrusion moulding where dedicated mixtures of tungsten heavy alloy powder and organic binder are formed and sintered to fullcondition. Foil material of thicknesses below 0.1 and sheet material of thickness up to 3 mm can be produced within metallic semifinished products. No polymeric binder residuals are left in the final material tight tolerances. The new tungsten heavy alloy y material (Densimet D176: D 92.5 wt.% W, rest Ni and Fe) is characterized by a liquid phase sintered microstructure without anisotropy as seen for conventionally rolled material. In tensilee tests the new material exhibited comparablee strength values, but significantly higher room temperaturee ductility in reference to a Densimet materiall produced by conventional rolling of
a sintered block. Due to the high tungsten content the material exhibits excellent shielding capability against Xray radiation which was expressed by a high equivalent lead thickness in the airkerma test. In comparative testing the D176 sheet material produces by the conventional and new processing route at tube voltages of 60 to 150kV, there was no influence of the manufacturing route on the shielding behaviour found. Foil and sheet based prototypes have been produced and passed qualification tests successfully. The application as 1Dcollimators in Xray detectors and shielding components in medical imaging diagnostics was demonstrated. References 1. W. Schatt (ed.), K.P. Wieters, B. Kieback (coauthor), Powder Metallurgy Processing and Materials, pp. 414416, EPMA, Shrewsbury, (1997) 2. D. Chaiat, High Density Tungsten Powder Filled Polymer, Proceedings of the Powder Metallurgy World Congress and Exhibition, Vol.3, EPMA, Shrewsbury, pp. 361367, (2010) 3. K. Yue et al., A new leadfree radiation shielding material for radiotherapy, Radiat Prot Dosimetry, 133 (4), 256260, (2009) 4. IPR WO2007147792A1 (20071227). H.C. Starck GmbH. DE 10 2006 029 101.8 20060622 5. IPR WO2008003108A1 (20080110). PLANSEE SE. AT GM 529/2006 20060706 6. E.B. Podgorsak (ed.), Radiation Oncology Physics: A Handbook for Teachers and Students, International Atomic Energy Agency, Vienna, (2005)