Laser-processed Layers Research Articles

Utilizing ultrafast lasers for postprocessing to improve mechanical properties of 3D-printed parts

Recent advances in additive manufacturing technologies have already led to the wide-scale adoption of 3D-printed parts in the aerospace, medical, automotive, tooling, and electronics industries. The expansion in choice of materials that can be processed, in particular, using fused deposition modeling (FDM), selective laser sintering/melting, and stereolithography, and the steady advancements in dimensional accuracy control, have extended the range of applications beyond rapid prototyping. However, additive manufacturing still has considerable limitations compared to traditional and subtractive manufacturing processes. This work addresses limitations associated with the as-deposited surface roughness of 3D-printed parts. The effects of roughness-induced stress concentrations on the mechanical strength were studied, and ultrafast laser postprocessing was utilized to reduce the surface roughness of 3D-printed parts. The samples were manufactured using a commercial desktop FDM system and standard ASTM flat dogbone geometries. The samples were then postprocessed with a high-repetition-rate ultrafast Yb-fiber laser using a multi-layer scan approach. This novel postprocessing method enables high-efficiency material removal without inducing excessive thermal residual stresses into the material and, therefore, is suitable for postprocessing thermally sensitive materials, such as PLA and other polymers as well as parts with engineered porosity. In this work, we vary laser process parameters, such as average power and number of laser-processed layers, to achieve various levels of surface roughness. Values of tensile strength of the specimens were compared between 3D-printed samples featuring initial roughness and laser postprocessed samples with different values of surface roughness. The results indicate that the laser-processed samples exhibit an almost 10% increase in tensile strength depending on specific laser processing parameters.

Pulsed laser processed NiCrFeCSiB/WC coating versus coatings obtained upon applying the conventional re-melting techniques: Evaluation of the microstructure, hardness and wear properties

In the present work, the influence of pulsed laser re-melting on the structure, hardness and wear resistance of Ni-WC metal matrix composite (MMC) coatings was investigated. The NiCrFeCSiB/40%WC powder was used for experiments. The microstructural analysis upon applying the scanning electron microscopy, energy dispersive spectroscopy and X-ray diffractometry was conducted along with microhardness measurements and wear resistance tests. The obtained properties of laser-processed layers were compared with the analogical coatings re-melted using three different conventional heating techniques. The evolution of the structure in Ni-WC layers during heating was studied and it was found that upon using conventional heating techniques, the optimal microstructure of the WC containing Ni-based coating that provided the highest hardness (~880 HK) and the best wear resistance was obtainable in a narrow range of heating duration. An incomplete re-melting process results in an absence of metallurgical bond between coating and substrate. When overheated, tungsten carbides dissolve in metal matrix. The laser processing provided stable ultrafine W-rich dendrites in Ni-rich matrix microstructure of deposited layer, which morphology did not change significantly with variation of the process parameters. The size of the finest tungsten-rich particles was about 200 nm and the hardness reached ~990 HK, providing 12–31% improvement as compared with the best results of induction, furnace, and flame heating. The wear rate of laser processed coating was 12% (by mass loss) and 42% (by thickness of removed layer) lower as compared with the highest results for conventionally heated coatings.

Effect of scanning speeds on microstructure and wear behavior of laser-processed NiCr–Cr3C2–MoS2–CeO2 on 38CrMoAl steel

Self-lubricating wear-resistant NiCr–Cr3C2–MoS2–CeO2 layers were fabricated on 38CrMoAl extruder screws by laser processing. The effect of scanning speeds on microstructure, phases, microhardness, and wear behavior was investigated. The obtained results indicate that the laser-processed layers had fine and nonuniform microstructures with undissolved MoS2 particles distributed on the matrix. With an increase of the laser-scanning speeds, the microstructures changed from hypoeutectic to hypereutectic, volume fraction of martensite increased, microhardness increased, and thickness and friction coefficients of the layers decreased. Wear resistance of the optimized layer was increased by 29.76 times compared with that of the substrate. The undissolved MoS2 was separated from the matrix on loading. In addition to the grain-refining and solution-strengthening effects, oxide films formed on the surface of the layers shielded them and enhanced their wear resistance. The crack or fracture behavior of the laser-processed layers on loading was determined by its toughness, which also had an important effect on the wear behavior of the processed layers.

Surface modification of cp-Ti using femtosecond laser micromachining and the deposition of Ca/P layer

Femtosecond laser processing is employed to create regular patterns and bioactive layer on the surface of pure titanium. Surface morphology and microstructure of the laser-processed layer are characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that the processing with varied laser energies builds a surface with three-order roughness from nanometer scale to micrometer scale. Selected area electron diffraction indicates that some kind of TiO layer emerges on the surface after femtosecond laser machining. The bioactivity of TiO layer is further evaluated by soaking it in simulated body fluid (SBF). SEM observation and EDX analysis show that Ca/P layer is rapidly formed on the surface of TiO layer after SBF soaking. It implies that TiO layer with unique three-order roughness has good bioactivity.

Effect of Laser Process Parameters on the Surface Absorbtivity

Hardening laser surface offers new possibilities for the enhancement of the mechanical resistance of superficial layers. Essential for the laser treatment processes is the efficiency at which the incident laser power is coupled into the work piece. The aim of the work was to study the effect of some factors such as spot shape, beam angle of incidence and surface coating on the steel surface absorptivity. Samples made of carbon steel were laser processed using a CW CO2 - 1,2 kW laser unit. Different process parameters (beam power, spot dimension and traverse speed), were used. The microstructure of laser hardened layers which have been investigated by optical microscopy. The mechanical characterization of the layers has been done by hardness measurements. Correlation has been established between the structure of the laser-processed layers and the process parameters.

Evaporation effect on laser induced solid–liquid phase transitions in CdTe and HgCdTe

Numerical simulation of laser induced phase transitions (melting, solidification, evaporation) in a near-surface region of CdTe and HgCdTe was carried out. The possibility of solidification from the outer surface of the melted layer caused by a vaporization of volatile component (Cd, Hg, etc.) was shown. This allows to explain experimentally observed high defectiveness of the laser-processed layer.

Laser surface melting and alloying of type 304 L stainless steel

Microstructures of laser surface melted (LSM) and alloyed (molybdenum and tantalum) type 304L stainless steel have been studied. 6-ferrite was found to be the primary solidification phase for these alloys. Chemical analysis of the laser-processed layers indicated no compositional variation between the as-received and laser-processed layers in terms of major alloying elements, i.e. chromium and nickel. However, laser processing resulted in a lowered manganese content, by about 15%. Furthermore, TEM studies showed laser processing to introduce extensive amorphous Mn-Si precipitation into the microstructure.

Tribology and Hardness of Excimer‐Laser‐Processed Titanium Layers on Cubic Zirconia

We have examined the wear and friction and surface hardness of excimer‐laser‐processed Ti layers on cubic zirconia substrates. The film exhibits a complex array of cracking following processing that is related to the crystallographic orientation of the substrate. The friction between the laser‐processed layers and both steel and ruby pins is reduced by approximately one‐third relative to that of untreated zirconia. In the untreated case, wear is characterized by pin wear and debris, whereas the laser‐processed layer wears by film transfer to the pin. The surface hardness of the processed layer is lower than that of both the untreated zirconia and the deposited Ti film. Indentation tests indicate that the surface is brittle following processing.

Laser-processed electrodes consisting of amorphous nickel-niobium-platinum group metal surface alloys on niobium

Laser processing was applied to a specimen composed of a bulk niobium substrate covered by electrodeposited nickel and platinum group metals in order to form amorphous surface alloys having excellent electrocatalytic activities. The bulk niobium substrate acts as an electric conductor instead of thin amorphous alloys of a high electric resistance. Laser processing leads to alloying of electrodeposited metals with niobium, and hence the niobium content of the laser-processed layer is determined by the laser-processing conditions. A wide area is vitrified only when the niobium content was 35–40 at.% in spite of the fact that nickel-niobium alloys containing 25–65 at.% niobium become amorphous by conventional methods for preparation of amorphous alloys such as melt spinning. Vitrification of a wide area is performed by overlapped traverses by the laser beam and often results in crystallization in the heat-affected zone in the previously vitrified phase. Thus the laser processing must be carried out so as to form the surface alloy having a very high glass-forming ability, and the appropriate laser-processing conditions must be determined for each alloy family. The laser-processed amorphous alloy electrodes exhibit excellent electrocatalytic activities when their surfaces are activated chemically.

Laser melt injection of BN powders on tool steels I:Microhardness and structure

A 5 kW continuous wave CO 2 gas laser, operating in an oscillating square beam mode, was utilized to melt inject hexagonal BN powders into the surfaces of AISI T1 and M 2 tool steels. Optical metallography, scanning electron microscopy, Auger spectroscopy and Vickers' microhardness tests were employed to characterize the melt layers. Both the hardness and the BN concentration of laser-processed layers were increased with an increase in the number of superimposing laser passes. A Vickers hardness of 2150 HV (maximum) was obtained in the specimen that was BN injected ten times. Microstructural features included fine cellular-dendritic structures and complex precipitates of metal boronitrides. The melt layers were nearly free from cracks and pores.

Some corrosion characteristics of stainless surface alloys laser processed on a mild steel

Preparation of corrosion-resistant ferritic and austenitic surface alloys on a mild steel was attempted by laser processing of chromium-plated mild steel and chromium and nickel-plated mild steel. The passivating ability increased with increasing number of laser irradiations due to an improvement of homogeneity of laser-processed surface layer with increasing number of meltings. However, there is an optimal number of irradiations, since further increase in number of laser irradiation meltings lowered both the passivating ability and pitting potential due to a decrease in chromium content in the laser-processed layer by evaporation.

Reflection-factors of porcelain enamel and various metals

Preparation of corrosion-resistant ferritic and austenitic surface alloys on a mild steel was attempted by laser processing of chromium-plated mild steel and chromium and nickel-plated mild steel. The passivating ability increased with increasing number of laser irradiations due to an improvement of homogeneity of laser-processed surface layer with increasing number of meltings. However, there is an optimal number of irradiations, since further increase in number of laser irradiation meltings lowered both the passivating ability and pitting potential due to a decrease in chromium content in the laser-processed layer by evaporation.