Influence of laser heat treatment on microstructure and properties of surface layer of Waspaloy aimed for laser-assisted machining

In this study, a nickel-based superalloy, Waspaloy, was laser heat treated with diode laser. Single laser tracks were manufactured with different laser beam power densities between 63 and 331 kW/cm², and scanning laser beam speed ranged from 5 to 100 m/min. It was found that laser heat treatment of Waspaloy causes decrease in material hardness—the microhardness in laser tracks is about 300 HV0,1 while the microhardness of substrate is ranged from 300 to 600 HV0,1—which is a positive phenomenon for laser-assisted machining of investigated material. Impacts of laser heat treatment parameters on laser tracks properties were identified for obtaining multiple laser tracks with the most homogenous thickness. Moreover, roughness of heated layers was measured to specify surface quality after laser heat treatment. Multiple laser tracks were produced using different scanning laser beam speed and distances between laser tracks ranged from 0.125 to 1 mm. It was found that if scanning laser beam speed is 75 m/min and distance between laser tracks is equal to or lower than 0.25 mm, in microstructures of multiple laser tracks, cracks are occurring. The most suitable laser heat parameters for obtaining heated layers, and which can be used for laser-assisted machining, were identified as laser beam power density 178.3 kW/cm², scanning laser beam speed 5 m/min, and distance between laser tracks 0.125 mm.     

Modeling of vapor-to-melt ratio in laser cutting aluminum alloy sheet

To study the regular pattern of vapor-to-melt ratio in laser cutting sheet metal, a physical model of vapor-to-melt ratio is developed to demonstrate the material remove forms of vaporization-melt in cutting area and the state of energy and mass flow in the molten layer. Variation of vapor-to-melt ratio with laser power and cutting velocity is obtained by laser cutting of 6063 aluminum alloy sheet. The 0.5-mm sheet thickness is carried out on a JK701H Nd:YAG pulse laser cutting system by simulating under the regression correction of cut radius. Observation on the cut samples with different parameters (65 W, 85 W, 105 W varied with laser power increasing, and 2.2 mm/s, 2.0 mm/s, 1.8 mm/s with decreasing of beam cutting speed) and the calculations show that vapor-to-melt ratio increases (0.595–1.995, 0.672–2.631, 0.787–4.171) with laser power (65 W–110 W) and decreases with cutting velocity (1.8 mm/s–2.4 mm/s). At the same time, the laser cutting quality increases with vapor-to-melt ratio and the decrease with thickness of residual molten layer. The results show good agreement between vapor-to-melt ratio model and experiments. The analysis verifies that this model is feasible and it makes contribution to laser precision cutting.     

Numerical modeling and experimental study of thermally induced residual stress in the direct diode laser heat treatment of dual-phase 980 steel

The direct diode laser application has been found useful in the localized heat treatment of metal parts because of its wider beam and more uniform energy distribution with respect to other lasers with Gaussian-like energy distribution. In this study, an uncoupled thermomechanical finite element model is developed to study the temperature field and thermally induced stress evolution in high-strength dual phase (DP) 980 steel during its direct diode laser heat treatment. Thermal analysis results are experimentally validated through thermocouples and then input into a mechanical model as transient temperature loading in order to acquire the thermally induced stresses and strains. The effect of martensite phase transformation on residual stress distribution in heat-treated DP980 steel is considered. An X-ray diffraction technique is used to measure the residual stress distribution at the top surface of the heat-treated coupons of DP980 steel. The numerical results show that compressive stresses are located at the laser–material interaction zone. After heat treatment, tensile stresses are retained at the heat-treated DP980 steel coupons. There is qualitative agreement between the numerically predicted and experimentally measured residual stresses. The effect of the overlapping ratio on the residual stress and hardness of the heat-treated DP980 steel is also experimentally and numerically investigated.     

Modelling of back tempering in laser hardening

Back tempering is one of the most critical problems in laser hardening of extended surfaces. In this type of treatment, several laser tracks are slightly overlapped to obtain a uniform hardened surface. Due to the overlapping, tempered zones are generated on the treated surface with the consequent lack of uniformity in the surface hardness. In this work, a regression model was developed to estimate the loss of hardness due to the tempering effect as a function of the thermal cycle. A specific test, named laser surface treatment test, was designed and executed to reproduce the hardness reduction due to the tempering effect. An analytical thermal model was developed to evaluate the thermal cycle undergone by the material during this test. By the results of the laser surface treatment test combined with the analytical model, a prediction model was estimated. Good agreement was found between predicted and measured hardness decrease, and the identified model could be integrated in a numerical code to evaluate the optimal process parameters.     

Fe modeling of the apparent spot technique in circular laser hardening

Circular laser hardening is the laser surface treatment used in the case of cylindrical workpieces. The single-track treatment is a particular case of circular laser hardening used when only one revolution of the workpiece is executed since the treatment of a narrow surface is required. As a result, an annular narrow hardening track is obtained. During the laser hardening, the initial and final parts of the workpiece are overlapped and treated twice. The main drawback of this treatment is the back-tempering effect focused on the overlapping zone. This phenomenon leads to a hardness decrease in the overlapping zone. To avoid this problem, a new technique called apparent spot (AS) was introduced by the authors. The aim of the AS technique is to increase in a fictitious way the dimensions of the laser spot. In the case of circular laser hardening, this technique results into a high-speed rotation (up to 1,000 rpm) of the cylindrical workpiece instead of the traditional low speed. So, a uniform hardening zone without overlapping and back tempering is obtained. However, despite these benefits, there is still a lack of knowledge about the physics of this treatment in particular referring to the thermal cycle that affects the workpiece. In order to enhance the knowledge of this technique in this work, the AS was modeled via the FE approach. DEFORM software was used to model the circular laser hardening process. The software was firstly validated by a comparison with experimental results. Once the software reliability was tested, a regression model was estimated to predict the surface temperature within the treatments. Good agreement was found between the prediction model and the numerical results.     

A study on the influence of laser power on microstructural evolution and tribological functionality of metallic coatings deposited on Ti-6Al-4V alloy

Metallic Ti–Co binary coatings were fabricated on titanium alloy (Ti–6Al–4V) substrate by laser surface cladding technique using a continuous wave RofinSinar 4 kW Nd: YAG laser. The influence of laser power on microstructure, hardness and tribological performance of Ti–Co laser clad coatings on titanium alloy (Ti–6Al–4V) was examined. Laser powers of 750 and 900 W were varied with constant scan speed of 1.2 m/min. A beam size of 3 mm and argon shield gas flow rate of 1.2 L/min were set as the operating laser parameters. Phase identification and morphological studies of the coatings were carried out using X-ray diffractometry (XRD) and scanning electron microscopy (SEM), respectively. Based on the results of laser process optimisation, it was observed that both laser powers produced clad coatings with good metallurgical bond with no cracks or pores in the coatings. With respect to the substrate (Ti–6Al–4V), the microstructure, hardness and friction/wear behaviour of Ti–Co coatings on Ti–6Al–4V substrate were enhanced obviously.     

Influence of high-power diode laser heat treatment on wear resistance of a mold steel

The effect of hardness on wear loss and wear behavior during fretting was studied. A high-power diode laser was used to achieve the surface hardening of a mold steel (AISI P20-improved) at temperatures of 1000 and 1200 °C. A hardness increment was detected in laser heat-treated specimens, which may be attributed to phase transformation from ferrite to martensite, influencing wear loss and fretting wear behavior. In the fretting test results, smaller wear scars and less wear loss were observed for laser heat-treated specimens in comparison to those of base metal. Moreover, relatively more stable friction coefficient profiles were obtained for the laser heat-treated specimens due to uniform contact characteristics at two contacting surfaces. The effectiveness of the proposed technique was verified by the morphology of the wear scars of the treated specimens, which had a smooth appearance and minor abrasion grooves.

     

Microstructural and hardness investigation of tool steel D2 processed by laser surface melting and alloying

Several techniques can be used to improve surface properties of metals. These can involve changes on the surface chemical composition such as alloying or on the surface microstructure, such as hardening. In the present work, melting of the surface by a 9 kW CO₂ CW laser of wavelength 10.6 μm was used to alter surface features of D2 tool steel. Carbon powder and nitrogen gas were used as sources of alloying elements during laser processing. The effect of various laser parameters (power and speed) on the microstructure and hardness of D2 tool steel was investigated. Laser powers from 1 to 8 kW and laser speeds from 5 to 15 mm/s were employed. It was found that as the laser power increases, the hardness of the melted zone decreases while that of the heat-affected zone increases. On the other hand, the depth of both of melted and heat-affected zones increases with power.     

Design of energy optimization for laser polymer joining process

Different laser heat inputs were applied on the gray-colored acrylonitrile butadiene styrene (ABS) plastic using fixed laser power and variable scanning speeds to join ABS- and polycarbonate (PC)-based polymers. Experiments with a laser power between 6 and 8 W and a scanning speed of 1,500, 3,000, and 4,500 mm/min were used for the joining. Heat-affected zone (HAZ) and melt zone measurements were performed to find the joining energy threshold, and the mechanical properties of welds were evaluated. At the low scanning speed, the total heat input at the given area resulted in carbonization damage on the surface. However, energy distributed laser beam joining process by galvanometers resulted in secure and sound weld joining quality. Damage threshold was calculated as 127 J/cm² with relatively less sensitivity of scanning speed. However, the ablation threshold was measured to be 215, 281, and 424 J/cm² for the scanning speed of 4,500, 3,000 and 1,500 mm/min, respectively.     

Features of electromagnetic methods for testing the wear resistance of medium-carbon structural steel subjected to laser or bulk hardening and tempering

Features of applying attachable eddy-current transducers of two types (with a flat end surface and a protruding ferrite rod core with localities 5–6 and 3–4 mm in diameter, respectively) for testing the structural state, hardness, and abrasive wear resistance of structural steel 45X (0.45 mass % C and 0.85% Cr), which was hardened under the action of continuous laser radiation, have been studied. The feasibilities of the eddy-current and coercimetric techniques for evaluating the wear resistance of a medium-carbon steel subjected to laser or bulk hardening and tempering in the temperature range 75–600°C have been studied.     

Effect of process parameters in nanosecond pulsed laser micromachining of PMMA-based microchannels at near-infrared and ultraviolet wavelengths

This paper presents investigations on the effects of nanosecond laser processing parameters on depth and width of microchannels fabricated from polymethylmethacrylate (PMMA) polymer. A neodymium-doped yttrium aluminium garnet pulsed laser with a fundamental wavelength of 1,064 nm and a third harmonic wavelength of 355 nm with pulse duration of 5 ns is utilized. Hence, experiments are conducted at near-infrared (NIR) and ultraviolet (UV) wavelengths. The laser processing parameters of pulse energy (402–415 mJ at NIR and 35–73 mJ at UV wavelengths), pulse frequency (8–11 Hz), focal spot size (140–190 μm at NIR and 75 μm at UV wavelengths) and scanning rate (400–800 pulse/mm at NIR and 101–263 pulse/mm at UV wavelengths) are varied to obtain a wide range of fluence and processing rate. Microchannel width and depth profile are measured, and main effects plots are obtained to identify the effects of process parameters on channel geometry (width and depth) and material removal rate. The relationship between process variables (width and depth of laser-ablated microchannels) and process parameters is investigated. It is observed that channel width (140–430 μm at NIR and 100–150 μm at UV wavelengths) and depth (30–120 μm at NIR and 35–75 μm at UV wavelengths) decreased linearly with increasing fluence and increased non-linearly with increasing scanning rate. It is also observed that laser processing at UV wavelength provided more consistent channel profiles at lower fluences due to higher laser absorption of PMMA at this wavelength. Mathematical modeling for predicting microchannel profile was developed and validated with experimental results obtained with pulsed laser micromachining at NIR and UV wavelengths.     

Dual-beam laser welding of ultra-thin AA 5052-H19 aluminum

Welding of 0.05 mm (0.002 inch) thin AA 5052-H19 aluminum samples in lap-joint configuration was conducted autogenously (no filler metal) using dual lasers that included Nd:YAG and diode with a zero inter-beam spacing. The 70-ns pulsed Nd:YAG (1064 nm) laser acted as the welding tool while the continuous wave diode (810 nm) laser with interaction times of 40–120 ms served to improve the light absorption characteristics of aluminum through preheating and oxidation effects. The microstructure, composition, flaws, and hardness of the joint were evaluated by scanning electron microscopy, energy dispersive X-ray analysis, X-ray diffraction, and micro-indentation hardness test. The dual-beam welding technique was also compared with single-beam (Nd:YAG) welding. Results of parametric effects are displayed in the form of processing maps. Deeper penetration, better weld quality (less humping and cutting), and increased hardness were observed in dual-beam welds when compared with single-beam welds. The most astounding result was a nearly 200% increase in hardness over the base metal in dual-beam welding. This can be explained by the oxygen pickup in a dual-beam weld due to longer heating and amorphous microstructure of aluminum oxide as revealed by energy dispersive X-ray spectrum and X-ray diffraction respectively.     

Microstructure and hardness of fiber laser deposited Inconel 718 using filler wire

A continuous wave 5 kW fiber laser welding system was used to deposit INCONEL® alloy 718 (IN718) layers in conduction mode by applying filler wire with a composition similar to the parent metal, which was extracted directly from a scrapped, service-exposed IN718 aerospace component. The quality of the deposits was characterized in both the as-deposited and fully heat-treated conditions in terms of the macrostructure, defects, microstructure, and hardness. Integral deposits with no visible porosity were obtained using the fiber laser deposition technique. In the as-deposited clad zone, weld metal liquation cracking led to the presence of minor microcracks in the lower layer beads near the layer interface. The crack healing behavior observed after post-clad heat treatment of the IN718 deposits supports the marked potential of using the laser deposition technique by filler wire addition to manufacture and repair/remanufacture superalloy components for aerospace applications.     

Experimental analysis of applicability of a picosecond laser for micro-polishing of micromilled Inconel 718 superalloy

An increasing number of recent technological advancement is linked to the widespread adoptions of ultra-short picosecond (ps) pulsed laser in various applications of material processing. The superior capability of this laser is associated with the precise control of laser–material interaction as an outcome of extremely short interaction times resulting in almost-negligible heat affected zones. In this context, the present study explores the applicability of a picosecond laser in laser micro-polishing (LμP) of Ni-based superalloy Inconel 718 (IN718). The specific research goals of the present study constitute determination of melting regime—a mandatory phase for LμP, establishing the concept of polishability of the spatial contents of the initial surface topography and experimental demonstration of the process capability of a ps laser for potential micro-polishing applications. The initial surface topography was prepared by micromilling operation with a step-over of 50 μm and scallop height of 2 μm. The LμP experiments were performed at five different levels of fluence associated with the melting regime by changing the focal offset, a parameter denoting the working distance between workpiece surface and focusing lens focal plane. The LμP performance was evaluated based on the line profiling average surface roughness (R _a) spectrum distributed at different spatial wavelength intervals along the laser path trajectory. Furthermore, additional statistical metrics such as material ratio and power spectral density functions were analyzed in order to establish the process parameters associated with best achievable surface finish. The applicability of ps LμP was demonstrated in two regimes—1D (line) and 2D (area) polishing. During 1D LμP, significant (～52 %) improvement of the surface quality was achieved by reducing an R _a value from 0.50 μm before polishing to an R _a value of 0.24 μm across the laser path trajectory on initially ground surface. In addition, an initially micromilled area of 4.5?×?4.5 mm was LμPed resulting in the reduction of an areal topography surface roughness (S _a) value from 0.435 to 0.127 μm (70.8 % surface quality improvement).     

An experimental and coupled thermo-mechanical finite element study of heat partition effects in machining

A better understanding of heat partition between the tool and the chip is required in order to produce more realistic finite element (FE) models of machining processes. The objectives are to use these FE models to optimise the cutting process for longer tool life and better surface integrity. In this work, orthogonal cutting of AISI/SAE 4140 steel was performed with tungsten-based cemented carbide cutting inserts at cutting speeds ranging between 100 and 628 m/min with a feed rate of 0.1 mm/rev and a constant depth of cut of 2.5 mm. Cutting temperatures were measured experimentally using an infrared thermal imaging camera. Chip formation was simulated using a fully coupled thermo-mechanical finite element model. The results from cutting tests were used to validate the model in terms of deformed chip thickness and cutting forces. The coupled thermo-mechanical model was then utilised to evaluate the sensitivity of the model output to the specified value of heat partition. The results clearly show that over a wide range of cutting speeds, the accuracy of finite element model output such as chip morphology, tool–chip interface temperature, von Mises stresses and the tool–chip contact length are significantly dependent on the specified value of heat partition.     

Process simulation and optimization of laser tube bending

A 3D thermomechanical finite element analysis model for laser tube bending is developed based on the software MSC/Marc. The processes of single- and multi-scan are analyzed numerically. The gradient and development of the temperature between the laser scanning side and the nonscanning side leads to the changing complexity of the stress and strain. Consequently, the length of the laser scanning side becomes shorter than that of nonscanning side after cooling. The length difference between both sides makes the tube produce the bending angle. The relationship between the number of scans and the bending angle is about in direct ratio. The bending angle induced by the first irradiated time is largest. Meanwhile, the finite element simulation is integrated with the genetic algorithm. Aiming at different process demands, corresponding objective functions are established. Laser power, beam diameter, scanning velocity, and scanning wrap angle are regarded as design variables. Process optimizations of maximum angle bending and fixed angle bending after single laser scan are realized. Groups of optimized process parameters can be obtained according to different optimization objectives. The bending angle can approach to the maximum when the laser power, spot diameter, scanning velocity, and scanning wrap angle are 381.24 W, 3.37 mm, 16.34 mm/s, and 123.1°, respectively. When the laser power, spot diameter and scanning velocity are 426.12 W, 4.9 mm, 14.31 mm/s respectively, a fixed angle bending can be achieved.     

Finite element simulation and process optimization for hot stretch bending of Ti-6Al-4V thin-walled extrusion

Ti-6Al-4V is widely utilized to manufacture airframe component structures with curvature, because of its excellent strength to weight ratio, outstanding resistance to corrosion, and inherent thermal and electrical compatibility with carbon fiber composite. Hot stretch bending (HSB) is an effective technology to manufacture these kinds of structures. When comparing the thin-walled extrusion with the thick-walled one, however, it is more difficult to form. The reason is that the local temperature of extrusion decreases more because of the heat transfer between extrusion and die. In this study, the material properties of Ti-6Al-4V were measured experimentally, such as the tensile property within the temperatures from 873 to 1023 K and the strain rates from 0.0005 to 0.005 s^?1, the stress relaxation behavior in a wide range of temperatures (773–973 K) and prestrains (0.7–10%), as well as the heat transfer rule between Ti-6Al-4V (extrusion material) and asbestos cement (die material) under different pressures (8–25 MPa). The heat transfer coefficients (HTCs) were determined by an inverse analysis procedure, which was based on the comparison between measured and calculated temperature. Then, the coupled thermomechanical finite element (FE) model considering the effect of heat transfer was established. The influence of preheating temperature of die, initial temperature of extrusion, and dwell time on spring-back was researched based on orthogonal array testing strategy (OATS). The optimized parameters were verified by process test. It was showed that the established FE model could be used to predict spring-back within a relative deviation of 8.05%.     

Understanding the process parameter interactions in multiple-pass ultra-narrow-gap laser welding of thick-section stainless steels

In laser welding, typical welding penetration depths are in the order of 1–2 mm/kW laser power. The multipass laser welding technique, based on the narrow-gap approach, is an emerging welding technology that can be applied to thick-section welds by using relatively low laser power, but the process is more complicated since it is necessary to introduce filler wire to narrow-gap weld configurations. The aim of this work was to understand significant process parameters and their interactions in order to control the weld quality in ultra-narrow-gap (1.5 mm gap width) laser welding of AISI grade 316L stainless steel. A 1-kW IPG single-mode fiber laser was used for welding plates that were 5 to 20 mm in thickness using the multiple-pass narrow-gap approach. Design of experiments and statistical modelling techniques were employed to understand and optimise the processing parameters. The effects of laser power, wire feed rate, and welding speed on the weld homogeneity, integrity, bead shape, gap bridgability and surface oxidation were studied. The results were evaluated under different optimising constraints. The results show that the models developed in this work can effectively predict the responses within the factors domain.     

Experimental study on the effect of varying focal offset distance on laser micropolished surfaces

Laser micropolishing (LμP) is an innovative part-finishing process that reduces machining roughness by melting a thin layer of material on the micromilled surface using a focused laser beam. The quality of the final polished surface is dependent upon the part material, initial surface topography and roughness, and the energy density of the beam. The focal offset distance (FOD) is one critical parameter that controls the amount of energy delivered to the workpiece. The impact of varying the FOD on final laser-polished surface quality is investigated by performing a series of experiments on carefully prepared AISI H13 test samples with known initial surface roughness and waviness due to the milled track periodicity. Three well-defined polishing regimes were observed when adjusting the FOD for a Q-switched Nd/YAG LμP system between 1.3 and 2.9 mm. Given the same initial micromilled surface geometry, each LμP regime (i.e., short FOD, <1.8 mm; long FOD, >2.2 mm; and intermediate FOD) reduced the surface roughness and periodic waviness in a distinct manner. For a micromilled sample with a 33-μm periodicity, the LμP with FOD of >2.2 mm was determined to be the most effective regime by improving surface quality by 39.7 %. The affects of repetitive exposure to the beam and increasing the applied laser power on improving surface quality are also investigated for the 3 LμP regimes.     

Influence of cryosoaking period on wear characteristics and surface topography of M35 tool steel

Cryogenic treatment affects tool steels wherein alteration in microstructural features like phases, uniform precipitation of carbides is observed. In this work, improvement in wear resistance of cryotreated material with microstructural features and surface roughness of material has been correlated. Samples of AISI M35 steel were hardened at 1200°C, followed by triple tempering at 555°C in the salt bath, subsequently subjected to cryogenic treatment at minus 185°C for varying cryosoaking period (4–32?h) followed by soft tempering at 100°C. Such samples were characterized for hardness, microstructure, carbide density, wear rate and surface roughness. A correlation of carbide density and roughness has been established with wear resistance.