Vittorio Alfieri

Investigation and Optimization of Laser Welding of Ti-6Al-4 V Titanium Alloy Plates

Titanium alloys are employed for several applications, ranging from aerospace to medicine. In particular, Ti-6Al-4V is the most common, thanks to an excellent combination of low density, high specific strength and corrosion resistance.Laser welding has been increasingly considered as an alternative to traditional techniques to join titanium alloys. An increase in penetration depth and a reduction of possible welding defects is achieved indeed; moreover a smaller grain size in the fused zone is benefited in comparison to either TIG and plasma arc welding, thus providing an increase in the tensile strength of the welded structures.The aim of this work is to develop the regression model for a number of responses which are crucial for the feature of the joint. The study was carried out on 3 mm thick Ti-6Al-4V plates; a square butt welding configuration was considered employing a disk-laser source. A 3-level factorial plan was hence arranged in a face-centred cubic scheme. The responses were analyzed referring to the governing parameters. Then, an optimization was carried out via statistical tools, in order to find the optimal welding set-up for the alloy under examination.Copyright

Dimensional analysis for the definition of the influence of process parameters in selective laser melting of Ti–6Al–4V alloy

This article aims to use the methodology of dimensional analysis to determine the influence of process parameters in selective laser melting of metal powders. The complexity of the selective laser melting process, in fact, does not allow a full evaluation of a physical model, involving all the influencing variables. Through dimensional analysis, the article intends to find out an appropriate definition of a set of non-dimensional groups in order to represent the output parameters. The article, therefore, focuses on the impact of the most influential ones, referring to the production of samples made of Ti–6Al–4V powder using a laser sintering machine EOSINT M270. Specimens are examined in terms of their relative density, given its importance in the design and production of elements for dental implants. Adequate exposure strategies based on the results provided may allow careful designs in order to provide tailored porosity to enhance biological fixation and achieve long-term stability.

Manufacturing of Porous Biomaterials for Dental Implant Applications through Selective Laser Melting

The paper discusses the possibility of manufacturing dental implants through Selective Laser Melting (SLM) of a Ti-6Al-4V alloy powder. Among all possible biomaterials, this alloy is widely used in biomedical applications due to high biocompatibility. Selective Laser Melting allows to obtain biomaterials with peculiar characteristics in terms of porosity gradient, roughness, customized geometry, and mechanical properties. Influence of input process parameters on porosity and analysis of Selective Laser Melting capabilities in implant dentistry have been focused. Porosity is a key parameter in dental implants as it affects stiffness, which is related to Young’s modulus. Ti-6Al-4V bulk material presents a Young’s modulus of 110 GPa, whereas the bone one ranges from 10 to 26 GPa. The relative difference of mechanical properties causes the phenomenon of stress shielding, which has a detrimental effect on the longevity of dental implants. Total porosity is important in reducing the effective modulus of porous metals. Biomaterials specimens obtained during experimental phase have been examined in terms of porosity (in inverse ratio to relative density), microstructure, microhardness and roughness. According to test results discussed in this paper, Selective Laser Melting is proved to be an efficient technology for the construction of Ti-6Al-4V dental implants, because biomaterials with adequate properties can be obtained changing processing parameters. Other fabrication techniques fail to produce biomaterials for dental implants with the desired features.

Experimental analysis of selective laser melting process for Ti-6Al-4V turbine blade manufacturing

The present work focuses on the use of Selective Laser Melting (SLM) technique for manufacturing of near-net-shape aircraft component prototypes with Ti-6Al-4V titanium alloy, which has already successfully employed for the production of turbine blades since it combines mechanical properties with excellent wear resistance. The main characteristic of SLM is layer manufacturing which allows to obtain complex shaped elements using three dimensional computer aided design data, with the addition of particular features like channels or cavities which can not been easily obtained with traditional technologies. The other key aspect in comparison with investment casting is shorter post-processing. The feasibility of manufacturing turbine blades with mentioned process using a laser sintered machine EOSINT M 270 (Titanium version) is analysed. The first experimental phase has dealt with the definition of processing parameters which would guarantee laser sintered part maximum density. Preliminary specimens have been manufactured to define any material-dependent scaling value to control dimensional shrinkage. Afterwards a prototype of a turbine blade has been produced using optimal process parameter set. The element positioning and support definition are discussed as they influence the overall job time and the need of post processing operations. Further analyses have been carried out to check the whole structure of the prototype using X-rays and Fluorescent Penetrant Inspection, aiming to point out possible imperfections; no defects have been detected. Furthermore, laser sintered part dimensional inspection has been successively performed via coordinate measuring machine. Eventually, the microstructure of the prototype has been examined.

Reduction of Surface Roughness by Means of Laser Processing over Additive Manufacturing Metal Parts

Optimization of processing parameters and exposure strategies is usually performed in additive manufacturing to set up the process; nevertheless, standards for roughness may not be evenly matched on a single complex part, since surface features depend on the building direction of the part. This paper aims to evaluate post processing treating via laser surface modification by means of scanning optics and beam wobbling to process metal parts resulting from selective laser melting of stainless steel in order to improve surface topography. The results are discussed in terms of roughness, geometry of the fusion zone in the cross-section, microstructural modification, and microhardness so as to assess the effects of laser post processing. The benefits of beam wobbling over linear scanning processing are shown, as heat effects in the base metal are proven to be lower.

Investigation on Porosity Content in 2024 Aluminum Alloy Welding by Yb:YAG Disk Laser

Aluminum alloy 2024 is extensively used in automotive and aerospace industries, but its application is limited due to the susceptibility to generate porosity during the welding process. Nevertheless, benefits from laser welding are clearly demonstrated. In addition, the use of a disk laser allows to obtain significant reduction in focus diameter and increased beam quality compared to traditional rod or slab lasers. The aim of the work is to discuss porosity formation as influenced by the thermal input provided, so bead-on-plate specimens in different conditions have been prepared. Porosity content is examined in relation to the fused zone extent and discussed considering interaction between laser and material. Higher thermal inputs are beneficial in full penetrative welds.

Additive manufacturing by means of laser-aided directed metal deposition of 2024 aluminium powder: Investigation and optimization:

Directed metal deposition by means of laser beam is investigated in this article. The process is receiving increasingly interest in the frame of additive manufacturing to the purpose of maintenance, repair and overhaul of condemned products when severe conditions hindering the working order have been experienced. Minimal distortion, reduced heat-affected zones and better surface quality are benefited in comparison with conventional techniques. Namely, metal feeding of 2024 aluminium powder is considered to produce clad traces on 2024 aluminium plates, aiming to give grounds for repairing damaged real components using materials with same or similar features with respect to the parent metal. A fibre-delivered disc laser and a three-way feeding nozzle are used. The responses are discussed in terms of geometry, microstructure and microhardness both in the fusion zone and in the heat-affected zone; the optimization is conducted via desirability functions, based on proper technical constraints upon numerical modelling. Reparation of real parts, where cracks are machined to produce V-grooves to be filled, is aimed.

Study on the Factors Affecting the Mechanical Behavior of Electron Beam Melted Ti6Al4V

In this study, a mechanical characterization has been performed on EBM built Ti-6Al-4V tensile samples. The results of tensile tests have shown a different behavior between two sets of specimens: as built and machined ones. Supporting investigations have been carried out in order to physically explain the statistical difference of mechanical performances. Cylindrical samples which represent the tensile specimens geometry have been EBM manufactured and then investigated in their as built conditions from macrostructural and microstructural point of view. In order to make robust this study, cylindrical samples have been EBM manufactured with different size and at different height from build plate. The reason of this choice was arisen from the need of understanding if other factors as the massivity and specific location could affect the microstructure and defects generations consequently influencing the mechanical behavior of the EBMed components. The results of this study have proved that the irregularity of external circular surfaces of examined cylinders, reducing significantly the true cross section withstanding the applied load, has given a comprehensive physical explanation of the different tensile behavior of the two sets of tensile specimens.

Investigation and optimization of disk-laser welding of 1 mm thick Ti-6Al-4V titanium alloy sheets

Ti-6Al-4V joints are employed in nuclear engineering, civil industry, military, and space vehicles. Laser beam welding has been proven to be promising, thanks to increased penetration depth and reduction of possible defects of the welding bead; moreover, a smaller grain size in the fusion zone is better in comparison to either TIG or plasma arc welding, thus providing an increase in tensile strength of any welded structures. In this frame, the regression models for a number of crucial responses are discussed in this paper. The study has been conducted on 1 mm thick Ti-6Al-4V plates in square butt welding configuration; a disk-laser source has been used. A three-level Box-Behnken experimental design is considered. An optimum condition is then suggested via numerical optimization with the response surface method using desirability functions with proper weights and importance of constraints. Eventually, Vickers microhardness testing has been conducted to discuss structural changes in fusion and heat affected zone due to welding thermal cycles.

Disk-laser welding of Hastelloy X cover on René 80 turbine stator blade

Nickel-base alloys, such as Hastelloy X and René 80, are among the most common ones for aerospace applications, due to their mechanical strength at high temperatures and oxidation resistance properties, although processing for missile and space vehicle applications requires extensive fusion and resistance welding for fastening. Laser welding using a Yb:YAG disk laser in continuous mode emission is investigated in this paper for overlap joining of Hastelloy X plates on René 80 samples resulting from waste turbine blades. An explorative study is carried out in order to find an appropriate processing window as well as discussing bead features and common issues. Special fixtures for clamping have been specifically developed and tested. A 3-factors study with power, welding speed and focus position as governing parameters has been arranged; 2 levels have been chosen for each factor. Geometric features, defects and indications are discussed referring to the parameters main effects.