Papers by Author: Claudio Testani

Abstract: Ti6Al4V is one of the best known and studied titanium alloy for the optimization of the thermo-mechanical treatments. The Ti-forgings represent a valid opportunity for the aircraft manufacturers and designers because of high tensile and fatigue properties. Nevertheless the total-cost reduction of the manufacturing-chain requires both: the ability to manufacture nearer-shaped components by mean of forging-process-modification and less final machining (material scraps). Even if Ti6Al4V is a well known alloy, any process parameters modification introduced still represents a challenge for the metallurgists and manufacturers.The idea, at the base of the present work, has been the feasibility study of forging experiments in the Beta-field using Hot Isostatic Pressed (HIP) powders billets. The preliminary compression tests has been carried out in laboratory and the results have been validated in a industrial Forging-Workshop. The deformation behavior of Ti6Al4V HIPped powders during high temperature deformation tests is reported. Laboratory compression and tensile tests have been coupled with relaxation tests in order to achieve robust data about strain rate sensitivity m-coefficient and activation energy Q.The obtained results have been fitted for the assessment of generalized exponential deformation law. The final result is a “Dorn model” that takes into account and compares all the results from the three different deformation tests: compression, tensile and relaxation. The deformation tests have been carried out at temperatures ranging from 1173 K up to 1373 K and strain rate from 0,01 s^-1 up to about 1 s^-1, trying to describe the high temperature complex shape forging operations.Finally the recorded deformation curves has been used for modeling by means of FEM DeformTM code the deformation process and microstructure evolution by means of an Avrami type law.

Abstract: Ti6Al4V is probably the best known and studied titanium alloy, not only for aerospace applications. Nevertheless the deformation behavior still represents a challenge if any modification in the deformation process is required or introduced. This work deals with deformation behavior description of Ti6Al4V HIPped powders during high temperature deformation tests carried on in the Beta-region. Laboratory compression and tensile tests have been coupled with relaxation tests in order to achieve robust data about strain rate sensibility m-coefficient and activation energy Q. These results have been fitted for the assessment of a more general exponential deformation law. The final result is a “Dorn model” that takes into account and compare all the results from the three different laboratory techniques: compression, tensile and relaxation with a statistical correlation coefficient R_d²=0,78. The deformation tests have been carried out at temperatures ranging from 1173 K up to 1373 K and strain rate from 0,01 s-1 up to about 1 s-1, trying to describe the high temperature complex shape forging operations. The final results has been used and are in use for modeling the forging precursors and dies-shapes to optimize industrial small scale forging tests.

Abstract: Titanium alloys are finding an increasing use in the aeronautical field, due to their characteristics of high mechanical properties, lightness and corrosion resistance. Moreover these alloys are compatible with the carbon fibre reinforced plastics that are also finding a wide use in the aeronautical field. On the other hand the use of these alloys implies some drawbacks, for example titanium alloys are often considered more difficult to form and generally have less predictable forming characteristics than other metallic alloys such as steel and aluminum. In this paper was studied both the microstructure evolution and the mechanical properties of a Ti-6Al-4V rolled bar after hot forging. The thermo-mechanical response of a Ti-6Al-4V alloy was studied in elevated temperature compression tests (CT). Furthermore numerical simulations were carried out in order to do a comparison between numerical data and experimental results. The simulations were carried out using an implicit commercial code able to conduct coupled thermo-mechanical-microstructural analysis of hot forming processes of metal alloys.

Abstract: The external structures of supersonic vehicles are exposed in several critical regions to temperatures that could reach 1000°C. This limit is higher than any safe operative service not only for titanium alloys but also for commercial nickel superalloys. The simplest way to improve titanium and nickel matrices temperature behavior (i.e: strength and fatigue resistance) is to introduce a strengthening phase in its matrix. These class of materials are known as: Metal Matrix Composites (MMC). It is possible reinforce both titanium and nickel superalloys by mean of high temperature resistant ceramic fibers.

Abstract: Titanium-metal-matrix composites (Ti-MMC) are materials with very large specific resistance and potential operative temperature up to 800° C. At present these composites are produced by Hot Isostatic Pressing (HIP), a reliable but expensive manufacturing method. To cut production costs, Centro Sviluppo Materiali SpA (CSM) has developed and patented an experimental plant for co-rolling at high temperature sheets of titanium alloy and silicon carbide monofilaments fabrics. The experimental Roll Diffusion Bonding (RDB) pilot plant permits a reduction of process costs of about 40% with respect to the HIP process. This work reports the results of microstructural and mechanical examinations carried out on composites realized by RDB and HIP. The comparison shows that the fibre-matrix interface is stable in both the composites while the mechanical properties of RDB composite are better due to its smaller grain size and high dislocation density.

Abstract: The paper reports the results of an extensive characterization of the Ti6Al4V-SiC_f composite produced by hot isostatic pressing (HIP) to assess its capability to withstand the in-service conditions of turbine blades operating at middle temperatures in aeronautical engines. The microstructure of composite, in as-fabricated condition and after long-term heat treatments (up to 1,000 hours) in the temperature range 673-873 K, has been investigated by means of different techniques. Particular attention was paid to the micro-chemical evolution of fibre-matrix interface which is scarcely affected also by the most severe heat treatments examined here. This leads to stable mechanical properties as evidenced by hardness, tensile and FIMEC instrumented indentation tests. Therefore, the composite can operate at the maximum temperature (873 K) foreseen for its aeronautical applications without remarkable modifications of its microstructure and degradation of mechanical properties. The mechanical characterization has been completed by internal friction and dynamic modulus measurements carried out both at constant and increasing temperature, from 80 to 1173 K.

Abstract: Titanium-alloy matrix composites (TMC) are nowadays one of the material class with the highest specific resistance from room temperature up to 800° C. Centro Sviluppo Materiali SpA (CSM) efforts have been focused on the developing of an innovative solution to reduce the process costs. The new approach consists in an experimental “diffusion bonding” plant for co-rolling at high temperature sheets of titanium alloy and silicon carbide monofilaments fabrics. The result is a process cost reduction of about 40% respect to HIP process. The experimental pilot plant has been proposed for patent with n° 2006A000261 on may 2006. This paper describes the pilot plant and the process results. The metallographic examination on products shows full bonded samples (100 mm wide and 1500 mm long) obtained in a work field that is at least 100 times faster than that of HIP. High temperature tensile tests have been carried on Roll Diffusion Bonded specimens and the results are reported in comparison with those obtained by Isostatic Pressing (HIP) and Thermal- Spraying (TS) processes on the same composite.

Abstract: Roll Diffusion Bonding (RDB) is a new process, developed at C.S.M., for producing Ti composites reinforced by long fibres. The prototypal “diffusion bonding” plant permits to co-roll at high temperature in superplastic rolling field (under temperature and strain rate control) foils of titanium alloy and fabrics made of SiC monofilaments. This study evidenced that the Ti6Al4V-SiCf composite produced by roll-bonding exhibits superior mechanical properties with respect the same material prepared by Hot Isostatic Pressing (HIP) owing to the smaller grain size and the higher dislocation density.

Abstract: The composite, consisting of Ti6Al4V matrix reinforced by unidirectional SiC fibres (SCS-6), has been investigated by mechanical spectroscopy at temperatures up to 1,173 K. For comparison, the same experiments have been performed on the corresponding monolithic alloy. The internal friction (IF) spectrum of the composite exhibits a new relaxation peak superimposed to an exponentially increasing background. This peak, which is not present in the monolithic alloy, has an activation energy H = 186 kJ mol-1 and a relaxation time 0 = 2.3 x 10-15 s. The phenomenon has been attributed to a reorientation of interstitial-substitutional pairs in the  phase of Ti6Al4V matrix around the fibres. This explanation is supported by the results of micro-chemical characterization carried out by X-ray photoelectron spectroscopy (XPS) combined with Ar ion sputtering.

Abstract: Ti6Al4V-SiCf composite, manufactured by Hot Isostatic Pressing (HIP) at Centro Sviluppo Materiali, has been submitted to long-term heat treatments (up to 1000 hours) at 400 and 600°C. The mechanical properties of the material, in as-fabricated condition and after heat treatments, have been investigated by instrumented indentation (FIMEC), dynamic modulus, tensile and fatigue tests. For comparison some experiments have been carried out also on the monolithic Ti6Al4V alloy. Results show that heat treatments, also the most severe examined here, do not produce remarkable variations of mechanical characteristics. In agreement with the microstructure examinations presented in part I, this behaviour, quite promising for future aeronautical applications, can be primarily ascribed to the stability of fibre-matrix interface.