PurposeThe aim of the paper is the study of the change in the mechanical properties (and in particular in ductility), with the microstructure, of a biomedical Ti‐6Al‐4V alloy produced by different variants of selective laser melting (SLM).Design/methodology/approachTi‐6Al‐4V alloy produced by different variants of SLM has been mechanically characterized through tensile testing. Its microstructure has been investigated by optical observation after etching and by X‐ray diffraction analysis.FindingsSLM applied to Ti‐6Al‐4V alloy produces a material with a martensitic microstructure. Some microcracks, due the effect of incomplete homologous wetting and residual stresses produced by the large solidification undercooling of the melt pool, are observable in the matrix. Owing to the microstructure, the tensile strength of the additive manufactured parts is higher than the strength of hot worked parts, whereas the ductility is lower. A pre‐heating of the powder bed is effective in assisting remelting and reducing residual stresses, but ductility does not increase significantly, since the microstructure remains martensitic. A post‐building heat treatment causes the transformation of the metastable martensite in a biphasic a‐b matrix, with a morphology that depends on the heat treatment. This results in an increase in ductility and a reduction in strength values.Originality/valueThe study evidenced how it is possible to obtain a fully dense material and make the martensite transform in Ti‐6Al‐4V alloy through the variation of the SLM process. The stabilization of the microstructure also results in an improvement of the ductility.

PurposeThe purpose of this paper is the microstructural and mechanical characterization of a biomedical Ti‐6Al‐4V alloy produced by electron beam melting, and the study of the stability of the as‐built microstructure upon heat treatment.Design/methodology/approachTi‐6Al‐4V alloy produced by electron beam melting has been mechanically characterized through tensile and fatigue testing. Its microstructure has been investigated by optical observation after etching and by X‐ray diffractometry analysis. The stability of the microstructure of the as‐built material has been deepened carrying out suitable heat treatments, after an analysis by dilatometry test.FindingsThe microstructure of a Ti‐6Al‐4V alloy produced by electron beam melting has a very fine and acicular morphology, because of the intrinsically high‐solidification rate of the process. This microstructure is very stable, and the traditional thermal treatments cannot modify it; the microstructure changes significantly only when an amount of strain is introduced in the material. However, the mechanical properties of the alloy produced by electron beam melting are good.Originality/valueThe paper provides evidence of the microstructural stability of the material produced by electron beam melting. Even if the microstructure of the as‐built material is not recommended by the specific ISO standard, the related mechanical properties are fully satisfactory. This is a significant indication from the point of view of the production of Ti‐6Al‐4V orthopaedic and dental prostheses by electron beam melting.

Selective laser melting (SLM) is a rapid prototyping technique based on melting and solidification of powder layers to build up a 3-D solid body. [1][2][3] It was developed as a prototyping technology, but is attracting ever-increasing interest as a rapid manufacturing technology for the production, for instance, of orthopedic prostheses made of titanium and cobalt alloys [4,5] and die inserts made of low alloys and maraging steels for the plastics industry. [6,7] Because of the large solidification undercooling, SLM results in the formation of either very fine or metastable microstructures, which sometimes do not find a correspondence in the alternative technologies and in the international standards, as in the case of titanium alloys. [8] An in-depth investigation on the effect of the microstructure on mechanical properties is necessary to evaluate whether or not such a peculiar microstructure is acceptable for practical applications.SLM is used for the production of surgical tools, as well. In this case, a stainless steel combining mechanical strength, wear resistance and corrosion resistance is the best choice. This requirement is satisfied by the precipitation-hardening (PH) stainless steels. Among them, the 17-4 PH grade is a high-chromium steel with a martensitic microstructure (resulting from the transformation of austenite on cooling), which has to be heat treated (solution annealing and aging) to promote the precipitation of Cu phases. [9] In this material, the large solidification undercooling may prevent the formation of martensite in the as-built material, leading to a metastable austenitic microstructure.Metastable austenite may transform into martensite on straining, resulting in the well-known transformation-induced plasticity (TRIP) phenomenon. TRIP is exploited in some steel grades, namely the low alloyed Si-Mn steels as well as the high alloyed Mn-Si-Al steels, to improve their plastic ductility in cold forming. TRIP is also displayed by austenitic stainless steels. Its occurrence depends on the stacking fault energy (SFE) of austenite: [10] on decreasing SFE, deformation occurs by perfect and partial dislocation gliding, by twinning and then by martensitic transformation. With a SFE below 15-20 mJ m À2 , austenite transforms to e and a' martensite, resulting in an increase in strain hardenability and, in turn, ductility.An increase in toughness due to the presence of retained austenite in a 17-4 PH stainless steel was reported by Nakagawa and Miyazaki; [11] up to 30% austenite was stabilized in the microstructure by a special heat treatment. However, a mostly austenitic 17-4 PH stainless steels has never been investigated.In this paper, a study on the microstructure of a 17-4 PH stainless steels, produced by SLM, and on its effect on the tensile properties is presented. The as-built microstructure contains a large amount of metastable austenite. Tensile tests show a large plastic ductility coupled to significant strain hardenability, resulting in a high tensile strength around 1 GPa. The trans...

The paper, through the discussion of an experimental investigation, considers a combined approach based on artificial neural networks and genetic algorithms for structural damage identification. A reduced scale three-storey steel spatial frame was instrumented by a series of 12 accelerometers and progressively damaged by cutting one of its columns just above the first storey. Accelerations induced by ambient vibrations were recorded as the frame was progressively damaged, and the deepness of the cut was taken as the entity of the damage. At every damage level the modal properties (natural frequencies and modal shapes) of the steel frame were evaluated through a neural network based approach. Subsequently, two error functions that measure the differences between the experimental results and those calculated from a finite element model of the steel frame were defined and a genetic algorithm was employed for damage detection. Results of the experimentation (where damage was known as both location and extent) were compared with the results of the optimization algorithm in order to verify its ability to match the actual damage.

Electron Beam Melting (EBM) is attracting large interest among the manufacturers of surgical implants as a near-net shape technology. Titanium alloy Ti6Al4V is widely used in the biomedical field thanks to its high biocompatibility, corrosion resistance and mechanical properties. The chemistry and microstructural features of EBM Ti6Al4V indicate lower machinability in comparison with wrought Ti6Al4V. Aim of the paper is to present a comparison between the machinability of wrought and EBM Ti6Al4V in semi-finishing external turning, by quantifying the effects of the cutting speed and the feed rate. Tool wear, surface integrity, chip morphology and microstructural analysis have been used to compare and assess the machinability of Ti6Al4V delivered in the two conditions.