Beam Scanning Velocity Research Articles

INFLUENCE OF PARAMETERS OF OPERATING CONDITIONS OF SELECTIVE LASER MELTING ON SPALL FRACTURE KINETICS OF 12Cr18Ni10Ti STEEL

The results are presented with regard to experimental studies of dynamic strength characteristics of samples made of 12Cr18Ni10Ti steel powder. They were obtained through selective laser melting with various parameters of a technological process at shock-wave compression up to pressures of ~7 GPa. Critical parameters of a process, in addition to powder characteristics, include laser operating conditions: a laser spot diameter, laser power, a laser beam scanning velocity, as well as a powder layer thickness, protective atmosphere, etc. It has been demonstrated that an increase in the scan laser power and a decrease in a powder layer thickness bring about a decrease in a number of internal defects in initial structures of samples. The results are given, which compare strength characteristics of these steels with properties of steel produced by a traditional technique of hot rolling. Shock-wave experiments were carried out using a light gas gun-type facility, which makes it possible to accelerate flat impactors to speeds of ~700 m/s; internal wave processes in samples were reproduced when recording a rate of movement of the sample's free surface via a PDV laser interferometer; a degree of spall fracture was determined by the help of a metallographic analysis of samples recovered in tests. It has been showed that steel samples made through a selective laser melting technique have high spall strength and a lower degree of damage compared to hot-rolled steel under the same conditions of high-speed shock loading. According to the results of the metallographic studies, the presence of internal defects in initial structures of samples associated with a choice of operating conditions of a manufacturing process does not affect a degree of their spall damage. At the same time, a wave pattern of shock wave propagation differs significantly for samples with and without defects.

Microstructure and hardness of electron beam melted Ti-6Al-4V titanium alloy in dependence of the scanning speed

The effect of the beam scan velocity upon microstructure and hardness of Ti-6Al-4V titanium alloy produced by means of Electron Beam Melting (EBM) was investigated in this work. Five levels of scan velocity were used during the experimental campaign, ranging from 121 to 697 mm/s, and keeping all other process parameters fixed. It was found that a finer microstructure is generated with the increase of the beam speed, since the characteristic dimensions of both the prior columnar b grains and of the α laths decrease for higher beam speeds. In particular the finer microstructure was observed at a scan speed of 697 mm/s while the coarser at 121 mm/s. On the other side, the hardness of the EBMed samples increase with the increase of the scanning speed and a maximum hardness of about 400 HV was measured at a scan speed of 697 mm/s. This value is strictly associated to the finer microstructure that was obtained at this value of the scan speed. In particular, it was observed a 17% increase of the hardness going from a scan speed of 121 to one of 697 mm/s.

Femtosecond large-area fabrication of multi-phase titanium oxide LIPSS on thin films

The effect of the laser beam scanning velocity and overlap on the large-area fabrication of Laser-Induced Periodic Surface Structures (LIPSS), on 400 nm thickness titanium films, is investigated. The experiments were performed in ambient air with well below ablation threshold laser fluence by using a femtosecond laser at a wavelength of 1030 nm with a pulse duration of 270 fs, operating at a repetition rate of 20 kHz.For this purpose, single-line experiments were performed to study the LIPSS formation process as a function of fs-laser-beam scanning velocity. Based on these results, the fabrication of large-area LIPSS pattern was investigated by unidirectional scanning of the fs-laser beam across the sample surface using different lateral spacing. The results show the formation of LSFL and TLIPSS, with different orientation with respect to the polarization of the incident radiation. The analysis of the chemical composition, by microRaman spectroscopy, indicates the formation of a mixture of TiO2 (anatase phase) and Ti2O3 (rutile phase) for high scanning velocities and TiO2 (rutile phase) for low scanning velocities. The chemical composition, spatial period, and depth of the LIPSS show a clear dependence with the laser-beam scanning velocity and overlap.

Processing of Ti–13Nb–13Zr powder by selective laser melting under a relatively high oxygen atmosphere

Ti–13Nb–13Zr (TNZ) parts were produced by selective laser melting under an atmosphere with a relatively high O content using a comprehensive combination of laser powers and laser beam scanning velocities. The influence of processing conditions on the density, microstructure, interstitial element contents (N and O), and hardness of as-printed TNZ parts were studied. The N and O contents in the TNZ parts increased on increasing the energy density (ED). Despite the high O content in the chamber during laser processing, it was possible to obtain TNZ parts with high density and relatively low N and O contents, depending on the ED applied. Two different types of microstructure were observed in the TNZ parts: α′ martensite and α/α′ + β phases. Depending on its microstructure, the TNZ parts presented interstitial solid solution hardening and second phase hardening.

High-speed laser micro polishing of TiAl6V4

In the present paper, laser micro polishing of TiAl6V4 is investigated with main focus on increasing the area rate by using high pulse energy. First, the influence of the laser beam size, the scanning velocity, and the track distance on the final roughness is examined since these parameters determine the area rate. The results of these investigations are used to identify a set of process parameters that allow increasing the area rate and achieving a surface roughness comparable to state-of-the-art laser micro polishing. The results show that the laser beam size must exceed a critical size; otherwise, smoothing of the surface is limited, which leads to higher final roughness. The scanning velocity and the track distance do not have a great effect on the final roughness for the investigated regimes. However, with decreasing track distance, the number of undesirable surface structures, presumably resulting from evaporating vanadium compounds, is reduced. Additionally, the heat input into the surface decreases with increasing laser beam size, scanning velocity, and track distance. The results of these investigations are used to significantly increase the area rate to 19.1 cm2/s, which is a factor of 64 times faster than the state of the art. The final surface roughness is comparable to state-of-the-art laser micro polishing in this case.

Threshold properties of high modulus carbon fiber reinforced plastic composite with picosecond laser processing

The picosecond laser processing thresholds and morphology characteristics of polyacrylonitrile-based high modulus carbon fiber reinforced cyanate ester composite (M55/BS-4) and an asphalt-based high thermal conductivity carbon fiber reinforced plastic (K600/5418) were studied. Diameter-regression method was used to test and compare the ablation threshold, threshold incubation effect, and single-pulse thresholds for each composite were predicted. The influence of incident energy fluence (0.7-25 J/cm2) and beam scanning velocity (0.2-5 m/s) on the incision quality was analyzed. The results show that the great difference in the fiber thermal conductivity leads to significant quantitative difference in processing threshold and morphology. Using the highest scanning speed (5 m/s) available and fluence equivalent to 3.2 times the single-pulse threshold (i.e., about 8 J/cm2), carbon fiber and resin can be almost synergically removed, characterized by uniform slit inlet widths and neat cutting edge. The use of higher scanning speeds coupled with appropriate processing energy is expected to further improve the quality of processing.

Laser-induced scanning transfer deposition of silver electrodes on glass surfaces: A green and scalable technology

A pulsed laser ablation backwriting technique with high repetitive rates is implemented for the fabrication of silver coatings on glass surfaces. This method enables geometrical constraint-free deposition of metallic coatings. These exhibit sufficiently low electrical resistance to be used as electrodes in dielectric barrier discharge (DBD) plasma elements.Ambient air deposition of metallic silver electrodes on standard glass slides is explored using a sub-ns UV laser source, combined with hybrid beam scanning methods. The green nature of the overall deposition process includes a preliminary irradiation scan to homogenise the target surface before the subsequent backwriting step. Metal transfer is achieved by combining two phenomena within a simple beam scanning process: LIRT (laser-induced reverse transfer) of silver from the target to the glass, with a partial and secondary LIFT (Laser-Induced Forward Transfer) of silver from the glass to the target.Appropriate selection of pulse energy and pulse repetition rates, beam scanning velocities and target motion enable the growth of sufficiently thick Ag deposits on glass with the required low electrical resistivity and nearly 2D constraint-free geometry. This method avoids the use of vacuum and liquids, resulting in a cheap, facile and green methodology for the deposition of silver electrodes onto transparent substrate surfaces.

Experimental and Simulation Analysis of Effects of Laser Bending on Microstructures Applied to Advanced Metallic Alloys

The aim of this work is the analysis of laser beam forming (LBF) in the bending of two relevant materials used in the transportation industry—interstitial-free (IF) steel and AA6013 high-strength aluminum alloy. Our experiments and numerical simulations consider two different operating scenarios achieved by varying the laser beam scanning velocity using linear paths. The material behavior during this process is described via a coupled thermomechanical-plasticity-based formulation that allows prediction of temperature profiles and bending angles. Metallography, glow discharge optical emission spectroscopy, and X-ray diffraction are used for microstructure characterization. In addition, microstress analyses are performed in order to study the stress behavior of the irradiated zones. It is found that LBF mainly induces grain growth and melting in the case of high surface temperatures. Before melting, the materials developed compressive stresses that could be useful in preventing cracking failures. The resulting bending angles are predicted and experimentally validated, indicating the robustness of the model to estimate LBF effects on advanced alloys. The present analysis relating bending angles together with temperature and microstructure profiles along the thickness of the sheets is the main original contribution of this work, highlighting the need for further modeling refinement of the effects of LBF on advanced alloys to include more microstructural properties, such as grain boundary diffusion and surface roughness.

Influence of Microstructure and Chemical Composition on Microhardness and Wear Properties of Laser Borided Monel 400.

In this study, wear properties of Monel 400 after laser alloying with boron are described. Surfaces were prepared by covering them with boron paste layers of two different thicknesses (100 µm and 200 μm) and re-melting using diode laser. Laser beam power density was equal to 178.3 kW/cm2. Two laser beam scanning velocities were chosen for the process: 5 m/min and 50 m/min. Surfaces alloyed with boron were investigated in terms of wear resistance, and the surface of untreated Monel 400 was examined for comparison. Wear tests were performed using counterspecimen made from steel 100Cr6 and water as a lubricant. Both quantitative and qualitative analysis of surfaces after wear test are described in this paper. Additionally, microstructures and properties of obtained laser alloyed surfaces are presented. It was found that the wear resistance increased from four to tens of times, depending on parameters of the laser boriding process. The wear mechanism was mainly adhesive for surfaces alloyed with initial boron layer 100 µm thick and evolves to abrasive with increasing boron content and laser beam scanning velocity. Iron particles detached from counterspecimens were detected on each borided surface after the wear test, and it was found that the harder the surface the less built-ups are present. Moreover, adhered iron particles oxidized during the wear test.

THE INFLUENCE OF BORON IN THE SURFACE LAYER ON THE STRUCTURE AND THE TRIBOLOGICAL PROPERTIES OF IRON ALLOYS

This paper presents two methods of introducing boron into the surface layer of iron alloys, namely diffusion boronizing by means of the powder method and laser alloying with a TRUMPF TLF 2600 Turbo CO2 gas laser. Amorphous boron was used as the chemical element source. As regards diffusion drilling, the influence of temperature and time on the properties of the layer was tested. During the laser alloying, the influence of the thickness of the boriding paste layer as well as the power and laser beam scanning velocity was determined. How the carbon content in steel and alloying elements in the form of chromium and boron influence the structure of the surface layer was tested. To achieve this object, the following grades of steel were used: C45, C90, 41Cr4, 102Cr6, and HARDOX boron steel. The microhardness and wear resistance of the obtained boron-containing surface layers were tested. A Metaval Carl Zeiss Jena light microscope and a Tescan VEGA 5135 scanning electron microscope, a Zwick 3212B microhardness tester, and an Amsler tribotester were used for the tests. The structure of the diffusion- borided layer consists of the needle-like zone of FeB + Fe2B iron borides about 0.15 mm thick, with a good adhesion to the substrate of the steel subjected to hardening and tempering after the boriding process. After the laser alloying, the structure shows paths with dimensions within: width up to 0.60 mm, depth up to 0.35 mm, containing a melted zone with a eutectic mixture of iron borides and martensite, a heat affected zone with a martensitic-bainitic structure and a steel core. The microhardness of both diffusionborided and laser-borided layers falls within the range of 1000 – 1900 HV0.1, depending on the parameters of the processes. It has been shown that, apart from the structure and thickness of the layer containing boron and microhardness, the frictional wear resistance depends on the state of the steel substrate, i.e. its chemical composition and heat treatment. The results of testing iron alloys in the borided state were compared with those obtained only after the heat treatment.

Laser Alloying Monel 400 with Amorphous Boron to Obtain Hard Coatings.

In this study, Monel 400 is laser heat treated and laser alloyed with boron using diode laser to obtain adequate remelting and to improve the microhardness Single laser tracks were produced on the surface with three different laser beam scanning velocities: 5, 25, and 75 m/min. In order to enrich Monel 400 with boron surfaces were covered with initial layers of two different thicknesses before the process: 100 μm and 200 μm. In all experiments, laser beam power density was equal to 178 kW/cm2. Produced laser tracks were investigated in areas of microstructure, depth of remelting and microhardness. It was found that remelted zones are mainly composed of dendrites and the more boron is present in the laser track, the dendritic structure more fragmented is. Depth of remelting and microhardness depend not only on the laser beam scanning velocity but also on thickness of the initial boron layer. While microhardness of Monel 400 is equal to approximately 160 HV0.1, microhardness up to 980 HV0.1 was obtained in areas laser alloyed with boron.

Theoretical model for ablation of thick aluminum film on polyimide substrate by laser etching

The ablation mechanism of thick aluminum film on polyimide by a 1064 nm Nd:YAG laser etching was investigated. By establishing a theoretical model based on interaction of laser and material, the effects of laser parameters including laser power, equivalent laser pulse number, and scanning velocity of laser beam were studied. The results show that when a laser beam with fluence higher than the etching threshold of aluminum irradiated onto the target surface, the etched depth for a single laser pulse is a definite value; the equivalent laser pulse number can be derived when a polyimide plate coated with aluminum film was selected as an object of study. Namely, an optimized etched result can be obtained which corresponds to certain laser fluences and beam scanning velocities. By substituting the calculated results of laser processing parameters into a finite element simulation, the etched results of aluminum film on polyimide could be achieved. The simulated and experimental results present a good coincidence, and the results can be used to set practical laser processing parameters.

Microstructure and selected properties of Monel 400 alloy after laser heat treatment and laser boriding using diode laser

This study concentrates on describing effects of laser heat treatment of Monel 400 and laser alloying its surface with boron. Surfaces without and with initial boron layers of two different thicknesses (100 and 200 μm) were processed using diode laser. Laser beam power density was constant and equal to 178.3 kW/cm2. To determine the influence of laser beam scanning velocity on final properties of treated surfaces, laser beam scanning velocity was set on four different values: 5, 25, 50, and 75 m/min. Microstructures of pure Monel 400 and Monel 400 alloyed with 100 μm boron content are composed of dendrites. Areas laser alloyed with 200 μm boron layer contain mainly nickel borides. Boron addition in Monel 400 surface results in microhardness increase in which the level depends on boron content and the laser beam scanning velocity. Increasing the thickness of initial boron layer and speeding up the laser beam lead to obtain higher microhardness. On the other hand, areas laser alloyed with 200 μm boron layer using laser beam scanning velocity equal to 75 m/min contain deep cracks which propagate from the surface through the produced layer. Furthermore, it was found that the depths of laser heat-treated areas depend significantly on the boron content. As the result of differences in thermal properties between Monel 400 and boron, depth of re-melted zones in some conditions does not lower with increasing laser beam scanning velocity.

3D multi-layer grain structure simulation of powder bed fusion additive manufacturing

In powder bed fusion (PBF) additive manufacturing, powder layers are locally melted with a laser or an electron beam to build a component. Hatching strategies and beam parameters as beam power, scan velocity and line offset significantly affect the grain structure of the manufactured part. While experiments reveal the result of specific parameter combinations, the precise impact of distinct parameters on the resulting grain structure is widely unknown. This knowledge is necessary for a reliable prediction of the microstructure and consequently the mechanical properties of the manufactured part.We introduce the adaption of a three-dimensional model for the prediction of dendritic growth for use with PBF. The heat input is calculated using an analytical solution of the transient heat conduction equation. Massively parallel processing on a high-performance cluster computer allows the computation of the grain structure on the scale of small parts within reasonable times.The model is validated by accurately reproducing experimental grain structures of Inconel 718 test specimens manufactured by selective electron beam melting. The grain selection zone within the first layers as well as the subsequent microstructure in several millimeters build height is modeled in unprecedented level of detail. This model represents the cutting-edge of grain structure simulation in PBF and enables a reliable numerical prediction of appropriate beam parameters for arbitrary applications.

Study of Selective Laser Melting of intermetallic TiAl powder using integral analysis

Abstract Selective laser melting (SLM) of intermetallic Ti–48Al–2Cr–2Nb powder is studied using different and complementary methods: (a) experimental parametric analysis (variation of laser power, beam scanning velocity, etc.); (b) metallography including X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM); (c) optical monitoring by infra-red (IR) camera, high speed pyrometer and (d) mathematical modelling. The advantage of the present approach is based on parallel application of diverse methods of analysis using the same SLM set-up and feedstock powder. Also the novelty is related to the preheating of the entire manufacturing module up to 450 °C which is applied before and during the manufacturing of the samples. XRD analysis shows domination of the α2-Ti3Al phase in the initial powder, heat treated powder before SLM and in the developed samples. Some minor peaks of γ-TiAl are identified in SLM parts. EDS analysis confirms the effect of Al evaporation, which intensifies with laser energy input. IR-camera is used for optical monitoring of a single track formation. The geometry of the thermal emission field for different processing parameters is compared with the track geometry. The intensification of hydrodynamic instabilities accompanied by material rejection from the zone of powder consolidation with beam scanning velocity is found. For the first time the evaporation of Al for elevated laser energy input is clearly detected by the IR-camera. Mathematical modelling shows that the same elementary volume of SLM sample can be remelted several times during its fabrication depending on the SLM processing parameters and beam scanning strategy. The maximum values of calculated cooling rates are in the order of 106 °C/s. For the first time the thermo-cycling during SLM parts fabrication is confirmed experimentally by pyrometer measurements.

Experimental Analysis of Laser and Scanner Control Parameters During Laser Polishing of H13 Steel

Laser polishing is a manufacturing process in which a small amount of material is melted via laser irradiation and molten pool is then redistributed to create a smoother surface finish/superior surface quality. The focus of this study is to generate more comprehensive understanding of the effects that laser and scanner control parameters have on the formation of laser polished lines. A parameter known as “Laser On/Off delay” is varied along with the laser power to study the impact that these parameters have on the synchronization between laser power and beam scanning velocity. It was determined through experimental analysis that the “Laser On delay” parameter plays a significant role on the formation of the laser polished lines, essentially in a region that is outside to the widely characterized “steady state” zone of constant track width. A set of experiments was conducted to identify the combined effect of transient (acceleration/deceleration) phases for laser power and speed on the terminal geometry of the polished line. When the optimal transient combination of power and speed was used, surface quality improvement by 83% (areal surface roughness (Sa) reduction from 1.35 μm to 0.23 μm) was obtained.

Experimental investigation of laser forming process to produce dome-shaped products

The laser forming process is a new non-contact forming process which does not involve hard tooling. The bending occurs by a scanning laser beam causing localized heating resulting in localized plastic deformation which gives controlled bending with no spring back on cooling. A line bend is achieved by scanning in a straight line over the surface; more complex shaped surfaces, e.g., domes and pillow shapes require more complex scan patterns. In this manuscript, the forming of a dome-shaped structure from a flat circular aluminum sheet by a “spider” like scan pattern is discussed. The effect of varying the laser power, beam diameter, scan velocity, and number of scan passes on the dome height is shown experimentally. The results show that the dome height rises with increase in laser power or number of passes and a decrease in the beam diameter and scan velocity. A statistical fit equation using analysis of variance (ANOVA) is derived to predict the dome height as a function of these variables.

Height prediction of dome-shaped products in laser forming process

Laser forming is a thermal forming process, which uses laser beam irradiation to produce desired final forms. Initially, many researchers have investigated deeply in bending sheet metals using a straight scan pass. In this manuscript, a spider scanning strategy is considered to produce the dome-shaped aluminum sheet products. In this regard, laser forming process is investigated numerically and then some experiments are performed in order to validate the output results of numerical simulations. Full factorial design of experiments and analysis of variance are conducted to obtain an equation for predicting dome height of sheet metals. In addition, the effects of process parameters including laser power, scan velocity, beam diameter, and sheet thickness on dome height are studied. Moreover, the effects of heat flux, and line energy is investigated. The results show that with increasing laser power, line energy, and heat flux, dome height increases and with increasing beam diameter, sheet thickness, and scan velocity, dome height decreases notably. In addition, a final equation is presented which accurately predicts the dome height as a function of input process parameters.

Surface quality, microstructure, mechanical properties and tribological results of the SKD 61 tool steel with prior heat treatment affected by the deposited energy of continuous wave laser micro-polishing

SKD 61 tool steel specimens hardened by heat treatment are subjected to continuous wave laser micro-polishing (CWLμP). A total of 46 specimens are classified into three groups in order to find the operating conditions to minimize the areal average surface roughness (Sa) via a three-stage process. Deposited energy per area (DE, unit: J/mm2) is defined as a composite parameter of laser power (P), laser beam scanning velocity (V), and focused spot diameter (SS). The parameters of Sa and the sum of the thickness sum of the melt zone (MZ) and heat-affected zone (HAZ), as well as the mechanical properties of hardness (H) and reduced modulus (Er), are then expressed as a function of DE. Appropriate choices of laser operating conditions allow the Sa corresponding to the DE value at about 70J/mm2 to be the smallest for these specimens. Due to the complex behavior involved in the thermocapillary flow, the only controlling parameter that Sa increases monotonically with is the hatch distance. Decreases in focal offset (FO) and laser power (P) cause a decrease in the highest amplitude of the polished surface in the spatial frequency analyses. The highest amplitude cannot decrease if the scanning velocity is excessively high or low. The core roughness depth (Sk) and the reduced peak height (Spk) and valley depth (Svk) are found to be linearly proportional to Sa. The H and Er values of the specimens are lowered by increasing the applied deposited energy, although DE is not the sole governing parameter. The mean values of H and Er in the MZ, HAZ, and base material are found to increase by increasing the depth beneath the top surface of a specimen. The smaller wear rate in the specimens of CWLμP compared to that of the as-received specimen can be attributed to the lubrication effect arising in the tribo-contacts of specimens with a grain size greater than the critical value. The differences in several properties between the as-received specimens with and without heat treatment are compared and discussed.

Influence of laser beam fluence on surface quality, microstructure, mechanical properties, and tribological results for laser polishing of SKD61 tool steel

In the present study, the surface polishing of SKD61 tool steel specimens was carried out using a microsecond fiber laser system. The operating conditions of laser controlling factors and fluence for the minimization of the areal average surface roughness (Sa), wear rate and friction coefficient are determined through the planned arrangements of three stages including the experimental design method. The Sa value of the polished surface obtained from the measurements in the x direction perpendicular to the laser scanning direction was effectively reduced. However, that from the measurements parallel to the scanning direction strongly depends on the overflow probability of the melt flow. In the x direction, the highest peak after polishing was shifted to have a frequency different to that of the as-received specimen. The ripple frequency associated with the highest peak is affected by the laser beam scanning velocity and pulse frequency only. In the y direction, the amplitude of the highest peak after polishing is possibly greater than that of the as-received specimen if the melt overflowing the grinding mark stems left behind after polishing was solidified locally. However, its frequency is kept the same as that of the residual stems. A small Sa value can be obtained if the polished surface has small residual stems and lacks significant melt overflow. The thicknesses of the melt zone (MZ) and the heat-affected zone (HAZ) are strongly dependent on both the applied fluence and single-style controlling factor. Increases in laser power and pulse duration increase the MZ thickness, whereas increases in scanning velocity and pulse frequency decrease the MZ thickness. The formations of MZ and HAZ have the hardness (H) and the reduced modulus (Er) lower than those of the as-received specimen. This characteristic is proved to be consistent with the iron phases and their volume fraction formed in these two zones. The wear rate of a specimen depends on the total thickness and the microstructures of these two zones. An increase in laser power and decrease in scanning velocity can decrease the wear rate. The friction coefficient decreases with decreasing Sa of a polished surface.