Characterization of Electroless Nickel–Boron Deposit from Optimized Stabilizer-Free Bath

Conventional electroless nickel–boron deposits are produced using solutions that contain lead or thallium, which must be eliminated due to their toxicity. In this research, electroless nickel–boron deposits were produced in a stabilizer-free bath that does not include any toxic heavy metal. During processing, the plating rate increased from 10 to 14.5 µm/h by decreasing the concentration of the reducing agent, leading to increased bath stability. The thickness, composition, roughness, morphology, hardness, wear, and corrosion resistance of the deposits were characterized. The new deposit presents an excellent hardness of 933 ± 56 hv50, 866 ± 30 hk50, and 12 GPa from the instrumented indentation test (IIT), respectively, which are similar to that of hexavalent hard chromium coating. Moreover, by using both potentiodynamic polarization and salt spray tests it was shown that the coating presents higher corrosion resistance as compared to standard nickel-boron coatings. The new deposit exhibits properties close to those of the conventional electroless nickel–boron deposits. Therefore, it could replace them in any industrial applications.

Methodology to evaluate strength properties of steel by single instrumented indentation test

The instrumented indentation test (IIT) is an attractive non-destructive testing technique. Determining accurate strength properties of steel using IIT is still challenging. In this paper, a new methodology is proposed to acquire the yield and ultimate tensile strength from a single IIT. This method extracts true stress-strain curves from IIT results. Acquired stress-strain curves indicate that the initial yield stress is not repeatable. This is caused by the inhomogeneous deformation of IIT specimens. Based on the obtained true stress-stain curves, corresponding yield strength, and ultimate tensile strength are calculated through theoretical derivation. The results show that the strength has a convergent tendency. On basis of this phenomenon, the strength is determined with an extrapolating method. Finally, the strength properties of Q345R are investigated to verify the reliability of this method. It is found that the strength determined from IIT and conventional tensile tests shows good agreement. The proposed method is effective in predicting strength properties from a single instrumented indentation test.

Verification of radial displacement correction effect in instrumented indentation testing

The radial displacement had been ignored in the analysis of unloading curve of instrumented indentation test. There are two representative methods of radial displacement correction, proposed by Hay et al. [J. Mater. Res., 14, 2296, (1999)] and Chudoba-Jennett [J. Phys. D, 41, 215407, (2008)]. In this study, we examine the effect of radial displacement corrections in finite element analysis and experiments.

Determination of an Objective Criterion for the Assessment of the Feasibility of an Instrumented Indentation Test on Rough Surfaces

The influence of roughness on the results of indentation testing was investigated using a semianalytical model. This model used simulated surfaces that were described using three standard roughness parameters: the root-mean-square deviation Sq, the wavelength (or cut-off of Gaussian high-pass filter), and the fractal dimension. It was shown that Sq had the largest effect on the determination of the macrohardness, while the surface wavelength and fractal dimension had negligible effects at the scale of investigation. The error of determination of the macrohardness rose with the increase of the ratio Sq/hmax where hmax was the maximum indentation depth: Sq/hmax ratios lower than 0.02 were required to obtain a systematic error of the macrohardness lower than 5%, whatever the examined material mechanical properties (in elasticity and plasticity).

Finite Element Simulation of the Instrumented Indentation Test to Estimate the Mechanical Properties for ASTM516-G70 and AISI1010 steels

Instrumented indentation technique at micro-scales has become more popular to determine mechanical properties of materials like hardness, modulus of elasticity, and yield strength. It is introduced as a method that finds the stress-strain curve, instead of the traditional tensile test. Furthermore, it gives a possibility to determine the mechanical properties for small specimens and material under operation in the field. Several researchers have attempted to evaluate this method experimentally and to investigate the factors affecting it by using a different shape of indenters, and different types of materials. In the same regard, this research work is conducted to evaluate this method experimentally and by finite element simulation methods. Two types of industrially significant steels were selected; they are namely ASTM516-G70, AISI1010 steel; and two shapes of indenters, blunt and sharp (Spherical, and Vickers) were used. The finite element simulation has been performed by ABAQUS simulation program, and its results were then compared with the experimental test results obtained from Nanovea instrumented indentation test machine. The results obtained have demonstrated good agreement between the experimental and the finite element simulation results within 5 % difference for young’s module, and 7.7 % for yield strength whereas excellent agreement is observed in the elastic region and the beginning of the plastic region for the engineering stress-strain curve. Finally, it is to be emphasized that the obtained results are more applicable for the tested materials, and further research is recommended to accommodate other materials as well and to confirm the generality of this method.

Micro-Macro Relationship between Microstructure, Porosity, Mechanical Properties, and Build Mode Parameters of a Selective-Electron-Beam-Melted Ti-6Al-4V Alloy

The performance of two selective electron beam melting operation modes, namely the manual mode and the automatic ‘build theme mode’, have been investigated for the case of a Ti-6Al-4V alloy (45–105 μm average particle size of the powder) in terms of porosity, microstructure, and mechanical properties. The two operation modes produced notable differences in terms of build quality (porosity), microstructure, and properties over the sample thickness. The number and the average size of the pores were measured using a light microscope over the entire build height. A density measurement provided a quantitative index of the global porosity throughout the builds. The selective-electron-beam-melted microstructure was mainly composed of a columnar prior β-grain structure, delineated by α-phase boundaries, oriented along the build direction. A nearly equilibrium α + β mixture structure, formed from the original β-phase, arranged inside the prior β-grains as an α-colony or α-basket weave pattern, whereas the β-phase enveloped α-lamellae. The microstructure was finer with increasing distance from the build plate regardless of the selected build mode. Optical measurements of the α-plate width showed that it varied as the distance from the build plate varied. This microstructure parameter was correlated at the sample core with the mechanical properties measured by means of a macro-instrumented indentation test, thereby confirming Hall-Petch law behavior for strength at a local scale for the various process conditions. The tensile properties, while attesting to the mechanical performance of the builds over a macro scale, also validated the indentation property measurement at the core of the samples. Thus, a direct correlation between the process parameters, microstructure, porosity, and mechanical properties was established at the micro and macro scales. The macro-instrumented indentation test has emerged as a reliable, easy, quick, and yet non-destructive alternate means to the tensile test to measure tensile-like properties of selective-electron-beam-melted specimens. Furthermore, the macro-instrumented indentation test can be used effectively in additive manufacturing for a rapid setting up of the process, that is, by controlling the microscopic scale properties of the samples, or to quantitatively determine a product quality index of the final builds, by taking advantage of its intrinsic relationship with the tensile properties.