Laser-affected Zone Research Articles

Localized Laser Heat Treatment on AISI 1045 Steel Using a LPBF Laser

ABSTRACTLaser heat treatment (LHT) can provide local microstructure modification for metallic materials. This study investigates the viability of LHT on AISI 1045 steel using a laser designed for laser powder bed processing (LPBF) with a very small beam diameter compared to existing literature. Energy density models from the literature are analyzed and adapted to improve consistency across varying laser properties, mainly focusing on spot size and laser–material interaction time. Fifteen different laser parameter schematics are evaluated, comparing hardness versus depth relations as well as laser‐affected zone (LAZ) profiles. Through adapting the energy density model, new laser parameters are calculated and demonstrated to produce a valid localized heat treatment.

Periodic surface structure of 4H-SiC by 46.9 nm laser.

This paper presents an experimental study on the laser-induced atomic and close-to-atomic scale (ACS) structure of 4H-SiC using a capillary-discharged extreme ultraviolet (EUV) pulse of 46.9 nm wavelength. The modification mechanism at the ACS is investigated through molecular dynamics (MD) simulations. The irradiated surface is measured via scanning electron microscopy and atomic force microscopy. The possible changes in the crystalline structure are investigated using Raman spectroscopy and scanning transmission electron microscopy. The results show that the stripe-like structure is formed due to the uneven energy distribution of a beam. The laser-induced periodic surface structure at the ACS is first presented. The detected periodic surface structures with a peak-to-peak height of only 0.4 nm show periods of 190, 380, and 760 nm, which are approximately 4, 8, and 16 times the wavelength. In addition, no lattice damage is detected in the laser-affected zone. The study shows that the EUV pulse is a potential approach for the ACS manufacturing of semiconductors.

Study on surface thermal oxidation of silicon carbide irradiated by pulsed laser using reactive molecular dynamics.

Pulsed lasers are a powerful tool for fabricating silicon carbide (SiC) that has a hard and brittle nature, but oxidation is usually unavoidable. This study presents an exploration of the oxidation mechanism of 4H-SiC in oxygen and water under different temperatures via reactive force field molecular dynamics. Single pulse irradiation experiments were conducted to study the oxygen content of the laser-affected zone through energy dispersive x-ray spectrometry. The results show that laser-induced thermal oxidation is a complex dynamic process with the interactions among H, C, O, and Si atoms. The oxidation zone includes an oxide layer, a graphite layer, and a C-rich layer. With an increase in oxygen concentration, the amorphous oxide layer changes from silicon oxide to silicon dioxide. In addition, the formation of carbon clusters at the interface between SiOx and C-rich layers promotes the desorption of the oxide layer. The mechanism revealed in this study provides theoretical guidance for high-quality processing of 4H-SiC at atomic and close-to-atomic scales.

Engineering austenite/martensite mesostructured materials by controlled localised laser treatments in a Fe–Ni–C alloy

Localised laser treatments enable the creation of sophisticated austenite/martensite mesostructures in Fe–Ni–C steel with the potential of achieving enhanced mechanical performance. The control of phase topology is essential to modify the properties of these structures on demand and requires a profound understanding of the effect of the processing parameters on the development of the different phases upon the application of laser treatment. In this work, the microstructure evolution under exceptional gradients in temperature and heating rates is thoroughly investigated. The extent of the laser-affected zone and the heat input were tailored by varying laser parameters and specimen thickness, based on a model that considers transient material properties and the coupling between temperature and microstructure. The predicted temperature fields resulted in a complex interplay between martensite to austenite phase transformation and martensite tempering. Considering the high heating rates of up to 25000 K/s and the observed microstructures, it is suggested that austenite was formed by a pseudo-displacive mechanism and subsequently fully recrystallised in the zones most directly affected by the laser heat source. A smooth strength transition from austenite to martensite, affected by the laser parameters, could be exploited for more effective deformation mechanisms and improved material mechanical properties.

Molten pool behavior and solidification characterization in steady magnetic field assisted laser-MIG hybrid welding of aluminum alloy

Microstructure evolution is highly correlated with the melt flow characterization in welding. In this study, a 3D heat flow model with consideration of magnetic field was constructed to describe the temperature field, the flow and keyhole behaviors in the molten pool during the laser-MIG hybrid welding. The role of magnetic flux density in the molten pool dynamics and microstructure evolution was explored. A moving hybrid heat source model including Gaussian surface heat source model and Gaussian rotary body heat source model as well as double ellipsoidal heat source model was carried out to elucidate the laser and MIG arc energy distribution. Decent agreement is realized between the measured weld geometry and predicted weld pool dimension. Numerical results indicate that an expanded molten pool is clearly observed under magnetic field. As the magnetic flux density increases, the downward flow velocity of the molten pool is enhanced. Moreover, visible difference of oscillation growth period for keyhole depth is produced and this leads to a variation in the time it takes for the molten pool to stabilize. Significant grain refinement appearing in both arc-affected zone at the weld upper part and laser-affected zone at the weld lower part is documented although the refinement degree varies at different zones. This is attributed to the formation of many initial nuclei or crystallites, originated from the enhanced flow of the molten pool with the assistance of magnetic field. This study provides a perspective in understanding the physical phenomena involved in magnetic field assisted laser-MIG hybrid welding and lays the foundation for weld microstructure optimization.

Ultra-Short-Pulse Laser Filaments for Float Glass Cutting: Influence of Laser Parameters on Micro Cracks Formation

Utilizing ultra-short-pulse laser filamentation of glass is one of the latest developed techniques to cut glass. In comparison with other techniques the superior benefits are a small heat-affected zone, a quasi-non-gap cut, and the possibility to free from cut. However, despite the large interest in the laser/glass interaction for various laser sources and pulse durations, the process of cleaving and the underlying mechanisms are fairly undescribed. In this study, we utilized a Nd:YAG laser with an average power of 100 W, a center wavelength of 1,064 nm, and a pulse duration of 12 ps via a specially designed optic to generate laser filamentation in soda-lime- and borosilicate glass with varying Pitch and Burst parameters. The filaments and cut edge are analyzed with scanning electron- and laser scanning microscopy to study the topological phenomena and roughness. Photoelastic measurements were done to assess the stress distribution within the glass and show cumulative interactions at the filament extremity. From the observations made, phenomenological models are proposed to describe the initial micro crack formation considering the shock wave and thermal influence caused by the laser pulses. Two types of micro cracks are identified, with radial cracks passing through the filament and bypass cracks that form around the laser-affected zone. Models are elaborated for both glass types to evaluate the influence of a parameter change on the micro crack formation and the cleaving guidance. The different behavior between soda-lime- and borosilicate glass, with their distinct physical properties, helps to interpret the influence of laser parameters on the micro crack formation and the cleaving guidance.

Laser sintering of PA12 particles studied by in-situ optical, thermal and X-ray characterization

The microstructure of products manufactured by selective laser sintering (SLS) is known to be highly dependent on various process and material parameters. The latter thus also affect the final part properties. While most work has focused on ex-situ characterization of the printed parts, little is known about the time-dependent microstructure development during sintering. In this work, we present direct observations of the microstructural evolution during laser sintering of polyamide 12 (PA12) particle doublets by in-situ synchrotron wide angle X-ray diffraction (WAXD), using our in-house developed laser sintering setup. Simultaneously, the neck growth between the particles and the temperature are captured via optical and infrared microscopy. We show that isothermal crystallization experiments under quiescent conditions are not sufficient to describe crystallization in a non-isothermal process like SLS. The enhanced crystallization kinetics in small particles suggests that both temperature and flow play a role. This finding was corroborated by the critical Weissenberg numbers estimated from rheological reptation and Rouse time scales. Furthermore, a microstructure survey has been carried out by microtoming thin slices of the sintered doublets. Both optical and atomic force microscopy reveal significant differences in the crystalline structure of the laser-affected zone as compared to the un-sintered region.

Structural changes during femtosecond laser percussion drilling of high-aspect-ratio diamond microholes

We studied the structural and elemental evolutions during the femtosecond laser percussion drilling of high-aspect-ratio diamond microholes. Microholes 225-μm-deep having an aspect ratio of 15 were drilled with an exposure time of 100 s and a laser power of 60 mW. It is found that when the specimen was machined by femtosecond laser at low power and short exposure time, the laser-affected zone (LAZ) may be still solid in the interior rather being void, even it reached the bottom of the diamond. A clear crack appeared between the solid portion and pristine diamond. Elemental analysis revealed that oxygen was incorporated into the solid portion of the LAZs, and its atomic percentage reached 6.5% for a laser power of 10 mW at initial position and decreased as the depth increased in the solid portion. The wall of the void contained nearly no oxygen. Furthermore, nanoripples were observed on the sidewall surface of the hole.

Investigation on laser-MIG hybrid-welded joint for 5A06 aluminum alloy: effect of the laser heat input on grain size and microhardness

5A06 aluminum alloy plates with a thickness of 6.9 mm are welded by laser-metal inert gas (MIG) hybrid welding. To investigate the influence of laser heat input on grain size and microhardness, optical microscope observation, scanning electron microscope (SEM) and microhardness test are conducted. According to the microstructure distribution characteristics, the weld seam can be divided into arc-affected zone and laser-affected zone. The crystalline morphology from the weld boundary to the weld center is coarse columnar dendrites, finer columnar dendrites and equiaxed dendrites. In addition, the calculation and comparison of the grain size indicate that, the grain size Zl of the coarse columnar dendrites and the fine columnar dendrites in the laser-affected zone has increased, compared to both of the arc-affected zone. Due to the mechanical stirring effect on the upper molten pool, the finer equiaxed dendrites are formed at the center of arc-affected zone. It is worth noting that the grain size inhomogeneity has been significantly improved due to the decrease of laser heat input. The average microhardness of the arc-affected zone in case 1 is significantly lower than that of the other three cases due to the higher laser heat input. The laser line energy of 90 J/mm is more suitable for achieving the excellent microhardness property of laser-MIG hybrid welded joint for the 6.9 mm 5A06 aluminum alloy.

Investigation of Fracture Behavior and Mechanism in High-Speed Precise Shearing for Metal Bars with Prefabricated Fracture-Start Kerfs.

A laser-assisted high-speed shearing (LAHSS) method has been proposed for metal bars, which prefabricates equally spaced fracture-start kerfs by Nd:Yag laser to make stress concentration, and applies a high-speed load to complete fracture separation. Comparative tests were conducted for Q235, 40Cr, and 304 steel bars, and the effects of fracture-start kerfs and axial clearance were investigated on the fracture section. Moreover, the fracture behavior was demonstrated by numerical simulation, and the micro-fracture mechanism was revealed by fractographic analysis. The numerical simulation results show that the material damage concentrates along with the kerf tips with peak equivalent plastic strain, and the corresponding stress triaxiality drops to almost zero at the kerf tip, which reveals that the material is subjected to pure shearing at kerf tip; the Max. loading force is reduced by 15.2%–29.6%, and the impact energy is decreased by 29.8%–46.9% for the three types of bar material. The experimental results showed that the fracture-start kerfs effectively inhibited the plastic deformation stage, and higher precision blanks were obtained in the LAHSS test: roundness error improved from 2.7%–10.9% to 1.1%–2.6%, Max. bending deflection decreased from 1.3–3.4 mm to 0.4–1.0 mm, and flatness error dropped from 0.9–3.3 mm to 0.3–0.7 mm. The fractographic analysis reveals that the crack initiation is related to alternative V-shape micro-notches at the laser-affected zone; the predominant fracture mechanism involves mode II microvoid coalescence at the main fracture plane; smaller and less elongated dimples were formed in 40Cr steels due to higher number density of grains and pinning effect of second-phase particles compared to Q235 and 304 steel bars.

Laser surface treatment of α-β titanium alloy to develop a β -rich phase with very high hardness

• Laser surface treatment results microstructural gradient in laser affected zone (LAZ). • Plasma plume detachment at higher power result in surface roughness enhancement. • Beta enrichment in LAZ with higher hardness. • Rosette structure formation. The α - β titanium alloy VT31 is used in the aerospace industry due to its high strength to weight ratio and good mechanical properties at elevated temperatures. Surface microstructure plays an important role in the mechanical behavior of the material, especially fatigue. In this work, VT3-1 α - β titanium alloy surface microstructure was modified using laser surface heat treatment. Laser surface treatment (LST) was carried out at various laser powers (100−250 W) and scanning speeds (150−500 mm/min). Laser affected zones, surfaces and sub-surfaces were characterized using different techniques. Electron backscatter diffraction (EBSD) analysis of the laser affected zone exhibits a distinct microstructure across the depth of the specimen, and it changes with laser power and scanning speed. At a lower scanning speed and high laser power (200−250 W); almost complete β -containing microstructure was developed at the surface with very high hardness. At an intermediate power (150−200 W), a β -rich surface layer with dendritic features followed by a layer of martensitic microstructure was evident. Interestingly, at high power (250 W) a distinct shiny surface with a lower surface roughness was observed. Moreover, the microstructure under this condition corresponds to an α -phase (transformed microstructure) which is typical of melted and fast cooled α - β titanium alloy. Finite element analysis of the LST was also carried out to predict thermal history based on which the extent of the laser affected zone across the depth was predicted for various processing parameters and correlated with the hardness variation.

Effect of Thermal Fatigue on Microstructure and Mechanical Properties of H13 Tool Steel Processed by Selective Laser Surface Melting

Selective laser surface melting, which brings together the bionic theory and the laser process, is an effective way to enhance the thermal fatigue behavior of materials. In this study, in order to examine the relationship between the mechanical properties and thermal fatigue behavior of materials processed by selective laser surface melting, the tensile properties at room temperature and elevated temperature of treated specimens and untreated specimens after different numbers of thermal fatigue cycles were investigated and compared. Moreover, the microstructure evolution and the microhardness of the laser-affected zone were investigated after different numbers of thermal fatigue cycles. The results show that microhardness of the laser-melted zone gradually decreases with an increasing number of thermal fatigue cycles; the number of thermal fatigue cycles has little effect on the grain size in the laser-melted zone, and the percentage of low-angle grain boundaries decreases with an increasing number of thermal fatigue cycles. The strength of specimens gradually decreases, whereas the fracture elongation gradually increases with an increasing number of thermal fatigue cycles at room temperature and elevated temperature. In addition, the stress distribution on the specimen surface during tensile test was investigated using the finite element method, and the results indicate that the stress transfer exists between the laser-affected zone and the untreated zone.

Formation of Structure of an Annealed High-Speed Steel upon Laser Surface Melting

The effect of the time and energy parameters of pulsed laser treatment on the structure formation and hardness of high-speed steel after annealing upon surface melting has been studied. The results of the metallographic analysis and energy-dispersive X-ray microanalysis of the steel, as well as of the microhardness measurements, have been discussed. It has been shown that the laser-induced remelting of the annealed high-speed steel causes a significant refinement of both the solid solution and of the carbide constituent. The structure of the laser-affected zone and the morphology of dendrites depend on the laser-irradiation regimes.

Formation of Structure of a High-Speed Steel upon Laser Surface Melting

The effect of time and energy parameters of the pulsed laser treatment on the structure formation and hardness of high-speed steel upon surface melting after complete heat treatment has been studied. The results of the metallographic analysis and energy-dispersive X-ray spectroscopy of the steel, as well as of the microhardness measurements, have been discussed. It has been shown that the laser-induced remelting causes a significant refinement of both the dendritic structure of the solid solution and of the carbide constituent of the high-speed steel. The structure of the laser-affected zone and the morphology of dendrites depend on the laser treatment regimes.

飞秒脉冲激光对硅基多层膜损伤特性

In order to understand the ultrafast laser-induced damage mechanisms of typical imaging sensor's film structures, the damage characteristics of Si-based multi-layer films irradiated by a femtosecond pulsed laser were investigated, and the laser pulse fluence ranges and threshold conditions corresponding to various damage phenomena were evaluated. Si-based multi-layer films that were similar in structure of CCD were prepared by electron beam deposition. The damage characteristics of these films irradiated by a femtosecond pulsed laser with wavelength of 800 nm and pulse width of 100 fs under different pulse fluences and numbers were investigated using a metallurgical microscope. Experimental results showed that the laser-affected zone size increased linearly with pulse fluence in the range of 1.01 to 24.7 J/cm2. Surface damage caused by oxidation/amorphization, non-thermal ablation, and laser-induced plasma ablation could be observed in the laser irradiated zone, which tightly depended on the pulse fluence. Multi-layer damage could be observed and the damage probability increased from 1% to 51% in the pulse fluence range from 2.42 to 24.7 J/cm2. Irradiated by sequent pulses at a fluence of 1.01 J/cm2, the laser affected zone remained almost unchanged and the ablated depth increased with the pulse number. From the single pulse damage experiment data, the femtosecond pulse laser-induced surface damage threshold was evaluated to be 0.543 J/cm2 and laser-induced multi-layer stress damage threshold was linearly fitted to be 2.16 J/cm2. Sequent pulse irradiation with low fluence(≤ 1.01 J/cm2) also could lead to deep damage on the multi-layer film.

The Effects of Laser Surface Hardening on Microstructural Characteristics and Wear Resistance of AISI H11 Hot Work Tool Steel

AbstractThe present study deals with the effects of laser surface treatment on microstructure evolution and wear resistance of AISI H11 hot work tool steel in quenched and tempered condition. The most upper laser-affected zone is characterized by re-melted microstructure consisting of dendrite cells with fresh non-tempered martensite, retained austenite and inter-dendritic carbidic network. The subsolidus microstructure just beneath the re-melted zone represents the most laser surface hardened zone consisting of fresh non-tempered martensite with fine and coarse carbides as a result of overheating the original QT substrate microstructure. The highest microhardness values in the range from 775 to 857 HV were measured for the LSH microstructure and the most softened microstructure exhibited the minimum hardness of 530 HV. The laser treated samples showed the improvement of their surface wear resistance by 35%.

Revealing and Identifying Laser-Induced Damages in CIGSe Solar Cells by Photoluminescence Spectroscopy

Laser-based patterning for serial interconnection of chalcopyrite (i.e., Cu(In ${}_{\rm{x}},\text{Ga}_{\rm{1-x}}$ )Se2 or CIGSe) solar cells was obtained by 1) laser ablation using picosecond (ps) pulses, 2) local phase transformation using nanosecond (ns) laser pulses, and 3) conventional needle-based patterning. All three patterning approaches cause a modification of the material properties in the vicinity of the actual P2 scribing lines, which affects and limits the electrical functionality of the interconnection, and thus has to be considered for positioning the P3 scribe. Thus, the extension and the properties of the affected zone aside the P2 scribe was investigated through spectral and spatial photoluminescence (PL). From the depletion of the PL intensity when approaching the scribing line and a peak shift analysis it is concluded that the laser-affected zone is distinctively larger than visual inspections suggest. Even putatively ultrashort, nonthermal ps pulses cause material modifications which might be facilitating recombination losses and thus limiting solar cell efficiencies. For ps laser patterning the affected area is even larger than for the ns laser patterning, due to a modification of the band structure and to thermal decomposition. Evolving subpeaks at the low energy tails are found to originate from Cu-related flat defect levels, i.e., Cu vacancies ( $V_{{\rm{Cu}}}$ ) and antisites (Cu In), created upon laser impact. These findings provide insights into laser-based material modification and provide beneficial information for minimizing the dead area resulting from laser-based monolithic interconnection.

Rotational moulding of PEEK polymer liners with carbon fibre/PEEK over tape-placement for space cryogenic fuel tanks

PEEK polymers are investigated as replacement materials for metallic liners in composite overwrapped pressure vessels (COPVs) for fuel tank applications in space. A novel, integrally heated, rotational moulding tool has been developed to produce PEEK polymer liners, samples of which have then been overwrapped using CF/PEEK in a laser assisted tape-placement (LATP) process to produce demonstrator samples of a polymer lined COPV. Helium permeability testing has shown that the designs are capable of resisting leakage to acceptable levels for fuel storage, while X-ray CT scanning and cryogenic cycling have shown that the current design is capable of resisting crack growth over multiple cycles. Nano-indentation testing has shown that the LATP process has created a region of reduced modulus in the PEEK polymer at the surface of the liner where the CF/PEEK has been tape-laid. This laser-affected zone of reduced polymer modulus in the composite interface region has enabled an enhanced resistance to crack growth formations from thermal residual stresses in comparison to hot plate moulded test samples.

Fabrication of through-wafer 3D microfluidics in silicon carbide using femtosecond laser

We demonstrate a prototype through-wafer microfluidic structure in bulk silicon carbide (SiC) fabricated by femtosecond laser micromachining. The effects of laser fluence and scanning speed on the laser-affected zone are also investigated. Furthermore, the wettability of the laser-affected surface for the target liquid, mineral oil, is examined. Microchannels of various cross-sectional shapes are fabricated by the femtosecond laser and their effects on the liquid flow are simulated and compared. This fabrication approach offers a fast and efficient route to implement SiC-based through-wafer micro-structures, which are not able to be realized using other methods such as chemical etching. The flexibility of manufacturing 3D structures based on this fabrication method enables more complex structures as well. Smooth liquid flow in the microchannels of the bulk SiC substrate is presented. The work shown here paves a new way for various applications such as reliable microfluidic systems in a high-temperature, high radioactivity, and corrosive environment, and could be combined with SiC wafer-to-wafer bonding to realize a plethora of novel microelectromechanical (MEMS) structures.

Higher-resolution selective metallization on alumina substrate by laser direct writing and electroless plating

How to fabricate conductive patterns on ceramic boards with higher resolution is a challenge in the past years. The fabrication of copper patterns on alumina substrate by laser direct writing and electroless copper plating is a low cost and high efficiency method. Nevertheless, the lower resolution limits its further industrial applications in many fields. In this report, the mechanisms of laser direct writing and electroless copper plating were studied. The results indicated that as the decomposed products of precursor PdCl2 have different chemical states respectively in laser-irradiated zone (LIZ) and laser-affected zone (LAZ). This phenomenon was utilized and a special chemical cleaning method with aqua regia solution was taken to selectively remove the metallic Pd in LAZ, while kept the PdO in LIZ as the only active seeds. As a result, the resolution of subsequent copper patterns was improved significantly. This technique has a great significance to develop the microelectronics devices.