Surface nitriding of Ti–6Al–4V alloy with a high power CO2 laser

Surface nitriding of a Ti–6Al–4V alloy by laser melting in a flow of nitrogen gas has been investigated, with the aim of increasing surface hardness and hence improving related properties such as wear and erosion resistance. The effect of the scanning speed, nitrogen dilution, and nitrogen flow rate on microstructure, microhardness, and cracking of the nitrided layers was studied. Optical, scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction (XRD) were used to reveal the microstructure and to identify the phases formed. It is shown that smooth, deep, and crack-free nitride layers of a surface hardness ranging between 500 and 800 HV can be obtained by controlling the processing parameters. Cracks are present in the sample processed at slow scanning speed and high laser power. Dilution of the nitrogen gas with argon gas leads to a crack-free nitride layer at the expense of a reduction in surface hardness. Slow scanning speeds lead to the formation of a deep and hard surface layer, and increasing the nitrogen flow rate results in a rough surface with a slight increase in hardness.     

Microstructure and wear properties of TiCN/Ti coatings on titanium alloy by laser cladding

Titanium carbide nitride (TiCN) reinforced Ti coating was fabricated on the surface of Ti–6Al–4V alloy by laser cladding method. Microstructure and wear properties at the surface of the coating in atmosphere were investigated. Three zones can be distinguished of the coating: the clad zone (CZ), the heat affected zone (HAZ) and the substrate. The clad zone is composed of TiCN dendrites, TiO₂ and Ti. A metallurgical bonding between the coating and the substrate was obtained. The microhardness and wear resistance of the TiCN/Ti coating are significantly improved. The average hardness of the coating is about 3 to 6 times of that of the substrate. The friction coefficients of the substrate and the coating are 0.48 and 0.34 respectively. The friction coefficient of the Ti–6Al–4V substrate was insensitive to the normal load, while that of the cladded TiCN/Ti coating was very sensitive to the normal load. The wear mass losses of the cladded samples are much lower than that of the substrate whatever the normal load is.     

Laser surface alloying of an electroless Ni–P coating with Al-356 substrate

Laser surface alloying of an electroless plating Ni–P coatings on an Al-356 aluminium alloy was carried out using a 1-kW pulsed Nd:YAG laser. The microstructure, chemical composition and phase identification of the alloyed layer were determined using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and X-ray diffractometry (XRD), respectively. It was shown that laser surface treatment produced a relatively smooth, crack-free and hard surface layer. The hardness of the surface significantly increased due to the formation of the uniformly distributed fine Ni–Al intermetallic phases. The corrosion behaviour of the surface alloyed specimens in 3.5% NaCl solution at 23 °C was also determined by electrochemical techniques. The laser-alloyed surface showed an improved corrosion and pitting potential compared to the substrate as well as the plated Ni–P coating.     

Surface preparation of Ti–3Al–2.5V alloy tubes for welding using a fiber laser

Ti–3Al–2.5V tubes are widely used in aircraft hydraulic systems. Meticulous surface preparation before welding is necessary to obtain a sound weld involving these alloy tubes. Conventionally this is done by cleaning with environmentally malign toxic chemicals, such as, hydrofluoric acid and nitric acid. This paper describes the laser-cleaning process of the surface of these tubes with a fiber laser as a preparation for pulsed gas tungsten arc welding and results obtained. A simple one-dimensional heat equation has been solved to evaluate the temperature profile of the irradiated surface. It is shown that surface preparation by laser cleaning can be an environmentally friendly alternative process by producing acceptable welds with laser-processed tubes.     

Characteristics of deep penetration laser welding of dissimilar metal Ni-based cast superalloy K418 and alloy steel 42CrMo

Experimental trials of autogenous deep penetration welding between dissimilar cast Ni-based superalloy K418 and alloy steel 42CrMo flat plates with 5.0 mm thickness were conducted using a 3 kW continuous wave (CW) Nd:YAG laser. The influences of laser output power, welding velocity and defocusing distance on the morphology, welding depth and width as well as quality of the welded seam were investigated. Results show that full keyhole welding is not formed on both K418 and 42CrMo side, simultaneously, due to the relatively low output power. Partial fusion is observed on the welded seam near 42CrMo side because of the large disparity of thermal–physical and high-temperature mechanical properties of these two materials. The microhardness of the laser-welded joint was also examined and analyzed. It is suggested that applying negative defocusing in the range of Raylei length can increase the welding depth and improve the coupling efficiency of the laser materials interaction.     

Compact 500.9 nm laser based on doubly resonant intracavity sum–frequency mixing

A compact 500.9 nm laser was realized using doubly resonant intracavity sum–frequency mixing. An Nd:YAG crystal and an Nd:YVO₄ crystal were employed as the gain crystals. In two sub-cavities, 946 nm radiation from the Nd:YAG and 1064 nm radiation from the Nd:YVO₄ were mixed to generate 500.9 nm. In the overlapping of the two cavities, sum–frequency mixing was achieved in a type-II critical phase-matched KTP crystal. An output power of 78 mW at a wavelength of 500.9 nm was generated using a total incident pump power of 4 W and the output light exhibited low noise, with the root-mean-square value being 0.3%.     

Sheet metal welding using a pulsed Nd: YAG laser-robot

This paper presents a pulsed Nd: YAG laser-robot system for spot and seam welding of mild steel sheets. The study evaluates the laser beams behaviour for welding, and then investigates pulsed Nd: YAG laser spot and seam welding processes. High pulse power intensity is needed to initiate the key-hole welding process and a threshold pulse energy to reach full penetration. In seam welding, a weld consists of successive overlapping spots. Both high pulse energy and high average power are needed to keep the key-hole welding going. A 70% overlap is used to define overlapping spot welding as seam welding and to optimize process parameters because a high tensile strength joint compatible with the strength of the base material can be obtained when the overlap is ≥70%; at the same time a smooth seam with full penetration is obtained. In these cases, the joints in pulsed Nd: YAG laser welding are comparable in strength to those obtained with CO₂ laser welding. Robot positioning and motion accuracies can meet the demands of Nd: YAG laser sheet metal welding, but its cornering accuracy affects the welding processes. The purpose of the study is to evaluate the YAG laser-robot system for production in the automotive industry.     

Effects of microalloying with Cd and Ag on the precipitation process of Al–4Cu–0.3Mg (wt%) alloy at 200°C

The present work investigates the effects of individual and combined additions of Cd and Ag on precipitation processes in an Al–4Cu–0.3Mg (wt%) alloy. Analytical scanning transmission electron microscopy revealed that microalloying with Cd stimulates nucleation of θ′ phase on {001} planes and that Cd-rich particles form on the rim and broad facets of the θ′ platelets. We interpret these observations to suggest that Cd nucleates heterogeneously at the θ′– interface and that θ′ can also nucleate heterogeneously at the Cd– interface. In the quinary alloy, it was observed that Ag and Cd additions seem to work independently resulting in a fine and uniform dispersion of both Ω and θ′. Furthermore, the hardening effect of the {111} Ω phase appears to be more potent than other precipitates formed in this system since the hardness of the quinary alloy was intermediate between the Al–Cu–Mg–Ag and the Al–Cu–Cd alloys.     

YAG laser percussion drilling of a functional multi-layer thin plate

An Au-coated Fe–Ni alloy thin plate was laser drilled by a pulsed Nd:YAG laser. The influences of laser parameters on the diameter of perforation, the outer diameter of crater and the roundness of the perforation have been investigated by employing scanning electron microscopy. The diameter of perforation increases gradually with an increase in pulse width from 0.3 to 8 ms at the fixed average laser power and frequency, and increases with an increase in average laser power from 10 to 25 W at the fixed pulse width and frequency. Some craters are found around the perforations because of the heat affecting of laser beam to the non-irradiated zone. The diametric difference of the perforation between the incident and the effluent sides is very small under the shortest pulse width of 0.3 ms. Good roundness of the perforation can be achieved at either the lowest pulse width or the lowest laser power.     

Concurrent laser welding and annealing exploiting robotically manipulated optical fibers

Present investigation reports on the effects of incorporating pre- and post-heating on the mechanical properties of laser-welded joints, in normal air condition. Two common types of steels, i.e. mild steel, and stainless steel were welded with Lumonic's MS 830 Nd³⁺:YAG laser machine, with an output capacity of 400 W. Due to the low integrated energy input required for laser welded joints, the welded region are often cooled too rapidly via conduction to the surrounding material and atmosphere, which leads to hardness discontinuities in the fusion and heat affected zone. The effects of in-process laser annealing on the mechanical properties and microstructure of laser-welded joints are important in any manufacturing operation. To improve the poor weld characteristics, this work investigates the use of automated dual-beam delivery system to implement a pre- or post-heating technique, simultaneously with the welding process. The results show that proper selection of the control parameters for the pre- or post-heating can reduce the hardness of the weld significantly and improve the welded joints mechanical properties, such as higher tensile strength and better durability.     

Dissimilar autogenous full penetration welding of superalloy K418 and 42CrMo steel by a high power CW Nd:YAG laser

Experiments of autogenous laser full penetration welding between dissimilar cast Ni-based superalloy K418 and alloy steel 42CrMo flat plates with 3.5 mm thickness were conducted using a 3 kW continuous wave (CW) Nd:YAG laser. The influences of laser welding velocity, flow rate of side-blow shielding gas, defocusing distance were investigated. Microstructure of the welded seam was characterized by optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). Mechanical properties of the welded seam were evaluated by microhardness and tensile strength testing. Results show that high quality full penetration laser-welded joint can be obtained by optimizing the welding velocity, flow rate of shielding gas and defocusing distance. The laser-welded seam have non-equilibrium solidified microstructures consisting of γ-FeCr_0.29Ni_0.16C_0.06 austenite solid solution dendrites as the dominant and very small amount of super-fine dispersed Ni₃Al γ′ phase and Laves particles as well as MC needle-like carbides distributed in the interdendritic regions. Although the microhardness of the laser-welded seam was lower than that of the base metal, the strength of the joint was equal to that of the base metal and the fracture mechanism showed fine ductility.     

The mechanism of PEO process on Al–Si alloys with the bulk primary silicon

This study focuses on mechanism of ceramic coating on Al–Si alloys with bulk primary Si using plasma electrolytic oxidation (PEO) technology. Al–Si alloys with 27–32%Si in weight were used as substrates. The morphologies, composition and microstructure of PEO coatings were investigated by scanning electron microscopy (SEM) with energy dispersive X-ray system (EDX). Results showed that the PEO process had four different stages. The effect of bulk Si is greatly on the morphology and composition of coatings at first three stages. Anodic oxide films formed on Al and Si phases, respectively. When the voltage exceeded 40 V, glow appeared and concentrated on the localized zone of interface of Al and Si phase. Al–Si–O compounds formed and covered on the dendrite Si phase surface, and the coating on bulk Si, which was silicon oxide, was rougher than that on other phase. If the treatment time was long enough, the coatings with uniform surface morphologies and elements distribution will be obtained but the microstructure of inner layer is looser due to the bulk Si.     

An investigation on capability of hybrid Nd:YAG laser-TIG welding technology for AA2198 Al-Li alloy

This paper surveys the capability of the hybrid laser-arc welding in comparison with lone laser welding for AA2198 aluminum alloy experimentally. In the present research, a continuous Nd:YAG laser with a maximum power of 2000 W and a 350 A electric arc were used as two combined welding heat sources. In addition to the lone laser welding experiments, two strategies were examined for hybrid welding; the first one was low laser power (100 W) accompanied by high arc energy, and the second one was high laser power (2000 W) with low arc energy. Welding speed and arc current varied in the experiments. The influence of heat input on weld pool geometry was surveyed. The macrosection, microhardness profile and microstructure of the welded joints were studied and compared. The results indicated that in lone laser welding, conduction mode occurred and keyhole was not formed even in low welding speeds and thus the penetration depth was so low. It was also found that the second approach (high laser power accompanied with low arc energy) is superior to the first one (low laser power accompanied with high arc energy) in hybrid laser-arc welding of Al2198, since lower heat input was needed for full penetration weld and as a result a smaller HAZ was created.     

Active–passive mode-locking using Cr:YAG crystal as saturable absorber

Cr⁴⁺ : YAG crystal was used as a saturable absorber in an active–passive mode-locked Nd : YAG laser. The stable and complete mode-locked pulse train was achieved at 1064 nm. The duration of a single pulse was obtained from 0.8 to 2.4 ns by changing the Cr⁴⁺ : YAG crystal with different initial transmission, when there was the shortest pulse width. The mode-locking dynamic process of Cr⁴⁺ : YAG crystal was analyzed by the rate equations to explain the experimental result, and the effect of excited-state absorption was considered.     

Effects of relative positioning of energy sources on weld integrity for hybrid laser arc welding

This study is concerned with the effects of laser and arc arrangement on weld integrity for the hybrid laser arc welding processes. Experiments were conducted for a high-strength steel using a 4 kW Nd: YAG laser and a metal active gas (MAG) welding facility under two configurations of arc–laser hybrid welding (ALHW) and laser–arc hybrid welding (LAHW). Metallographic analysis and mechanical testing were performed to evaluate the weld integrity in terms of weld bead geometry, microstructure and mechanical properties. The morphology of the weld bead cross-section was studied and the typical macrostructure of the weld beads appeared to be cone-shaped and cocktail cup-shaped under ALHW and LAHW configurations, respectively. The weld integrity attributes of microstructure, phase constituents and microhardness were analyzed for different weld regions. The tensile and impact tests were performed and fracture surface morphology was analyzed by scanning electron microscope. The study showed that ALHW produced joints with a better weld shape and a more uniform microstructure of lath martensite, while LAHW weld had a heterogeneous structure of lath martensite and austenite.     

Experimental investigation and metallographic characterization of fiber laser beam welding of Ti-6Al-4V alloy using response surface method

In the present study, experimental investigations of fiber-laser-beam-welding of 5 mm thick Ti-6Al-4V alloy are carried out based on statistical design of experiments. The relationship between the process parameters such as welding power, welding speed, and defocused position of the laser beam with the output responses such as width of the fusion zone, size of the heat affected zone, and fusion zone area are established in terms of regression models. Also, the most significant process parameters and their optimum ranges are identified and their percentage contributions on output responses are calculated. It is observed that welding power and speed plays the major role for full penetration welding. Also, welding power shows direct effect whereas welding speed shows the inverse effect on the output responses. The bead geometry is influenced by the defocused position of the laser beam due to the change in power density on the workpiece surface. However, overall fusion zone area is unaffected. Mechanical characterization of the welded samples such as microstructural analysis, hardness, and tensile tests are conducted. It is noticed that the hardness value of the FZ is higher than the HAZ and BM zone due to the difference in cooling rate during welding which promotes the formation of α′ martensitic phase in the FZ. Also, an average hardness value in the FZ is compared for two different defocusing positions (i.e. 1 and 2 mm). It is found that hardness value is higher for 1 mm defocused position than 2 mm due the decrement in grain size below a critical range at 2 mm defocused position. The ultimate tensile strength and % elongation of the welded samples are degraded as compared to BM which can be further improved by post heat treatment.     

Comparison of microstructure and mechanical properties of ultra-narrow gap laser and gas-metal-arc welded S960 high strength steel

The microstructural characteristics and mechanical properties, including micro-hardness, tensile properties, three-point bending properties and Charpy impact toughness at different test temperatures of 8 mm thick S960 high strength steel plates were investigated following their joining by multi-pass ultra-narrow gap laser welding (NGLW) and gas metal arc welding (GMAW) techniques. It was found that the microstructure in the fusion zone (FZ) for the ultra-NGLW joint was predominantly martensite mixed with some tempered martensite, while the FZ for the GMAW joint was mainly consisted of ferrite with some martensite. The strength of the ultra-NGLW specimens was comparable to that of the base material (BM), with all welded specimens failed in the BM in the tensile tests. The tensile strength of the GMAW specimens was reduced approximately by 100 MPa when compared with the base material by a broad and soft heat affected zone (HAZ) with failure located in the soft HAZ. Both the ultra-NGLW and GMAW specimens performed well in three-point bending tests. The GMAW joints exhibited better impact toughness than the ultra-NGLW joints.     

Lasing characteristics of Rhodamine B and Rhodamine 6G as a sensitizer in sol–gel silica

We report lasing characteristics of Rhodamine B (Rh. B) in sol–gel silica under excitation with frequency-doubled Nd:YAG laser and sensitization with Rhodamine 6G (Rh. 6G). The principle of radiative energy transfer (from Rh. 6G to Rh. B) has been utilized as a longitudinally Rh. 6G laser (at 585 nm)-pumped Rh. B laser process in the same sample. Rh. B offers a high photostable and efficient laser dye in sol–gel silica sensitized with Rh. 6G; 75,000 shots as a laser half-lifetime of the sample and 24% efficiency at pumping intensity 0.1 J/cm² of 532 nm. Wavelength shift occurs from 606 to 630 nm in the Rh. B laser with increasing its concentration from 1×10⁻⁴ to 8×10⁻⁴ M. The measured optical gain for Rh. B sensitized with Rh. 6G in sol–gel silica is higher than that in ethanol. A new effect has been observed; at 1×10⁻⁴ M of Rh. B and 0.5×10⁻⁴ M of Rh. 6G mixture, the emitted color of laser is changed by changing the pump intensity of frequency-doubled Nd:YAG laser.     

CW/PW dual-beam YAG laser welding of steel/aluminum alloy sheets

The lap welding of JSC270CC steel and A6111-T4 aluminum alloys were carried out by a dual-beam YAG laser with the continuous wave (CW) and pulse wave (PW) modes. The microstructure of the welded joints were examined with SEM, EPMA while the properties were checked with microhardness tester and tensile testing machine. It was shown that the dual-beam laser welding can effectively reduce or avoid the formation of the blowholes in the welded joints. The PW laser beam penetrated the welding pool, leading to the root-shape structures with enhanced bonding strength at the weld interface. A 10 μm intermetallic compound (IMC) layer was generated at the interface. The shearing strength of lap joint was measured to be 128 MPa.     

Optical fibres for material processing lasers

Fibre transmission of high-power radiation has opened the laser a new dimension of material processing applications. The three-dimensional treatment revolutionised a far field of the production technology. One impressed example is the laser welding process in the automotive industry. The Nd–YAG solid state laser in combination with the optical fibre increase the automation and the flexibility of a whole industry. The next generation of Audis aluminium alloy car body is mainly welded by fibre-guided Nd–YAG laser beams. Since the past 10 years the maximum average laser power transmitted through optical fibres increase from some 100 W up to 10 kW. For industrial application only Nd–YAG lasers and recently the diode lasers are economically suited for fibre transmission.