Evaluation of microstructure and thermal history for TiC/Inconel 625 MMC deposition through pre-placed laser cladding method with and without the application of ultrasonic vibration

The current work presents the deposition of TiC/ Inconel 625 metal matrix composites (MMC) coating through pre-placed powder bed laser cladding method using a Yb-Fiber laser, under two different conditions, with and without the application of ultrasonic vibration to the work-piece with frequency ranging from 28 to 34 kHz. The disintegration of the TiC particles and its distribution in the matrix was observed under various process parameters (laser power, scan speed and vibration frequency), and the microstructural changes were correlated with the molten pool lifetime obtained from the temperature history of the clad layer, captured using an IR-Pyrometer. The laser cladding without the application of vibration was conducted under constant and variable line energy to study the effect of laser interaction time and laser power on the microstructural evolution under fixed and variable heat flux. Higher molten pool lifetime (due to slow scan speed) at constant laser power resulted in higher disintegration of the MMC particles and formation of dendrites, whereas higher laser power under constant heat flux also resulted the same. Further, the application of ultrasonic vibration, specifically a higher vibration amplitude (at lower vibration frequency) was found to be responsible for more uniform distribution of TiC particles due to breaking of agglomerates, which led to higher hardness

Real-time detection of cooling rate using pyrometers in tandem in laser material processing and directed energy deposition

A novel method of monitoring cooling rates in real-time using two pyrometers arranged in tandem has been demonstrated. First pyrometer monitors the temperature at the center of the molten pool, second monitors the temperature at its tailing end. The difference in two pyrometer signals provides the temperature gradient at 1 kHz frequency from which cooling rate is determined in real-time using Arduino interface. Effectiveness of this method in real-time monitoring of cooling rate during laser remelting and additive manufacturing by directed energy deposition with varying process parameters and layer number is demonstrated. © 2020 Elsevier B.V

A study on developing process-structure–property relationship with molten pool thermal history during laser surface remelting of Inconel 718

In the area of laser material processing, laser surface remelting has been found to be an effective method of improving surface properties such as hardness, wear, and corrosion resistance. However, the scale of improvement depends on the evolving microstructure and phases, which depend on the cooling rates. Therefore, in the present study, laser surface remelting of Inconel 718 was carried out and a process-structure–property relationship with respect to the cooling rates has been developed. During the laser surface remelting process, the molten pool thermal history i.e. cooling rate, molten pool lifetime, and solidification shelf time is monitored and estimated using an IR pyrometer. The evolution of microstructure is later correlated with these parameters. With an increase in scan speed, the cooling rate is found to increase resulting in transformation of microstructure from equiaxed grains to columnar epitaxial growth. Based on the results obtained, a process map is proposed to establish a particular type of microstructure with respect to the cooling rate. Further, the effect of cooling rate and microstructure on the surface hardness and specific wear rate has also been investigated. Both surface hardness and specific wear rate got reduced with decreasing cooling rate at a slower scan speed due to grain coarsening and an increase in elemental segregation or Laves phase formation. © 2022 Elsevier Lt

Laser surface polishing of NiCrSiBC – 60WC ceramic-metal matrix composite deposited by laser directed energy deposition process

Deposition of ceramic-metal matrix composite using laser directed energy deposition process presents multi-fold challenges. High melting point ceramic particles often remain partially melted and increase the roughness of the deposit, which essentially requires secondary finishing operation. Besides high surface roughness, the high gradient of thermal and physical properties between ceramic reinforcement and metal matrix introduces cracks in the composite. Therefore, in the present work, the effect of laser surface polishing and substrate heating on improving the surface quality of NiCrSiBC – 60WC ceramic-metal composite deposited by laser directed energy deposition process was investigated. The molten pool thermal history was monitored using an IR pyrometer during laser surface polishing. The effect of rate of heat input on heating rate, cooling rate, molten pool lifetime and peak temperature was investigated and correlated with the surface quality parameters viz. arithmetic surface roughness (Ra) and ten-point height (Rz). A combination of intermediate laser power and scanning speed (600 W and 2000 mm/min) resulted in proper spread of molten pool and rendered better surface finish. The surface roughness (Ra) was found to improve from 19.2 μm ± 1.36 to 1.75 μm ± 0.20. Further, different orientations of laser polishing (0°, 45°and 90°) with respect to the material deposition direction were examined, and 45° was found to yield better surface finish. Surface cracks were observed for all the cases irrespective of process parameters and cooling rates, which were mitigated by substrate pre-heating. © 2020 Elsevier B.V

Development of process maps based on molten pool thermal history during laser cladding of Inconel 718/TiC metal matrix composite coatings

In the present study, laser cladding of Inconel 718/TiC metal matrix composite coating on SS 304 was carried out using a 2 kW Yb-Fiber laser by preplaced and blown powder method. The molten pool thermal history during the deposition process was monitored using an IR pyrometer and correlated with the evolution of microstructure. The effect of various process parameters, viz. laser power ‘P’ (400 W – 1200 W), scanning speed ‘V’ (200 mm/min to 1400 mm/min) and powder feeding rate (8 g/min – 20 g/min) on the molten pool lifetime and cooling rate is investigated. Molten pool lifetime increased and cooling rate decreased with the decrease of scanning speed and increase of laser power. However, scanning speed had dominant effect on the molten pool lifetime and cooling rate compared to laser power. Moderate molten pool lifetime resulted in the formation of shell-core structure between Inconel 718 matrix and TiC particles. The longer molten pool lifetime (>0.45 s) resulted in complete decomposition of TiC particles rendering brittleness to the coating. The feasibility of detecting TiC particle decomposition in Inconel 718 matrix from the recorded thermal cycles based on multiple solidification shelfs is discussed. At the end, a process map based on a combined process parameter, PD/V2 (spot diameter D) and molten pool lifetime (τ) was developed. An optimum range of molten pool lifetime i.e. 0.25 s < τ ≤ 0.45 s was found to result in a good quality of coating with the formation of shell-core structure both in case of preplaced and blown powder laser cladding method. © 2020 Elsevier B.V

A study on in-situ synthesis of TiCN metal matrix composite coating on Ti–6Al–4V by laser surface alloying process

Laser surface alloying of Ti6Al4V substrate with in-situ synthesized TiCN was carried out by melting preplaced titanium and graphite powders in 3:1 ratio (wt%) in nitrogen atmosphere at different scan speeds in 400–1400 mm/min range, maintaining the laser power and the beam diameter constant at 1200 W and 3 mm respectively. Molten pool thermal history during the alloying process was recorded using an IR pyrometer. With the decrease in scan speed, the cooling rate was found to decrease, while the molten pool lifetime increased which caused densification of TiCN dendritic structure. Though it enhanced the hardness, coatings were found to fail exhibiting brittle fracture during the scratch test, while coatings processed at higher scan speeds (800 mm/min −1400 mm/min) were found to be intact. Further, coatings processed at higher scan speeds also exhibited better wear resistance. Linear polarization electro-chemical corrosion test was performed on the coatings and it was observed that even corrosion properties were better in case of coatings processed at higher scan speeds

High-temperature abrasive wear characteristics of H13 steel modified by laser remelting and cladded with Stellite 6 and Stellite 6/30% WC

The study is aimed to analyse the comparative behaviour of the high-temperature abrasive wear of H13 steel surfaces modified by laser melting and cladding with Stellite 6 and Stellite 6 + 30 wt% WC. 3-body abrasive tests were conducted at room temperature, 450 °C, 550 °C, and 650 °C. The microstructural evolution, microhardness, wear surface morphology and mechanisms, and various phases formed during laser surface modifications were also studied. The laser remelting of H13 steel surface increased its room temperature microhardness to 750 ± 35 HV0.01, whereas laser cladding of Stellite 6 powder yielded hardness of around 600 ± 20 HV0.01 in the clad layer; and Stellite 6/WC composite clad layer had marginally higher hardness than the Stellite 6 clad layer in the matrix and much higher hardness of ~3000 HV0.01 at the sporadically distributed WC particle sites. Though the room temperature microhardness of laser remelted H13 surface is the highest, the volumetric wear loss in it was comparable to that of the Stellite 6 cladding. However, Stellite 6/WC composite layer recorded a relatively less volumetric loss as WC particles resisted the abrasive wear. With increasing temperature, the wear loss in laser remelt surfaces increased at a fast rate, while that in Stellite 6 and composite clad layers varied marginally with no definite trend. Overall, Stellite 6/WC composite cladding performed better than others in the current temperature range. © 2021 Elsevier B.V

Effect of scan strategy and heat input on the shear strength of laser cladded Stellite 21 layers on AISI H13 tool steel in as-deposited and heat treated conditions

Coating of hard-facing, wear and corrosion resistant materials on working surfaces of engineering parts made of relatively lower grade materials plays an important role in their service life as this is dictated by the coating material rather than the substrate material. However, the quality of coating in terms of pores- and crack- free deposition, low dilution and good bond strength is vital for the working life of coatings. In this work, bond strength of laser cladded Stellite 21 layers deposited on AISI H13 tool steel with different scanning strategies and line energy is investigated by subjecting the clad-substrate interface to shear force. As Stellite 21 is deposited to improve surface property of high-temperature dies and also for refurbishing worn out dies, bond strength of cladded specimens subjected to a high thermal cycle was also measured. Three sets of shear test specimens of rectangular clad build-up were made with different deposition strategies. First two sets have same laser line energy, but interfacial layers have overlapped clad tracks deposited in two orthogonal directions to study the effect of track orientation on the bond strength. In the third set line energy was doubled by reducing the scan speed. Bond strength was more for shear force applied along the direction of clad tracks. Higher interfacial bond strength of clad tracks parallel to shear stress direction compared to transverse tracks could be resulting by the similar effect that the orientation of fibers in a composite have on its strength. Deposition with higher line energy also had higher bond strength because of more uniform overlapped clad interface. However, specimens subjected to heating up to 1000 °C followed by furnace cooling had much reduced bond strength due to grain coarsening. Results are supported by the micro-structure, micro-hardness, dilution and elemental analyses of cladding and fractured surfaces

Microstructure, mechanical, and corrosion properties of electron beam-welded commercially pure titanium after laser shock peening

Surface properties play a critical role in the structural integrity of any component and this becomes even more critical for weld joints. Laser shock peening (LSP) is one of the non-contact methods which is getting popular in industries to enhance surface properties for improving service life, mainly fatigue of engineering components. In the present study, electron beam welding (EBW) of commercially pure (cp) titanium was carried out at constant line energy with varying scan speeds ranging from 1000 to 1600 mm/min and welding currents from 25 to 40 mA. The influence of the EBW parameters on microstructure, surface micro-hardness, tensile, and fatigue strength was investigated. The effect of LSP on mechanical and corrosion properties of EBW cp titanium was studied. LSP induced a significant amount of compressive residual stresses at the surface through plastic deformation and led to significant improvement in micro-hardness (7–10%) at the sub-surface region of the fusion zone due to finer grain structure. Further, appreciable enhancement in tensile strength (~ 15%) and fatigue life (~ 43%) due to strain hardening and changes in microstructure like twinning within grains was realized. © 2021, The Author(s), under exclusive licence to Springer-Verlag London Ltd., part of Springer Nature

Performance of additive manufactured Stellite 6 tools in friction stir processing of CuCrZr sheet

In the recent time friction stir welding (FSW), a solid state welding process has rapidly gained attention for joining high melting point materials like Cu, Fe, Ti and their alloys apart from Al alloys due to its several advantages over fusion welding techniques. AISI H13, a versatile chromium–molybdenum hot work hardened steel, has been the most commonly used as a tool material for aluminium alloys. However, low tool life due to plastic deformation and wear at elevated temperatures is limiting its application in welding of high melting point materials. In the present work the performances of as-received, heat treated, laser remelted and Stellite 6 hardfaced H13 steel tools in friction stir processing (FSP) of CuCrZr have been investigated. Stellite 6 hardfaced FSW tools are developed by additive manufacturing (AM) process on H13 steel as a base material. In all these cases except the Stellite 6 hardfaced tool, the shoulder and pin are found to deform plastically with significant wear of shoulder along with the diffusion of CuCrZr into tool from tool pin-shoulder interface. However, tools developed by AM process are found to remain intact without any significant deformation or wear