Characteristics of Cr-B Coatings Produced on Vanadis® 6 Tool Steel Using Laser Processing

The paper presents the results of a study of the microstructure, chemical composition, microhardness and corrosion resistance of Cr-B coatings produced on Vanadis 6 tool steel. In this study, chromium and boron were added to the steel surface using a laser alloying process. The main purpose of the study was to determine the impact of those chemical elements on surface properties. Chromium and boron as well as their mixtures were prepared in various proportions and then were applied on steel substrate in the form of precoat of 100 µm thickness. Depending on the type of precoat used and laser processing parameters, changes in microstructure and properties were observed. Coatings produced using precoat containing chromium and boron mixture were characterized by high microhardness (900 HV0.05–1300 HV0.005) while maintaining good corrosion resistance. It was also found that too low laser beam power contributed to the formation of cracks and porosity.     

Investigation on Microstructure,Mechanical and Wear Properties of HVOF Sprayed Composite Coatings (WC–Co + CR) On Ductile Cast Iron

Recent work indicates that the high-velocity oxy-fuel (HVOF) thermal spraying WC–Co coatings have been used to enhance the wear resistance of various engineering components in a variety of industrial environments. In the present work, WC–Co powder, containing Cr particles in an amount of 10%, was deposited on ductile cast iron with the HVOF thermal spray coating technique. An investigation was conducted to determine the role of Cr particles in the WC–Co coating produced with the HVOF technique on microstructure, mechanical, and wear properties in a system of type: WC-Co coating/ductile cast iron. The microstructure of the HVOF-sprayed WC–Co + Cr coating was characterised by light microscopy, X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), and energy-dispersive X-ray spectroscopy (EDS). The analysis of the microstructure showed the formation of a coating with low porosity, compact structure, and good adhesion to the substrate with a typical lamellar structure composed of fine molten Cr particles and finely fragmented WC grains embedded in a Co matrix, reaching the size of nanocrystalline. The scratch test was applied for the analysis of the adhesion of coatings to the substrate. The erosion behaviour and mechanism of material removal was studied and discussed based on microstructural examinations. Moreover, the results were discussed in relation to the bending strength test, including cracks and delamination in the system of the WC–Co + Cr/ductile cast iron, as microhardness and erosion resistance of the coating. It was found that the addition of Cr particles to the WC–Co powder, which causes hardening of the binder phase is a key influence on increased mechanical and wear properties in the studied system. Additionally, due to the construction of nanostructured coatings, suitable proportion of hard and soft phases, the technique sprayed HVOF coatings have advantageous properties such as high density and good slurry erosion resistance.     

Investigation of the Implementation of Laser Surface Alloying of Cu with Cr–WC

This paper presents research on the microstructure and mechanical properties of an alloyed composite copper (Cu) surface layer, reinforced with a mixture of chromium–tungsten carbide (Cr–WC) powders. Copper alloying was performed using a high-power diode laser (HPDL). In the tests, three mixtures of powders with different percentage contents (75%Cr 25%WC, 50%Cr 50%WC, 25%Cr 75%WC) were injected into the melting pool during the laser surface alloying process. Microstructural evolution and the properties of the surface layer of copper after laser alloying were investigated. Structural investigations were performed using light microscopy, scanning and transmission electron microscopy (SEM, TEM) and X-ray diffraction (XRD). Microhardness and wear resistance of the modified surface layer were examined as well. After laser treatment the applied powders appear as uniformly distributed particles in the alloyed zone as well as nanoscale precipitates in the Cu matrix. Several types of precipitate characteristics, in terms of morphology, structure and chemical composition, were observed. Laser alloying of the surface layer modified the microstructure, which resulted in an increase in the hardness of the surface layers compared to the base material.     

Effect of Laser Processing Parameters on Microstructure,Hardness and Tribology of NiCrCoFeCBSi/WC Coatings

In the present study, pulsed laser post-processing was applied to improve the properties of the thermally sprayed NiCrCoFeCBSi/40 wt.% WC coatings. The powder mix was deposited onto a mild steel substrate by flame spray method and then the as-sprayed coatings were processed by Nd:YAG laser. The peak power density applied was between 4.00 × 10⁶ and 5.71 × 10⁶ W/cm², and the laser operating speed ranged between 100 and 400 mm/min, providing processing in a melting mode. Scanning electron microscopy, energy dispersive spectroscopy, Knop hardness measurements, and “ball-on-disc” dry friction tests were applied to study the effect of the processing parameters on the geometry of laser pass and microstructure, hardness, and tribology of the processed layers. The results obtained revealed that pulsed laser processing provides a monolithic remelted coating layer with the microstructure of ultrafine, W-rich dendrites in Ni-based matrix, where size and distribution of W-rich dendrites periodically vary across remelted layer depth. The composition of W-rich dendrites can be attributed to a carbide of type (W, Cr, Ni, Fe)C. The cracks sensitivity of coatings was visibly reduced with the reduction of power density applied. The hardness of coatings was between ~1070 and ~1140 HK0.2 and correlated with microstructure size, being dependent on the processing parameters. The friction coefficient and wear rate of coatings during dry sliding were reduced by up to ~30% and up to ~2.4 times, respectively, after laser processing.     

Microstructure and Abrasive Wear Resistance of Metal Matrix Composite Coatings Deposited on Steel Grade AISI 4715 by Powder Plasma Transferred Arc Welding Part 2. Mechanical and Structural Properties of a Nickel-Based Alloy Surface Layer Reinforced with Particles of Tungsten Carbide and Synthetic Metal–Diamond Composite

The article is the continuation of a cycle of works published in a Special Issue of MDPI entitled “Innovative Technologies and Materials for the Production of Mechanical, Thermal and Corrosion Wear-Resistant Surface Layers and Coatings” related to tests concerning the microstructure and mechanical properties of innovative surface layers made using the Powder Plasma Transferred Arc Welding (PPTAW) method and intended for work surfaces of drilling tools and machinery applied in the extraction industry. A layer subjected to tests was a metal matrix composite, made using powder based on a nickel alloy containing spherical fused tungsten carbide (SFTC) particles, which are fused tungsten carbide (FTC) particles and spherical particles of tungsten-coated synthetic metal–diamond composite (PD-W). The layer was deposited on the substrate of low-alloy structural steel grade AISI 4715. The results showed that the chemical composition of the metallic powder as well as the content of the hard phase constituting the matrix enabled the making of a powder filler material characterised by very good weldability and appropriate melting. It was also found that the structure of the Ni-WC-PD-W layer was complex and that proper claddings (characterised by the uniform distribution of tungsten carbide (WC)) were formed in relation to specific cladding process parameters. In addition, the structure of the composite layer revealed the partial thermal and structural decomposition of tungsten carbide, while the particles of the synthetic metal–diamond composite remained coherent. The deposited surface layer was characterised by favourable resistance to moderate dynamic impact loads with a potential energy of 200 J, yet at the same time, by over 12 times lower metal–mineral abrasive wear resistance than the previously tested surface layer made of cobalt-based composite powder, the matrix of which contained the hard phase composed of TiC particles and synthetic metal–diamond composite. The lower abrasive wear resistance could result from a different mechanism responsible for the hardening of the spherical particles of the hard phase susceptible to separation from the metal matrix, as well as from a different mechanism of tribological wear.     

Influence of Pulse Current Forward-Reverse Duty Cycle on Structure and Performance of Electroplated W–Cu Composite Coatings

Tungsten-copper (W–Cu) composites are widely used as electrical contact materials, resistance welding, electrical discharge machining (EDM), and plasma electrode materials due to their excellent arc erosion resistance, fusion welding resistance, high strength, and superior hardness. However, the traditional preparation methods pay little attention to the compactness and microstructural uniformity of W–Cu composites. Herein, W–Cu composite coatings are prepared by pulse electroplating using nano-W powder as raw material and the influence of forward-reverse duty cycle of pulse current on the structure and mechanical properties is systematically investigated. Moreover, the densification mechanism of the W–Cu composite coating is analyzed from the viewpoints of forward-pulse plating and reverse-pulse plating. At the current density (J) of 2 A/dm², frequency (f) of 1500 Hz, forward duty cycle (d_f) of 40% and reverse duty cycle (d_r) of 10%, the W–Cu composite coating rendered a uniform microstructure and compact structure, resulting in a hardness of 127 HV and electrical conductivity of 53.7 MS/m.     

Manufacturing Technology and Properties of Fe/TaC Metal Matrix Composite Coatings Produced on Medium Carbon Steel Using Laser Processing—Preliminary Study on the Single Laser Tracks

The paper presents study results of Fe/TaC metal matrix composite coatings produced on tool steel using laser processing of TaC pre-coat. The Fe/TaC coatings were produced in two steps. First, a pre-coat in the form of a paste based on tantalum carbide and water glass was made and then applied to the steel substrate. In the second step, the TaC pre-coat was remelted with steel substrate using a diode laser beam with a rated power of 3 kW. A constant scanning speed of the laser beam of 3 m/min and three types of laser beam power: 500 W, 800 W and 1100 W were applied. Tests were carried out on three different thicknesses of the TaC pre-coat: 30 µm, 60 µm and 90 µm. The influence of pre-coat thickness and laser beam power on the microstructure, chemical composition and microhardness were analyzed. A possibility of producing coatings with a characteristic composite structure was found, where the iron from the substrate became the matrix, and the introduced tantalum carbides—the reinforcing phase. It was found that too high power of the laser beam leads to complete melting of the introduced primary TaC particles. It was also found that the use of a thicker TaC pre-coat contributes to microhardness increase.     

Production and Characterization of Austenitic Stainless Steel Cast Parts Reinforced with WC Particles Fabricated by Ex Situ Technique

In this work, austenitic stainless steel specimens were locally reinforced with WC particles. The reinforcements were fabricated via an ex situ technique based on powder technology. Mixtures of WC, Fe, and M0101 binder were cold-pressed to obtain powder compacts. After debinding and sintering, the porous WC–Fe inserts were fixed in a mold cavity, where they reacted with liquid metal. Microstructural analysis was conducted for characterization of the phases constituting the produced reinforcement zone and the bonding interface. The results revealed that the reinforcement is a graded material with compositional and microstructural gradients throughout its thickness. The zone nearest to the surface has a ferrous matrix with homogeneously distributed WC particles and (Fe,W,Cr)₆C and (Fe,W,Cr)₃C carbides, formed from the liquid metal reaction with the insert. This precipitation leads to austenite destabilization, which transforms into martensite during cooling. A vast dissolution of the WC particles occurred in the inner zones, resulting in more intense carbides formation. Cr-rich carbides ((Fe,Cr,W)₇C₃, and (Fe,Cr,W)₂₃C₆) formed in the interdendritic regions of austenite; this zone is characterized by coarse dendrites of austenite and a multi-phase interdendritic network composed of carbides. An interface free of discontinuities and porosities indicates good bonding of the reinforcement zone to stainless steel.     

The Use of ZrO2 Waste for the Electrolytic Production of Composite Ni–P–ZrO2 Powder

Ni–P–ZrO₂ composite powder was obtained from a galvanic nickel bath with ZrO₂ powder. Production was conducted under galvanostatic conditions. The Ni–P–ZrO₂ composite powder was characterized by the presence of ZrO₂ particles covered with electrolytical nanocrystalline Ni–P coating. The chemical composition (XRF method), phase structure (XRD method) and morphology (SEM) of Ni–P–ZrO₂ and the distribution of elements in the powder were all investigated. Based on the analyses, it was found that the obtained powder contained about 50 weight % Zr and 40 weight % Ni. Phase structure analysis showed that the basic crystalline component of the tested powder is a mixed oxide of zirconium and yttrium Zr_0.92Y_0.08O_1.96. In addition, the sample contains very large amounts of amorphous compounds (Ni–P). The mechanism to produce the composite powder particles is explained on the basis of Ni²⁺ ions adsorption process on the metal oxide particles. Current flow through the cell forces the movement of particles in the bath. Oxide grains with adsorbed nickel ions were transported to the cathode surface. Ni²⁺ ions were discharged. The oxide particles were covered with a Ni–P layer and the heavy composite grains of Ni–P–ZrO₂ flowed down to the bottom of the cell.     

Comparison of Different Cermet Coatings Sprayed on Magnesium Alloy by HVOF

In the present study, two different cermet coatings, WC–CrC–Ni and Cr₃C₂–NiCr, manufactured by the high-velocity oxy-fuel (HVOF) method were studied. They are labeled as follows: WC–CrC–Ni coating—WC and Cr₃C₂–NiCr coating—CrC. These coatings were deposited onto a magnesium alloy (AZ31) substrate. The goal of the study was to compare these two types of cermet coating, which were investigated in terms of microstructure features and selected mechanical properties, such as hardness, instrumented indentation, fracture toughness, and wear resistance. The results reveal that the WC content influenced the hardness and Young’s modulus. The most noticeable effect of WC addition was observed for the wear resistance. WC coatings had a wear intensity value that was almost two times lower, equal to 6.5·10⁻⁶ mm³/N·m, whereas for CrC ones it was equal to 12.6·10⁻⁶ mm³/N·m. On the other hand, the WC coating exhibited a lower value of fracture toughness.     

W–Cu Composite with High W Content Prepared by Grading Rounded W Powder with Narrow Particle Size Distribution

In this study, the W (10–20%)–Cu composites were simultaneously fabricated using commercial, graded commercial, and graded jet-milled W powder. The results show that the W–Cu composites prepared with the graded jet-milled W powders have the highest density and best comprehensive performance due to the combined effect of the particle gradation and jet-milling treatment. Particle gradation is employed to increase the packing density of powders, thereby increasing the relative density of the compressed W skeleton, and the rounded powder with narrow particle size distribution after jet-milling treatment is used to reduce the enclosed pores formed during the process of compacting and infiltration. W–Cu composites with a high density of 16.25 g/cm³ can be directly obtained by conventional compacting at a low pressure of 300 MPa and following infiltration.     

Effect of Cr Content on Corrosion Resistance of Ni–xCr–Mo Laser-Cladding Coatings under H2S-Induced High-Temperature Corrosion Atmosphere

Ni–xCr–Mo laser-cladding coatings with varying Cr content of 10, 15, 20, 25, and 30 wt.% were fabricated using a self-assembled coaxial laser-cladding device. The H₂S-induced high-temperature corrosion tests under reductive atmosphere were conducted at 500, 550, and 600 °C. Subsequently, the influence of Cr content on the microstructural evolution and corrosion resistance of the Ni–xCr–Mo coatings was investigated. The experimental results revealed that 30 wt.% Cr is the limited maximum content that forms the suitable morphology of coatings without large prominent pores and cracks during the fabrication process, and 15 wt.% Cr corresponds to the critical minimum content for excellent corrosion resistance, as implied from the variation tendency of the corrosion weight-gain curves. Moreover, a two-layer structure of the corrosion scales was observed in the Ni–xCr–Mo laser-cladding coatings, which was primarily caused by the selective corrosion between the Ni and S and Cr/Mo and O.     

Prediction of Epitaxial Grain Growth in Single-Track Laser Melting of IN718 Using Integrated Finite Element and Cellular Automaton Approach

The mechanical properties of selective laser melting (SLM) components are fundamentally dependent on their microstructure. Accordingly, the present study proposes an integrated simulation framework consisting of a three-dimensional (3D) finite element model and a cellular automaton model for predicting the epitaxial grain growth mode in the single-track SLM processing of IN718. The laser beam scattering effect, melt surface evolution, powder volume shrinkage, bulk heterogeneous nucleation, epitaxial growth, and initial microstructure of the substrate are considered. The simulation results show that during single-track SLM processing, coarse epitaxial grains are formed at the melt–substrate interface, while fine grains grow at the melt–powder interface with a density determined by the intensity of the heat input. During the solidification stage, the epitaxial grains and bulk nucleated grains grow toward the top surface of the melt pool along the temperature gradient vectors. The rate of the epitaxial grain growth varies as a function of the orientation and size of the partially melted grains at the melt–substrate boundary, the melt pool size, and the temperature gradient. This is observed that by increasing heat input from 250 J/m to 500 J/m, the average grain size increases by ~20%. In addition, the average grain size reduces by 17% when the initial substrate grain size decreases by 50%. In general, the results show that the microstructure of the processed IN718 alloy can be controlled by adjusting the heat input, preheating conditions, and initial substrate grain size.     

Experimental Investigations of Micro-Meso Damage Evolution for a Co/WC-Type Tool Material with Application of Digital Image Correlation and Machine Learning

Commercial Co/WC/diamond composites are hard metals and very useful as a kind of tool material, for which both ductile and quasi-brittle behaviors are possible. This work experimentally investigates their damage evolution dependence on microstructural features. The current study investigates a different type of Co/WC-type tool material which contains 90 vol.% Co instead of the usual <50 vol.%. The studied composites showed quasi-brittle behavior. An in-house-designed testing machine realizes the in-situ micro-computed tomography (μCT) under loading. This advanced equipment can record local damage in 3D during the loading. The digital image correlation technique delivers local displacement/strain maps in 2D and 3D based on tomographic images. As shown by nanoindentation tests, matrix regions near diamond particles do not possess higher hardness values than other regions. Since local positions with high stress are often coincident with those with high strain, diamonds, which aim to achieve composites with high hardnesses, contribute to the strength less than the WC phase. Samples that illustrated quasi-brittle behavior possess about 100–130 MPa higher tensile strengths than those with ductile behavior. Voids and their connections (forming mini/small cracks) dominant the detected damages, which means void initiation, growth, and coalescence should be the damage mechanisms. The void appears in the form of debonding. Still, it is uncovered that debonding between Co-diamonds plays a major role in provoking fatal fractures for composites with quasi-brittle behavior. An optimized microstructure should avoid diamond clusters and their local volume concentrations. To improve the time efficiency and the object-identification accuracy in μCT image segmentation, machine learning (ML), U-Net in the convolutional neural network (deep learning), is applied. This method takes only about 40 min to segment more than 700 images, i.e., a great improvement of the time efficiency compared to the manual work and the accuracy maintained. The results mentioned above demonstrate knowledge about the strengthening and damage mechanisms for Co/WC/diamond composites with >50 vol.% Co. The material properties for such tool materials (>50 vol.% Co) is rarely published until now. Efforts made in the ML part contribute to the realization of autonomous processing procedures in big-data-driven science applied in materials science.     

Tribological Properties of Solid Solution Strengthened Laser Cladded NiCrBSi/WC-12Co Metal Matrix Composite Coatings

NiCrBSi, WC-12Co and NiCrBSi with 30, 40 and 50 wt.% WC-12Co coatings were produced on low carbon steel by laser cladding with an Nd:YAG laser with a multi-jet coaxial cladding-nozzle. The microstructure properties after WC-12Co alloying were investigated by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), electron backscatter diffraction (EBSD) and Vickers hardness tests. The resulting microstructures consisted of a γ-Ni and Ni₃B matrix, strengthened with Co and W, Ni₃Si, CrB, Cr₇C3, Cr₂₃C₆, WC/W₂C phases. In coatings with 30, 40 and 50 wt.% WC-12Co, a solid solution, strengthened multi-matrix NiCrWCo phase formed, which yielded a higher matrix hardness. Wear tests that monitored the friction coefficients were performed with a tribometer that contained a ball-on-disc configuration, Al₂O₃ counter-body and reciprocal sliding mode at room temperature. The major wear mode on the NiCrBSi coatings without the WC-12Co was adhesive with a high wear rate and visible material loss by flaking, delamination and micro-ploughing. The addition of WC-12Co to the NiCrBSi coating significantly increased the wear resistance and changed the major wear mechanism from adhesion to three-body abrasion and fatigue wear.     

Effect of Ag on Properties,Microstructure, and Thermostability of Cu–Cr Alloy

Cu–Cr-based alloys exhibit excellent electrical conductivity and strength, but their poor thermal stability limits their application in industry. In this paper, Cu–0.2Cr (at. %) and Cu–0.2Cr–0.12Ag (at. %) alloys were prepared to study the effect of Ag on the properties, microstructure, and thermal stability of the Cu–Cr alloy. Microstructure and precipitation were observed by an optical microscope (OM) and a transmission–electron microscope (TEM). After cold-drawing by 99.9% and aging at 450 °C for 2 h, the peak hardness and electric conductivity of the Cu–Cr alloy were 120.3 HV and 99.5% IACS, respectively, and those of the Cu–Cr–Ag alloy were 135.8 HV and 98.3% IACS, respectively. The softening temperature of the Cu–Cr alloy was 500~525 °C, and that of the Cu–Cr–Ag alloy was about 550 °C. The creep strains of the Cu–Cr and Cu–Cr–Ag alloys at 40 MPa and 400 ℃ for 50 h were 0.18% and 0.05%, respectively. Ag elements improved the thermal stability of the Cu–Cr alloy. Recovery and recrystallization occurred before the coarsening of precipitates during the softening process. Ag atoms mainly improved the softening resistance of the alloy by delaying recrystallization, and mainly increased creep resistance by preventing the increase in mobile-dislocation density.     

Process–Structure–Property Relationships of Copper Parts Manufactured by Laser Powder Bed Fusion

The process–structure–property relationships of copper laser powder bed fusion (L-PBF)-produced parts made of high purity copper powder (99.9 wt %) are examined in this work. A nominal laser beam diameter of 100 μm with a continuous wavelength of 1080 nm was employed. A wide range of process parameters was considered in this study, including five levels of laser power in the range of 200 to 370 W, nine levels of scanning speed from 200 to 700 mm/s, six levels of hatch spacing from 50 to 150 μm, and two layer thickness values of 30 μm and 40 μm. The influence of preheating was also investigated. A maximum relative density of 96% was obtained at a laser power of 370 W, scanning speed of 500 mm/s, and hatch spacing of 100 μm. The results illustrated the significant influence of some parameters such as laser power and hatch spacing on the part quality. In addition, surface integrity was evaluated by surface roughness measurements, where the optimum Ra was measured at 8 μm ± 0.5 μm. X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDX) were performed on the as-built samples to assess the impact of impurities on the L-PBF part characteristics. The highest electrical conductivity recorded for the optimum density-low contaminated coils was 81% IACS.     

Laser Boronizing of Additively Manufactured 18Ni-300 Maraging Steel Part Surface

The problem of insufficient wear resistance of maraging steels (MSt) has so far been solved mainly by the use of the thermochemical nitriding process, which has a number of limitations and disadvantages. In the present work, for MSt parts manufactured by laser powder bed fusion (LPBF), a more flexible laser alloying process was suggested as an alternative surface hardening process. The purpose of the present work is to give a better understanding on the possible hardening effect obtainable when amorphous boron is used as an alloying additive in relation with microstructural evolution and specific process parameters and to promote further development of this technology. For the alloying, a one kilowatt CO₂ laser was applied at 0.5–4.0 mm laser spot and 250–1500 mm/min laser operating speed, providing 50,955–796 W∙cm⁻² power density and 24.0–4.0 J∙mm⁻¹ heat input. Before laser processing, surfaces were covered with amorphous boron. The appropriate melt pool geometry was obtained at 0.5 mm laser spot, for which XPS analysis revealed an increase in boron concentration from ~3.1 to ~5.7 wt.% with a laser speed increase from 500 to 1500 mm/min. XRD analysis revealed domination of Fe₃B type borides along with the presence of FeB, Fe₂B, Ni₄B₃ borides, austenitic and martensitic phases. The microstructure of modified layers exhibited evolution from hypoeutectic microstructure, having ~630–780 HK0.5 hardness, to superfine lamellar nanoeutectic (~1000–1030 HK0.2) and further to submicron-sized dendritic boride structure (~1770 HK0.2). Aging of laser-boronized layers resulted in the change of phase composition and microstructure, which is mainly expressed in a plenty precipitation of Mo₂B₅ borides and leads to a reduction in hardness—more significant (by ~200–300 HK0.2) for hypoeutectic and hypereutectic layers and insignificant (by ~50 HK0.2) for near-eutectic. With the application of the laser boronizing technique, the hardness of MSt parts surface was increased up to ~three times before aging and up to ~2.3 times after aging, as compared with the hardness of aged MST part.     

Mechanical and Tribological Behavior of Mechanically Alloyed Ni-TiC Composites Processed via Spark Plasma Sintering

Titanium carbide (TiC) reinforced nickel (Ni) matrix composites were processed via mechanical alloying (MA) followed by spark plasma sintering (SPS) process. Mechanical alloying has gained special attention as a powerful non-equilibrium process for fabricating amorphous and nanocrystalline materials, whereas spark plasma sintering (SPS) is a unique technique for processing dense and near net shape bulk alloys with homogenous microstructure. TiC reinforcement varied from 5 to 50 wt.% into nickel matrix to investigate its effect on the microstructure and mechanical behavior of Ni-TiC composites. All Ni-TiC composites powder was mechanically alloyed using planetary high energy ball mill with 400 rpm and ball to powder ratio (BPR) 15:1 for 24 h. Bulk Ni-TiC composites were then sintered via SPS process at 50 MPa pressure and 900–1200 °C temperature. All Ni-TiC composites exhibited higher microhardness and compressive strength than pure nickel due to the presence of homogeneously distributed TiC particles within the nickel matrix, matrix grain refinement, and excellent interfacial bonding between nickel and TiC reinforcement. There is an increase in Ni-TiC composites microhardness with an increase in TiC reinforcement from 5 to 50 wt.%, and it reaches the maximum value of 900 HV for Ni-50TiC composites.     

Effect of Mg Addition on the Microstructure and Properties of a Heat-Affected Zone in Submerged Arc Welding of an Al-Killed Low Carbon Steel

To reveal the effect of Mg treatment on the microstructure evolution behavior in the actual steel welding process, the microstructure and properties of Al-deoxidized high-strength ship plate steel with Mg addition were analyzed after double-side submerged arc welding. It was found that the Al–Mg–O + MnS inclusion formed under 26 ppm Mg treatment could promote acicular ferrite (AF) nucleation in the coarse-grained heat-affected zone (CGHAZ) and inhibit the formation of widmanstätten ferrite and coarse grain boundary ferrite. In the fine-grained heat-affected zone (FGHAZ) and intercritical heat-affected zone (ICHAZ), polygonal ferrite and pearlite were dominant. Al–Mg–O+MnS cannot play a role in inducing AF, but the grain size of ferrite was refined by Mg addition. The impact toughness in HAZ of the Mg-added steel was higher than that of Mg-free steel. With the heat-input rising from 29.55 to 44.11 kJ/cm, it remained relatively stable in Mg-treated steel. From the fusion line to the base metal, the micro-hardness of the fusion zone, CGHAZ, ICHAZ and FGHAZ decreased to some extent after Mg addition, which means the cold cracking tendency in the welding weak zone could be reduced. Finally, the mechanisms of Mg-containing inclusion-induced AF were also systematically discussed.