Key Engineering Materials Vol. 813

Abstract: The contribution of the present work is to experimentally evaluate the wettability of Ti6Al4V solid surfaces manufactured via Electron Beam Melting (EBM), an innovative Additive Manufacturing (AM) process. The wettability behavior was quantified via goniometer, measuring the contact angle between a carefully deposited liquid drop (distilled water) and suitable solid surfaces. In order to study the influence of orientation and location in the build chamber on such properties several specimens were EBM manufactured with different growing direction.

Abstract: β-Ti (Ti–13Nb–13Zr) alloy was subjected to ultrasonic shotpeening (USSP) and a nanocrystalline layer of ~60 µm thickness was developed on the metastable Ti-13-Nb-13Zr alloy. In this investigation, the surface hardening and low cycle fatigue (LCF) behavior of the alloy were studied after USSP treatment. Compared to the un-shotpeened samples, the shotpeened specimens exhibit high surface hardness and an enhancement in fatigue life. A notable impact of USSP on the fatigue crack initiation and growth of the alloy was also observed. The results show that the crack initiation at free-surface was suppressed due to the formation of a nanograined microstructure and fatigue crack initiation site shifts from surface to inside of the material. Further, the microstructural analysis proves that the nanograin formation and compressive stresses imparted by ultrasonic shot peening treatment are helpful in significant improvement of fatigue life.

Abstract: Metal Additive Manufacturing technologies development is increasing in a remarkable way due to their great potential concerning the production of complex parts with tailored characteristics in terms of design, material properties, usage and applications. Among all, the most widespread technologies are the Powder Bed Fusion based technologies such as Selective Laser Melting and Electron Beam Melting. However, the high surface roughness of the as-built parts still represents one of the major limitations, making necessary the adoption of post-process finishing to match the technological requirements for most of the fields of application. In this scenario, Fluidised Bed Machining represents an emerging finishing technology that could overcome some of the limitations of the most common methods, especially in terms of feasibility for the treatment of complex parts thanks to the fluid-like mobility of the abrasive material. This work deals with the preliminary tests of the Fluidised Bed Machining of additive manufactured samples using alumina as the abrasive material, investigating the effects of a high abrasive/substrate hardness ratio condition. The experiments were carried out on small plates of AlSi10Mg alloy made through Selective Laser Melting technology, built in the vertical direction with respect to the building plate. The influence of the impact angle and treatment time were investigated under bubbling fluidization conditions. Surface morphology evaluations were carried out pre and post process by means of Confocal Microscopy and Scanning Electron Microscopy (SEM). Weight loss measurements were conducted to evaluate the material removal rates as well. Results show a small influence of the specific impact angle, a slight reduction of the surface roughness and an asymmetrical effect of treatment, acting mostly on the sintered powders forming the peaks of the as-built surface.

Abstract: TiN coatings are widely used in different applications for extending the lifetime of components due to their high hardness and good wear resistance. However, it is not convenient to deposit them on soft stainless steels. In this work, the wear and corrosion behavior of commercial TiN coatings deposited by Arc-PVD on nitrided and non-nitrided martensitic stainless steel was studied. Two different nitriding conditions were used, one at high temperature (HTN) and the other at low temperature (LTN). Nanohardness and microhardness were measured. The microstructure was characterized by OM, SEM, XRD and XPS. Pin on disk and erosion tests were carried out in order to evaluate their wear resistance. The corrosion behavior was analyzed in salt spray fog and electrochemical tests in NaCl solution and the adhesion was measured in Scratch and Rockwell C Indentation tests.The coating thickness was about 1.5 µm and its hardness of 34 GPa. The nitrided layers were 13 µm and 17 µm thick for LTN and HTN, the hardness was approximately 12 GPa for both nitrided samples. The nitrided layer improved TiN coating adhesion in the Scratch tests. The wear loss volume was similar for both duplex and only coated samples in pin on disk tests. Nevertheless, wear resistance was not good for the LTN or HTN + TiN coating system in the erosion tests. Regarding corrosion behavior, the coatings showed poor corrosion resistance and this could be related to the presence of porous defects, which allow the solution to reach and attack the substrate, thus producing coating detachment around the pits.

Abstract: Semi-crystalline polymers such as fluoropolymers are extremely difficult to coat with using solid-state deposition technique such as cold spray (CS) due to its high viscoelastic-viscoplastic behavior. Generally, fluoropolymers solid-state deposition using CS are characterized by a relatively low deposition efficiency (DE) as compared to metallic materials using this deposition technique. In this article, the study on the effect of hydrophobic fumed nanosilica (FNS) and fumed nanoalumina (FNA) in Perfluoroalkoxy (PFA) solid-state deposition using CS has been studied. This study incorporated certain parameters related to the CS process. In addition, powder modification technique using hydrophobic FNS and FNA as additive to feedstock has been studied as well. The results show a number of parameters affected the DE; particle size, traverse speed, gas temperature, and addition of hydrophobic fumed nanoparticles, indicated a better DE. Moreover, PFA coating produced in this manner, retained its hydrophobicity.

Abstract: Similar and dissimilar friction stir welds of AA5083 and AA6061 were produced modifying the location of the materials in the joint (advancing or retreating side) and travel speed (73 and 206 mm/min). Longitudinal and transverse residual stresses were evaluated by X-ray diffraction technique on the mid-length of each welded sample, in the transverse direction to the weld on a 42 mm distance. Unstrained lattice spacing (d₀) and full width at half maximum (FWHM) were also measured. A peak of tension stress at the edge of stirred zone was detected, describing an “M” like profile, which were more asymmetrical for dissimilar joints. Variations in d₀ and FWHM were also observed through the welded joint, which were related to microestructural changes.

Abstract: During the last decades, biomaterials have been deeply studied to perform and improve coatings for biomedical devices. Metallic materials, especially in the orthopedic field, represent the most common material used for different type of devices thanks to their good mechanical properties. Nevertheless, low/medium resistance to corrosion and low osteointegration ability characterizes these materials. To overcome these problems, the use of biocoatings on metals substrate is largely diffused. In fact, biocoatings have a key role to confer biocompatibility properties, to inhibit corrosion and thus improve the lifetime of implanted devices. In this work, the attention was focused on Hydroxyapatite-Chitosan (HA/CS) and Hydroxyapatite-Polyvinylacetate (HA/PVAc) composites, that have been studied as biocoatings for 304 SS based devices. Hydroxyapatite was selected for its osteoconductivity thanks to its chemical structure similar to bones. Furthermore, Chitosan and Polyvinylacetate are largely used yet in medical field (e.g. antibacterial agent or drug deliver) and in this work were used to create a synergic interaction with hydroxyapatite to increase the strength and bioactivity of coating. Biocotings were obtained by galvanic deposition process that does not require an external power supply. It is a spontaneous electrochemical deposition in which materials with different standard electrochemical potential were short-circuited and immersed in an electrolytic solution. Electrons supply for the cathodic reaction in the noblest material comes from oxidation of the less noble material. SEM, EDS, XRD and RAMAN were performed for chemical-physics characterization of biocoatings. Polarization and impedance measurements have been carried out to evaluate corrosion behavior. Besides, in-vitro cytotoxicity assays have been done for the biological features.

Abstract: Coatings with high entropy alloys of the AlCoCrFeNiV system were obtained by selective laser melting on low carbon steel substrates. The effect of the variation of the Fe and V contents as well as the laser processing parameters in the development of the coating were evaluated. The coatings were obtained from the simple powder mixtures of the high purity elemental components in a planetary ball mill. The coatings were obtained by using CO₂ laser with a power of 100 W, diameter of 0.16 mm, and scan speed varying from 3 to 12 mm/s. Phase constituents, microstructure and hardness were investigated by XRD, SEM, and microhardness tester, respectively. Wear resistance measurements were carried out by the micro-abrasion method using ball-cratering tests. The coatings presented good adhesion to the substrate and high hardness, of the order of 480 to 650 HV. Most homogeneous coating with nominal composition was obtained by using the higher scan speed, 12 mm/s. Vanadium addition increased hardness and gave rise to a high entropy alloy coating composed by BCC solid solutions. Ball cratering tests conducted on HEA layer showing improvement of material wear resistance, when compared to base substrate, decreasing up to 88% its wear rate, from 1.91x10^-6 mm³/Nmm to 0.23x10^-6 mm³/Nmm.

Abstract: The aim of the present study was to characterize two kinds of TiN/TiO₂ coatings in terms of topography, composition, and electrochemical stability after immersion in simulated body fluid (SBF). Micropatterning of the substrate (Ti-5Al-4V alloy) was done by using electron beam modification (EBM) by scanning electron beam while nanostructured TiN/TiO₂ films were deposited over EBM Ti5Al4V substrates using two physical vapor deposition techniques: 1) magnetron sputtering, and 2) cathodic arc and glow-discharge methods. When immersed for 7 and 14 days in SBF at static conditions (37±0.05 °C, pH 7.4), Ca/P ratio of the apatite deposits increased from approximately 1.5 up to near stoichiometric (1.67), respectively. After the initial decrease, the pH of the solution during soaking increased gradually reaching values close to 7.7 for both coatings. However, the weight gain of the samples with Arc coatings after the immersion period in SBF was nearly three times more than those with magnetron deposited coating. The electrochemical potentiodynamic tests performed in SBF indicated a shift in the corrosion potentials towards nobler direction after 7 and 14 days of immersion compared to non-immersed samples, whereas the corrosion current density was slightly increased.

Abstract: Selective Laser Melting (SLM) is one of the leader metal Additive Manufacturing (AM) processes thanks to its capability of coupling freeform design and environmental and economical sustainability to high mechanical properties. AlSi10Mg is a light weight Al-alloy with interesting processing properties and enhanced strength thanks to the presence of Mg, which, hence, finds application in several industrial fields. Furthermore, SLM allows overcoming those design constraints set by casting and melt spinning; however, SLM AlSi10Mg components require to be heat treated, both to strengthen the material and to engineer the microstructure. In this work, in order to assess the effectiveness of heat treatments on AlSi10Mg by SLM, an ad hoc analysis procedure based on statistical tools is applied in combination with indentation characterisation tests. In particular, to achieve full scale characterisation, traditional Brinell hardness and Instrumented Indentation Test (IIT) in macro and nano-range are considered. In particular, IIT is applied both at the lower end of macro range to provide consistency and statistically investigate relationship with Brinell scale and in the nano-range, enabling local, i.e. grain, and surface properties to be characterised.