Effect of Laser Remelting on the Microstructure and Corrosion Resistance of Plasma Sprayed Fe-based Coating

Fe-based amorphous and nanocrystalline coatings were fabricated by air plasma spraying. The coatings were further treated by laser remelting process to improve their microstructure and properties. The corrosion resistance of the as-sprayed and laser-remelted coatings in 3.5wt% NaC1 and 1 mol/L HCI solutions was evaluated by electrochemical polarization analysis. It was found that laser-remelted coating appeared much denser than the as-sprayed coating. However, laser-remelted coating contains much more nanocrystalline grains than the as-sprayed coatings, resulting from the lower cooling rate in laser remelting process compared with plasma spraying process. Electrochemical polarization results indicated that the laser-remelted coating has great corrosion resistance than the as-sprayed coating because of its dense structure.

Effect of laser remelting on microstructure and mechanical properties of CrMnFeCoNi high entropy alloy

Equiatomic CrMnFeCoNi high entropy alloy prepared by powder metallurgy was remelted by laser.The relative density and microstructure of fusion zone are evaluated.The nanoindentation tests are conducted to reveal the hardness difference of dendrite arms and interdendritic areas.Tensile tests are conducted to assess the mechanical properties of remelted HEA.After laser remelting,the number and morphology of voids changed significantly.Dendritic structure with face-centered cubic phase form in the fusion zone.Fe,Cr and Co are enriched in dendrite arm,while Mn and Ni are enriched in interdendritic area.Elements segregation led to a nanohardness difference between dendrite arm and interdendritic area.Local deformation occurs in interdendritic area during tensile tests and results in a fracture with directionality.

Laser Remelting of Plasma Sprayed NiCrAlY and NiCrAlY-Al_2O_3 Coatings

Two types of plasma sprayed coatings (NiCrAlY and NiCrAlY-A12O3) were remelted by a 5 kW cw CO2 laser. With increasing laser power and decreasing traverse speed in the ranges of 200-700 W and 5-30 mm/s respectively, the melted track grew in width and depth. In the optimum range of laser parameters, a homogeneous remelted layer without voids, cavities, unmelted particles and microcracks was formed. On the surface of remelted layers, Al203 and YAIO3 were detected. As a result of isothermal oxidation tests, weight gains of laser remelted coatings were obviously lower than that only plasma sprayed, especially laser remelted NiCrAlY-Al2O3 coatings. The effects of laser remelting and incorporation of A12O3 second phase in N1CrAlY matrix on high temperature oxidation resistance were discussed.

Multi-material additive manufacturing-functionally graded materials by means of laser remelting during laser powder bed fusion

Many processes may be used for manufacturing functionally graded materials.Among them,additive manufacturing seems to be predestined due to near-net shape manufacturing of complex geometries combined with the possibility of applying different materials in one component.By adjusting the powder composition of the starting material layer by layer,a macroscopic and step-like gradient can be achieved.To further improve the step-like gradient,an enhancement of the in-situ mixing degree,which is limited according to the state of the art,is necessary.In this paper,a novel technique for an enhancement of the in-situ material mixing degree in the melt pool by applying laser remelting(LR)is described.The effect of layer-wise LR on the formation of the interface was investigated using pure copper and low-alloy steel in a laser powder bed fusion process.Subsequent cross-sectional selective electron microscopic analyses were carried out.By applying LR,the mixing degree was enhanced,and the reaction zone thickness between the materials was increased.Moreover,an additional copper and iron-based phase was formed in the interface,resulting in a smoother gradient of the chemical composition than the case without LR.The Marangoni convection flow and thermal diffusion are the driving forces for the observed effect.

Strengthening behavior of AlCoCrFeNi(TiN)_(x) high-entropy alloy coatings fabricated by plasma spraying and laser remelting

High-entropy alloy(HEA)coatings are of great importance in the fabrication of wear resistance materials.HEA coatings containing ceramic particles as reinforcement phase usually have better wear performance.In this study,AlCoCrFe Ni(TiN)_(x)(x:molar ratio;x=0,0.2,0.4,0.6,0.8,1.0)HEA coatings were fabricated on Q235 steel by plasma spray first and then subjected to laser remelting.The experimental results confirm that plasma spray together with post laser remelting could result in the in-situ formation of TiN-Al_(2)O_(3) ceramic particles and cuboidal B2 phase in the AlCoCrFeNi(TiN)_(x) HEA coatings.The in-situ TiN-Al_(2)O_(3) and nano-cuboidal B2 precipitation phase strengthened the coatings and improved their wearresistance properties.Due to the dispersion of hard phase and nano-particles resulting from second heating,the microhardness of the Al Co Cr Fe Ni(Ti N)coatings significantly increased from 493 to 851 HV after laser remelting.For the same reasons,the wear-resistance performance was also significantly promoted after laser remelting.

Refined microstructure and enhanced mechanical properties of AlCrFe_(2)Ni_(2) medium entropy alloy produced via laser remelting

A Co-free as-cast AlCrAlCrFe_(2)Ni_(2)medium entropy alloy(MEA)with multi-phases was remelted by fiber laser in this study.The effect of laser remelting on the microstructure,phase distribution and mechanical properties was investigated by characterizing the as-cast and the remelted AlCrAlCrFe_(2)Ni_(2)alloy.The laser remelting process resulted in a significant decrease of grain size from about 780μm to 58.89μm(longitudinal section)and 15.87μm(transverse section)and an increase of hardness from 4.72±0.293 GPa to 6.40±0.147 GPa(longitudinal section)and 7.55±0.360 GPa(transverse section).It was also found that the long side plate-like microstructure composed of FCC phase,ordered B2 phase and disordered BCC phase in the as-cast alloy was transformed into nano-size weave-like microstructure consisting of alternating ordered B2 and disordered BCC phases.The mechanical properties were evaluated by the derived stressstrain relationship obtained from nano-indentation tests data.The results showed that the yield stress increased from 661.9 MPa to 1347.6 MPa(longitudinal section)and 1647.2 MPa(transverse section)after remelting.The individual contribution of four potential strengthening mechanisms to the yield strength of the remelted alloy was quantitatively evaluated,including grain boundary strengthening,dislocation strengthening,solid solution strengthening and precipitation strengthening.The calculation results indicated that dislocation and precipitation are dominant strengthening mechanisms in the laser remelted MEA.

Microstructure and properties of hybrid additive manufacturing 316L component by directed energy deposition and laser remelting

Arc additive manufacturing is a high-productivity and low-cost technology for directly fabricating fully dense metallic components.However,this technology with high deposit rate would cause degradation of dimensional accuracy and surface quality of the metallic component.A novel hybrid additive manufacturing technology by combining the benefit of directed energy deposition and laser remelting is developed.This hybrid technology is successfully utilized to fabricate 316L component with excellent surface quality.Results show that laser remelting can largely increase the amount ofδphases and eliminateσphases in additive manufacturing 316L component surface due to the rapid cooling.This leads to the formation of remelting layer with higher microhardness and excellent corrosion resistance when compared to the steel made by directed energy deposition only.Increasing laser remelting power can improve surface quality as well as corrosion resistance,but degrade microhardness of remelting layer owing to the decrease inδphases.

Microstructures and properties of Fe-Co-B-Si-Nb coating prepared by laser cladding and remelting

High power laser cladding of [ （ Fe0. 5 Co0. 5 ） 0. 75 B0. 2 Si0.05 ] 95. 7 Nb4. 3 powder mixture afier-remelting was performed to fabricate Fe-based metallic glass coating on the surface of steel of China Classification Society： Grade B （CCS-B）. Scanning electron microscopy （SEM）, X-ray diffraction （XRD）, transmission electron microscopy （TEM） with energy dispersive spectrometer （EDS）, Vickers hardness tester and corrosion resistance tester were employed to characterize microstructures and evaluate properties of this coating. According to the results of SEM, XRD and TEM, the cladding coating consisted of nanocrystalline embedded in amorphous phase. EDS data indicated that Nb segregated in the amorphous matrix. The results of hardness test revealed that the hardness of the top layer was higher than that of the inner layer of the coating. The coating exhibited excellent corrosion resistance in a 3.5% NaCl solution.

The crystallographic texture and dependent mechanical properties of the CrCoNi medium-entropy alloy by laser remelting strategy

Laser remelting(LR)has attracted widespread attention in recent years as an effective method to reduce internal defects and improve the surface quality of additively manufactured(AM)parts.In the present study,three different LR inter-layer scanning strategies(LR0,LR90 and LR45)and their effects on the porosity,microstructure,crystallographic texture and related mechanical properties of parts have been studied.Optical microscope,X-ray diffraction,and scanning electron microscope were used as characterization tools.In the LR90 sample,it shows obvious{111}<110>texture and strong<111>preferred orientation along the scanning direction(SD),while the 0°offset and the 45°rotation of LR scanning strategy form a finer microstructure and weak crystallographic texture.Meanwhile,the mechanical properties of the LR sample are improved compared with the sample only by laser metal deposition(LMD),and a combination of higher strength and optimal uniform elongation is obtained in the LR45 sample.The overall results show that a reasonable LR scanning strategy can reduce the anisotropy of AM parts and improve their mechanical properties.

Influence of Additive Silica on the Laser Melting of the Ceramic Coatings

The influence of additive silica on the microstructure of plasma sprayed Al2O3 and Al2O3+13 wt pct TiO2 ceramiccoatings at laser melting has been investigated in this study. At the laser melting, additive silica in Al2O3 ceramiccoating can reduce the stress of cooling shrinkage generated during solidification. Moreover, silica can render finersize of grains of the melting layer and form continuous glassy matter around the grain boundaries so as to reducefurther the cooling stresses and to suppress the formation and spreading of cracks. On the other hand, at the lasermelting, TiO2 reacts with Al2O3 and transforms into TiAl2O5. The latter new phase has great and anisotropiccoefficients of thermal expansion leading to big and asymmetrical stresses and thus to form cracks in the meltinglayer of Al2O3+13 wt pet TiO2 coating. Due to the fact that the influence of additive silica on the suppression of theformation of cracks is rather limited and cannot counterbalance the negative effect of TiAl2O5, thus the melting layerof Al2O3+13 wt pct TiO2 coating doped with 3 wt pct SiO2 cracks also. Nevertheless, TiO2 can greatly developthe wear resistance of the ceramic coating as sprayed or laser melted.

Oxidation Behaviour of Laser Remelted Cr-containing Steels

Laser surface remelting of steels with different Cr contents has been performed by using a CO_2 laser. The results of oxidation tests showed that the effects of laser remelting on the oxidation resistance of Cr-containing steels have close relation to the Cr contents and microstructures of the steels. The re- sistance to high temperature oxidation of 18-8 and HK40 at 1273 K can be obviously improved by laser remelting, whereas laser remelting showed little effect on the oxidation resistance of Fe-6Cr and Cr30 at 1173-1273 K.

Microstructure and Wear Resistance Properties of FeCrAl-Cu(Ni,Co)HEA Coatings Synthesized by Laser Remelting

FeCrAlCu,FeCrAlCuNi,FeCrAlCuCo,and FeCrAlCuNiCo high-entropy alloy(HEA)coatings were synthesized on the surface of 45#steel through cold spraying-assisted laser remelting.Results reveal that all four HEA coatings are composed of face-centered cubic+body-centered cubic phases.Additionally,the microstructure of the coatings consists of columnar dendrites.With the simultaneous addition of both Ni and Co elements,the columnar dendritic grains are gradually refined in the coating.Moreover,the FeCrAlCuNiCo HEA coating exhibits excellent friction performance with the coating hardness of 5847.7 MPa,friction factor of 0.45,and wear rate of 3.72×10^(−5) mm^(3)·N^(−1)·m^(−1).The predominant wear mechanism is the adhesive wear and abrasive wear.

EFFECT OF CeO_2 ON MICROSTRUCTURE AND WEAR RESISTANCE OF LASER REMELTED FeBSi COATING

The hardness and wear resistance of sprayed FeBSi coating after laser remelting were much improved by addition of 8 wt-% CeO_2.Microstructural observation on the FeBSi+CeO_2 coating revealed that the formation of martensite occurs,as well as the refined grains and the more eutectic and compounds with regular morphology are dis- tributed.While the FeBSi coating free from CeO_2 is a sharp constrast in microstructure.

On the Martensitic Transformation Temperatures and Mechanical Properties of NiTi Alloy Manufactured by Selective Laser Melting: Effect of Remelting

In this study, the infl uence of laser remelting on the relative density, martensitic transformation temperatures(MTTs), and mechanical properties of a NiTi alloy fabricated by selective laser melting(SLM) at a laser power between 15 and 75 W were investigated. A relative alloy density of approximately 99% was achieved in the power range of 45–60 W corresponding to the forming energy density range of 65.45–87.27 J/mm3. The MTTs increased with the increase in the energy density;thus, the initial contents of the B2 and B19′ phases of the SLM-produced NiTi alloy can be tailored by the utilized technique. However, the number of defects such as metallurgical pores and microcracks considerably increased at higher energy densities(> 87.27 J/mm3). Interestingly, the concentration of these defects was reduced by remelting in the energy density range of 21.82–65.45 J/mm3, while the alloy relative density increased to 99.7% ± 0.1% at a remelting energy density of 65.45 J/mm3. The results of tensile testing revealed that when the remelting energy was 75% or 100% of the forming energy input, the ultimate tensile strength and elongation of the alloy significantly increased. Therefore, the remelting strategy represents a promising route for manufacturing NiTi alloys with desired MTT ranges and mechanical properties.

Effects of substrate surface treatments on hybrid manufacturing of AlSi7Mg using die casting and selective laser melting

To balance the manufacturing cost and customizability of automotive parts,a hybrid manufacturing process combining die-casting and selective laser melting(SLM)is proposed:starting with a conventional cast substrate,SLM is utilized to add additional geometric elements on top of it.For this hybrid process,the first priority is to prepare a substrate surface suitable for the subsequent SLM addition of the top-on elements.In this study,the original cast surface of AlSi7Mg was processed by sandblasting,wire electro-discharge machining,and laser remelting,respectively.Then,additional AlSi7Mg components were built on both the original cast and treated surfaces through SLM.After hybrid builds,these surfaces and resultant interfaces were examined by optical and scanning electron microscopes.Results indicate that the defect-free metallurgical joint between the cast and additively added parts can be formed on all surfaces except for the one processed by electro-discharge machining.The observed epitaxial grain growth crossing the interface implies a strong connection between the cast and the SLMed component.Despite these benefits,also mismatches in microstructure,residual stress level and element distribution between the two parts are identified.After a comprehensive assessment,laser remelting with no additional machining is recommended as the optimal surface treatment preceding SLM fabrication,because of its user-friendly operation,low cost,and high industrial feasibility.

Microstructural evolution and hardness of as-cast Be-Al-Sc-Zr alloy processed by laser surface remelting

As-cast beryllium-aluminum(Be-Al)alloy exhibits a coarse microstructure with pore defects due to a large solidification interval,greatly limiting its mechanical properties.In this research,the relationship between laser surface remelting process and microstructure and hardness of as-cast Be-Al-Sc-Zr alloy was established.The experimental results demonstrated that a pore-free refined microstructure of remelted layer was obtained by controlling the parameter of effective laser energy input.The microstructure of as-cast Be-Al-Sc-Zr alloy consisted of equiaxed grains with Al phase forming a continuous frame wrapping Be phase,which was significantly refined in the remelted zone(from 25μm to 2μm).The Vickers hardness in the remelted zone(approximately 210 HV)was approximately 3 times that of as-cast Be-Al-Sc-Zr alloy.Analysis of the Vickers hardness and the Be phase size showed a good agreement with a Hall-Petch equation.In addition,transmission electron microscopy(TEM),auger electron spectroscopy(AES)and X-ray diffraction(XRD)analysis evidenced that Sc and Zr elements formed a single blocky phase Be13(Scx,Zr1-x),which was also greatly refined from 8μm to 1.5μm in the remelted zone.The results obtained in this study indicate that the laser surface remelting allowed refining the microstructure and further strengthening the Vickers hardness of Be-Al-Sc-Zr alloy.

Influence of laser surface remelting on microstructure and degradation mechanism in simulated body fluid of Zn-0.5Zr alloy

In this study,the Zn-0.5 wt%Zr(Zn-Zr)alloy was treated by laser surface remelting(LSR),and then the microstructure and degradation mechanism of the remelting layer were investigated and compared with the original as-cast alloy.The results reveal that after LSR,the bulky Zn(22)Zr phase in the original Zn-Zr alloy is dissolved and the coarse equiaxed grains transform into fine dendrites with a secondary dendrite arm space of about 100 nm.During the degradation process in simulated body fluid(SBF),the corrosion products usually concentrate at some certain areas in the original alloy,while the corrosion products distribute uniformly and loosely in the LSR-treated surface.After removing the corrosion products,it was found that the former suffers obvious pitting corrosion and then localized corrosion.The proposed mechanism is that corrosion initiates at grain boundaries and develops into the depth at some locations,and then leads to localized corrosion.For the LSR-treated sample,corrosion initiates at some active sites and propagates in all directions,corrosion takes place in the whole surface with distinctly uniform thickness reduction,while the localized corrosion and peeling of bulky Zn(22)Zr particles were eliminated.The electrochemical results also suggest the uniform corrosion of LSR-treated sample and localized corrosion of original sample.Based on the results,a new approach to regulate the corrosion mode of the biodegradable Zn alloy is proposed.

Rapid Directional Solidification with Ultra-High Temperature Gradient and Cellular Spacing Selection of Cu-Mn Alloy

The detailed laser surface remelting experiments of Cu-31.4 wt pct Mn and Cu-26.6 wt pct Mn alloys on a 5 kW CO2 laser were carried out to study the effects of processing parameters (scanning velocity, output power of laser) on the growth direction of microstructure in the molten pool and cellular spacing selection under the condition of ultra-high temperature gradient and rapid directional solidification. The experimental results show that the growth direction of microstructure is strongly affected by laser processing parameters. The ultra-high temperature gradient directional solidification can be realized on the surface of samples during laser surface remelting by controlling laser processing parameters, the temperature gradient and growth velocity can reach 106 K/m and 24.1 mm/s, respectively, and the solidification microstructure in the center of the molten pool grows along the laser beam scanning direction. There exists a distribution range of cellular spacings under the laser rapid solidification conditions, and the average spacing decreases with increasing of growth rate. The maximum, λmax, minimum, λmin, and average primary spacing, A, as functions of growth rate, Vb, can be given by,λmax=12.54Vb-0.61, λmin=4.47 Vb-0.52, λ=9.09Vb-0.62, respectively. The experimental results are compared with the current Hunt-Lu model for rapid cellular/dendritic growth, and a good agreement is found.

Effects of scanning speed on microstructure in laser surface-melted single crystal superalloy and theoretical analysis

Scanning speed is a critical parameter for laser process, which can play a key role in the microstruc- ture evolution of laser melting. In the laser melting of single crystal superalloy, the effects of scanning speed were investigated by experimental analysis and computational simulation. The laser was scanning along [710] direction on （001） surface in different speeds. Solidification microstructures of dendrites growth direction and the primary dendritic spacing were analyzed by metallograph. Besides, a planar interface during solidification was taken into attention, Experiment results indicated that the primary dendritic spacing and thickness of planar interface decrease with the increase of speed. Through simu- lation, distribution of dendrites growth velocity and thermal gradient along dendrite growth direction were calculated, and the simulation of dendrites growth direction agreed with the experiment results. Additionally, a constant value was acquired which can be used to predict the primary dendritic spacing. Moreover, according to curve-fitting method and inequality relation, a model was proposed to predict the thickness of planar interface.

Microstructure and Mechanical Properties of AlSi10Mg Alloy Manufactured by Laser Powder Bed Fusion Under Nitrogen and Argon Atmosphere

In order to study the effect of gas atmosphere on forming performance of laser powder bed fusion(LPBF),AlSi10 Mg alloy was prepared by direct forming and in situ laser remelting under the shielding gas of argon and nitrogen in this study,and its micro structure and properties were characterized and tested,respectively.The results show that the forming performance of AlSi10 Mg under nitrogen atmosphere is better than that of argon.Moreover,in situ laser remelting method can effectively enhance the relative density and mechanical properties of AlSi10 Mg,in which the densification is increased to 99.5%.In terms of mechanical properties,after in situ remelting,ultimate tensile strength under argon protection increased from444.85±8.73 to 489.45±3.20 MPa,and that under nitrogen protection increased from 459.21±13.77 to 500.14±5.15 MPa.In addition,the elongation is nearly doubled and the micro-Vickers hardness is increased by 20%.The research results provide a new regulation control method for the customization of AlSi10 Mg properties fabricated by LPBF.