Surface Engineering of Metals and Alloys

1. Introduction and Scope

Nowadays, the surface treatments of metals and alloys allow us to obtain from them the films or coatings with different physical and chemical properties from the substrate. The most often used surface modification methods are as follows: electropolishing, magnetoelectropolishing, plasma electrolytic oxidation, ion nitriding, functionalization of electrochemically reduced graphene oxide, laser cavitation peening, cold spray deposition, magnetic abrasive finishing, hydrophobic modification of graphene oxide, stress-corrosion cracking, gas metal arc welding, surface metallization, multiple abrasive waterjet peening, laser peening, machined milling, high-speed magnetic abrasive finishing, shape memory stainless steel, fiber laser surface melting, laser cladding, and protective coatings.

It must be pointed out that the present Special Issue in Metals is a continuation of the already closed subject Surface Treatment Technology of Metals and Alloys, in which some aspects of surface modification by selected methods were presented. That way, all researchers interested in widely understood surface engineering of metals and alloys are welcomed.

2. Contributions

Twenty research papers have been published in the Special Issue of Metals titled Surface Engineering of Metals and Alloys. All the presented subjects are multidisciplinary, and include shot peening treatment [1], electropolishing [2], low-pressure carburizing [3], oxi-nitrocarburization [4], abrasive blasting [5], hydrothermal treatments [6], laser cladding [7], plasma modification [8], low-temperature.

The ion nitriding behavior of AISI 316L austenite stainless steel was investigated at different nitriding times (2 h, 4 h, and 9 h) and temperatures (450 °C, 500 °C, and 550 °C). It was noted that the diffusion of nitrogen atomic species into the sample surface caused a transformation of the γ phase matrix into an expanded austenite (γN) phase, which is recognized with its high hardness and wear resistance. Furthermore, nitriding condition, chromium nitride (Cr1-2N) and iron nitride (ε-Fe2-3N and γ′-Fe4N) phases were detected, which can be detrimental to the corrosion resistance of the 316L austenite stainless steel. The γNphase was observed in all nitriding conditions, resulting in a significant increase in the surface hardness. However, decomposition of the γNphase with an increase in nitriding temperature eventually altered the surface hardness distribution in the nitriding layer [1].

Electrochemically reduced graphene oxide (ErGO) on CoCr can be functionalized with hyaluronic acid (ErGOHA). It was found out that macrophage assays showed better behavior on CoCrErGOHA than CoCr and CoCrErGO surfaces, based on their biocompatible, cytotoxic, and inflammatory responses. Comparative analysis of IL-10 cytokines showed that functionalization with HA induces higher values of anti-inflammatory cytokine, suggesting an improvement in inflammatory behavior [2].

Magnesium AM60 alloy after plasma electrolytic oxidation (PEO) was described. The potentiodynamic polarization curves indicated that the PEO-coated samples experienced a significant decrease of 99.9% in the corrosion rate compared to the base metal. The electrochemical impedance spectroscopy findings showed that PEO coating increased the corrosion resistance of the AM60 magnesium alloy by 1,071,870% compared to the base metal. In addition, the PEO coated samples showed superior adhesion to the substrate. Moreover, the PEO coating led to an improvement in the hardness value by 114% compared to base metal, coupled with insignificant change in the compressive properties [3].

In conventional cavitation peening, cavitation is generated by injecting a high-speed water jet into the water, and the impacts of cavitation collapses are utilized for mechanical surface treatment. The impact forces induced by laser ablation and laser cavitation collapse were evaluated with a polyvinylidene fluoride (PVDF) sensor and a submerged shockwave sensor, and the diameter of the laser cavitation was measured by observing a high-speed video taken with a camera. It was revealed that the impact of laser cavitation collapse was larger than that of laser ablation, and the peening effect was closely related to the volume of laser cavitation. In addition, the laser cavitation peening improved the fatigue strength of stainless-steel welds [4].

The use of cold spray deposition, coupled with diffusion-driven thermal postprocessing, is considered herein as a surface modification process such that near-surface microstructural, micromechanical, and microchemical property improvements can be procured for cost-effective and common aluminum alloy castings. In case of copper feedstock was employed alongside thermal postprocessing, diverse surface-based intermetallic compounds formed alongside exotic diffusion zones and severely oxidized regions, thus eliminating thermally activated copper cold-sprayed consolidations from future work too. In addition, both nickel and titanium cold spray surface modification processing demonstrated potential and promise if correct processing stages were performed directly and chronologically [5].

A numerical model of a magnetic abrasive finishing station, which can be analyzed using the finite element method (FEM) may be used for comparison the real values with measured once. It was found that within the assumed range of machining parameters, the surface layer of AISI 304L stainless steel improved, so that precision increased and better surface quality of manufacturing elements was achieved. In addition, the relative change of roughness parameters for the smallest concentration of abrasive grains (K = 16%) is negligible. Additionally, the effect of the machining gap width is increasingly significant if the concentration of abrasive grains increases [6].

Modified graphene oxide (GO) with hydrophilic octadecylamine (ODA) via covalent bonding can improve its dispersion in silicone-modified epoxy resin (SMER) coatings. It was found that ODA was grafted onto the surfaces of GO. The resulting ODA-GO material exhibited good hydrophobicity and dispersion in SMER coatings. The anticorrosive properties of the ODA-GO/SMER coatings were significantly improved due to the increased interfacial adhesion between the nanosheets and SMER, lengthening of the corrosive solution diffusion path, and increased cathodic peeling resistance [7].

The stress corrosion response and corrosion damage characteristics of aluminum alloy 2219, both the base material and a friction stir welding (FSW) counterpart upon exposure to exfoliation corrosion (EXCO) solution were studied. The results reveal that the test specimen containing an FSW joint reveals better electrochemical corrosion resistance than that taken from the base metal. The test specimen containing the FSW joint was found to be less resistant to stress corrosion damage than that taken from the base metal for the various levels of applied stress and exposure time to EXCO solution. This reveals that the test specimen containing the FSW joint is more susceptible to damage and degradation than the test specimen taken from the base metal [8].

Vanadium carbide (VC)-reinforced FeCrVC hardfacings have become important to improve the lifetime of tools suffering abrasive and impact loads, because the microstructural properties of such hardfacings enable the primary VCs to act as obstacles against the penetrating abrasive. By inserting an additional hot wire in the melt, an approach was developed to separate the material and energy input during gas metal arc welding (GMAW), and hence realised low dilution claddings. The carbide content could be increased, and a grain refinement was observed compared with conventional GMAW. These effects could be attributed to both the reduced dilution and heterogeneous nucleation [9].

Carbon fiber is mainly distributed in the shape of short fibers and continuous fiber bundles as the reinforcing phase in metal matrix composites, and it is seldom studied as braided rope shaped to reinforce the matrix. The surface of the carbon fiber braided rope treated with ultrasonic degumming contains many hydrophilic oxygen-containing functional groups, which can effectively improve the wettability between the carbon fiber braided rope and the aluminum matrix. Additionally, a copper coating can effectively inhibit the formation of Al4C3 brittle phase [10].

Non-contact assessment can be used for characterization of stainless austenitic steel surfaces after electrochemical polishing in a magnetic field. It was realized with the use of a modified angle-resolved scattering (ARS) method based on the analysis of angular distribution of the scattered light intensity. Parametric analysis oriented toward the calculation of selected key geometricand photometric parameters carried out allowed for characterization of the surface conditions of the assessed samples in terms of their scattering properties. The obtained experimental results confirmed the usefulness of the ARS method used in the presented studies, as well as the possibility of its practical use on a wider scale, especially in industrial applications [11].

Abrasive waterjet peening (AWJP) as an important 18CrNiMo7-6 steel surface strengthening method can effectively improve surface properties. Compared with the single AWJP, multiple AWJP can obviously increase the surface residual stresses (−1024 MPa to −1455 MPa) and the depth of maximum compressive residual stress (100 μm to 120 μm), as well as make the stress distribution more uniform. In terms of the surface roughness, multiple AWJP influences its uniform distribution and reduces the surface roughness (Sa = 0.69 μm), compared with a single AWJP (Sa = 2.96 μm), due to the smaller shot balls and a uniform deformation during multiple AWJP. In addition, the multiple AWJP increases the hardness by up to 15.9%, compared to the single AWJP [12].

Laser peening without coating (LPwC) involves irradiating materials covered with water with intense laser pulses to induce compressive residual stress (RS) on a surface. The thermo-mechanical effect of single laser pulse irradiation and introduce a phenomenological model to predict the outcome of LPwC was found out. To validate this model, RS distribution across the laser-irradiated spot was analyzed using X-ray diffraction with synchrotron radiation. Large tensile RSs were found in the spot irradiated by the single pulse; however, compressive RSs appeared around the spot. The compressive RSs around a subsequent laser spot effectively compensated the tensile component on the previous spot by controlling the overlap, which may result in compressive RSs on the surface after LPwC [13].

An analytical prediction model for residual stress during milling is established, which considers the thermal-mechanical coupling effect. Considering the effects of thermal-mechanical coupling, the residual stress distribution in the workpiece is determined by the stress loading history according to McDowell′s hybrid algorithm. Based on the analysis of the geometric relationship of orthogonal cutting, the prediction model for milling force and residual stress in the machined surface is established [14].

The micro-machining characteristics in a high-speed magnetic abrasive finishing are applicable for achieving the high surface accuracy and dimensional accuracy of fine ceramic bars that are typically characterized by strong hardness and brittle susceptibility. The obtained results showed that under variants of diamond abrasives sizing (between 1, 3 and 9 µm), 1 µm showed comparatively good values with regard to surface roughness and roundness within shortest processing time. The performance of ultra-precision processing linear controlling was possibly achievable for the stable region of diameter change, while linearly controlling diameters in the workpiece [15].

The microstructural characterization and corrosion-resistance behavior of Fe-Mn-Si-Cr-Ni alloy with shape memory effect was studied under different mechanical processing conditions and heat treatments, which were produced using conventional casting and routing methods to reduce costs and make production viable. The cast condition presented a dendritic structure and the presence of the secondary phases: ferrite-δ and Chi-X phase. The heat treatment eliminated phases, reincorporated elements in the matrix, and increased the austenitic grain. After the hot rolling process, the alloy exhibited a refined microstructure with recrystallized austenitic grains. It was found that the heat-treated samples presented better oxidation resistance than the other ones, while the hot-rolled samples showed repassivation of the pits, raising them to higher levels [16].

The surface melting of a NiTi superelastic alloy using a high-power laser Yb:Fiber can be divided on three regions (fusion zone (FZ), heat-affected zone (HAZ), base metal (BM)). It was found that the geometry of the molten pool could be controlled by the optimization of the laser parameters. In addition, the high laser fluence caused preferential volatilization of nickel, dynamic precipitation of intermetallic phases, including Ti₂Ni, Ni₃Ti, and Ni₄Ti₃, as well as solubilization of TiC in the matrix, which led to grain refinement. Thus, high laser fluence is a suitable technique to enhance mechanical properties such as hardness and Young’s modulus [17].

The next three papers are prepared as the review article on the following topics: laser cladding coatings on magnesium alloys [18], electrochemical polishing of metals and alloys in ionic liquids [19], and thin protective coating for wear protection in high-temperature application [20]. In the first review paper, laser cladding is proposed as an effective fabrication technique to obtain coatings on magnesium alloys, which has better mechanical and tribological properties, and in most cases, improved the corrosion behavior. It was also noticed that the main problem of laser cladding technique used on magnesium alloys is the very low melting point of these alloys, which results in the evaporation of the substrate and high dilution and alloying between the coating and the substrate. In addition, the authors focused on the possibilities of obtaining different material coatings on magnesium alloys fabricated by laser cladding [18]. The second review article is about electropolishing of metals in ionic liquids providing benign conditions for metal finish to obtain the required quality of the surface. That anodization of metals in acid-free ionic liquids often leads to smoothing the metal surface and obtaining mirror-like metal surfaces rather than etching and pitting. In the third article, a thin protective coating that can be applied by various coating deposition methods, was shown. In that review, the state-of-the-art development of the coating methods such as electrodepositing, radio frequency ion source implantation, electron beam implantation, plasma-sprayed coating deposition, flame-sprayed coating deposition, chemical catalytic reduction deposition, vacuum-diffused deposition, vapor deposition, chemical vapor deposition, physical vapor deposition, and plasma arc deposition were presented.

3. Conclusions and Outlook

Many topics related to the surface engineering of metals and alloys have been shown in the present Special Issue. It should be pointed out that the subject is interdisciplinary and includes, inter alia, topics such as electropolishing, magnetoelectropolishing, ion nitriding, plasma electrolytic oxidation, functionalization of electrochemically reduced graphene oxide, laser cavitation peening, cold spray deposition, magnetic abrasive finishing, hydrophobic modification of graphene oxide, stress corrosion cracking, gas metal arc welding, surface metallization, multiple abrasive waterjet peening, laser peening, machined milling, high-speed magnetic abrasive finishing, shape memory stainless steel, fiber laser surface melting, laser cladding, and protective coatings.

The issue of surface treatment is still open for researchers, because the top surface layers, which are fabricated during physical, chemical/electrochemical, and vacuum processes, have different properties from those of treated substrates.