The experiment study presents the influence of femtosecond laser shock peening (FsLSP) without a protective layer in the air on the surface hardness and surface mechanical property of NiTi shape memory alloy. Femtosecond laser shock peening is a new possibility of direct laser ablation without any protective layer under atmospheric conditions, which can produce intense shock waves with low pulse energy in the air. The average surface roughness values of the NiTi alloy samples were measured, because the surface roughness may affect its friction resistance. The results showed that the surface roughness of NiTi increased after femtosecond laser shock peening treatment. In comparison with the initial state, the coefficient of friction decreased and surface microhardness increased after femtosecond laser shock peening treatment with different FsLSP parameters. This improvement of wear properties may be attributed to the enhancement of surface microhardness and surface titanium oxide layer induced by the shock wave and laser ablation during FsLSP treatment.

Properties of diamond are extreme. Since the first successful synthesis of diamond in 1955, the use of synthetic diamond has widely spread into diverse industries (e.g. manufacturing, electronics and optics). However, being the hardest material known, the manufacture of diamond material into an engineered tool is extremely challenging. The polishing process remains a traditional mechanical method existing for over hundreds of years. The development of alternative ways of polishing diamond is an active subject of research and has recently been investigated in topics such as chemically assisted mechanical polishing or ion beam polishing. Laser polishing is another alternative and a state-of-theart laser polishing method is presented in this paper. A high-power femtosecond laser ablation process is developed to achieve a high throughput polishing process of polycrystalline diamond composite (PCD) wafers. Laser ablation trials are carried out with a femtosecond laser delivering over 80W average power on three different PCD grades synthesized by high-pressure/high-temperature. The role of the fluence is highlighted and the effect of the burst mode on PCD is demonstrated for the first time to the best of our knowledge. Eventually, the roughness of the initial surface on fine grain diamond material is reduced by two while the ablation rate is twice higher than the removal rate achieved by mechanical polishing.

Laser polishing (LP) is considered as one of the enabling technologies primed to replace time-consuming manual polishing operations. During laser polishing, a thin layer of material is melted as a result of laser irradiation. Since molten metal is characterized by the melt pool relocation capabilities, laser polishing results in a significant decrease of surface roughness. Experiments on flat stainless-steel metal plates with a 1200W fiber laser lead to a surface roughness as low as Sa = 320nm (90% roughness decrease) with a processing time of approximately 40s/cm².

The aim of this study is first to demonstrate the potential for the upscaling of process speed. Upscaling laws based on power density and energy density will be discussed. Experiments are carried out to assess the upscaling from 1200W to a 10kW fiber laser in order to improve processing time. We can expect to reach a processing time of 5s/cm², a diminution by 4 to 6 times comparing to the current process. Under appropriate process parameters, certain classes of metallic materials are suitable for LP and can reduce their average surface roughness by more than 90 %.

Micro laser shock peening (μLSP) with pulse energies well below 1 J proved to be a useful technique to obtain fatiguelife performances similar to those reported for traditional LSP processes on metallic bulk materials [1, 2]. However, it suffers from a lack of productivity as spot sizes are reduced and pulse overlaps are increased in order to obtain compressive residual stresses, deep below the surface of the bulk material. To overcome these limitations of μLSP, we have investigated strategies to scale up the productivity by increasing laser repetition rate while keeping constant the total amount of energy deposited on the peened surface [3-5]. We have built a laser processing cell to meet industrial grade applications. Complex surfaces are mounted on a KUKA robot to control the laser orientation and pulse overlap on the 3D workpiece surface. The pulse energy is provided by an 8 ns, Nd:YAG Laser, operating at 1064 nm, with a variable repetition rate from 10 to 100 Hz and delivering a maximum energy of 450 mJ/pulse on Al 2024-T351 samples with a thickness of 10 mm. We present high speed video analysis as diagnostic tool illustrating limitations in upscaling of repetition rates. As a proof of the μLSP effectiveness we present compressive residual stress profiles with up to -500 MPa peak and a return to zero down to 1.8 mm below the surface. This represents a 5-times improvement of the maximum stress depth, when compared to conventional peening processes widely used in the aeronautic industry.

Laser shock surface treatment applications, ie with plasma formation, require pulses of very high peak power and duration of the order of a few tens of nanoseconds. This processing technique are generally performed in free space due to the difficulties to inject such peak power in a single core fiber. We present a innovative method allowing to inject 500 mJ pulses at 1064 nm in a single core multimode fiber. This subsystem allows at once to secure the input interface of the fiber and to reduce the optical losses due to Brillouin scattering.

Fluorescence Imaging technology combined with a thin, linear-response fluorescent plate and a camera was developed for real-time 2D beam profiling of a raw beam of a high-power laser in the wavelength range from blue to near infrared. The input laser beam was slightly absorbed on the fluorescent plate in front top of the profiler body and generated 2D fluorescence image, that was precise reproduction of the laser beam profile at the place, was reimaged by a camera at the back side of the profiler. The laser beam that transferred the fluorescent plate was separated by a dichroic mirror and outputted from the body. >99% energy of the input laser beam transferred the fluorescent plate, then the heat generation in the fluorescent plate was very low. This made long-time, stable measurement of the high power, raw laser beam profile possible. >2MW/cm² damage threshold of the fluorescent plate and the efficient heat management made direct measurement of >100W laser beam profiles possible without any attenuators and any computer-aided image reconstructions. Speckle-free and interference fringe-free, high-resolution images were also successfully obtained by the incoherent fluorescence reimaging.

The measurement of depth of a laser capillary in industrial conditions by Optical Coherence Tomography technique is demonstrated in this work. This paper highlights the results achieved by the recent combination of ultrasound sensitive membrane-free optical microphone by XARION and PRECITEC’s OCT IDM system. Joint interpretation of the ultrasound spectrograms and the digging curves of the keyholes enables better understanding of the dynamics of the melted metal and determination of the ideal welding parameters.

This paper reports on an IRAD effort to develop material property allowables data of production grade additively manufactured silicon carbide (SiC) The SiC in this study is reaction bonded and manufactured via a molded, Ceraform® Silicon Carbide, or printed, Ceraprint Silicon Carbide, using production processes and equipment at the AOA facility in Devens, MA. We report statistically significant measurements of materials properties such as density, Young’s Modulus, Poisson’s Ratio, Modulus of Rupture, thermal conductivity, specific heat and coefficient of thermal expansion from 23 Kelvin to 298 Kelvin (-250 °C to +25 °C), for both the formed and printed SiC material. This paper reports on an IRAD effort to develop material property allowables data of production grade additively manufactured silicon carbide (SiC).

During welding the localized heat input results in high temperature gradients between the weld seam and the base material leading to residual stress. The residual stress is a result of two competing processes; thermal shrinkage of the material while cooling (resulting in tensile stress in the weld seam) and phase transformation induced volume expansion (resulting in compressive stress in the weld seam). Both processes superimpose to a resulting residual stress profile. To counteract the problems of residual stress and distortion, in the past few years low-transformation-temperature (LTT) materials have been successfully used as filler wire. Typically, LTT materials are highly alloyed Fe-based materials with levels of Cr and Ni that ensure that austenite transforms to martensite at reduced temperatures. This transformation is accompanied by large volumetric dilatation. The surrounding base material prevents this dilatation in the weld seam and compressive stress builds up while reducing residual stress and distortion. A way to use the LTT effect, other than using a LTT filler wire, is to combine dissimilar materials. By combining high alloy and low alloy materials a microstructure is formed in-situ that shows similar properties as a common LTT weld metal. The displacements after welding are always lower when using LTT filler material when compared to conventional wire, proving that LTT can be used to mitigate distortion during laser beam welding. In this paper the strain distribution by the use of digital image correlation is examined. The influence of dissimilar welding on the microstructure is considered and it is investigated whether the LTT effect can be reproduced with conventional filler wire.

In this paper we present advances in strategies for joining dissimilar materials like copper and aluminum to realize battery connections by implementing distinct laser welding strategies with a fast oscillating single mode laser beam. The high beam quality enables a stable in coupling into high reflective copper connectors with low spatter events. To achieve joints with high electric conductivity a beam oscillation pattern is executed in order to create adequate cross-sections to enables high current flow. This is crucial for fast battery charging and extended lifetime of the batteries. The mechanical stability is achieved in a micro structured manner in such a way wherein the created intermetallic phase zone between copper and aluminum is minimized by implementing very accurate and very small metal mixture through compression of isotherms and melt flow guidance. TRUMPF’s new generation of the product line TruFiber together with matching scanner optics (PFO series) have been implemented to demonstrate the above-mentioned strategy.

We report on industrial high-power lasers in the green wavelength regime. By means of a thin disk oscillator and a resonator-internal nonlinear crystal for second harmonic generation we provide 2000 W continuous-wave laser radiation at a wavelength of 515 nm, transported by a fiber with a 150 µm core diameter and NA 0.1. Application tests show that this laser is perfectly suited for copper welding due to the superior absorption of the green wavelength compared to IR, which allows the production of spatter-free weld spots and seams with an unprecedented reproducibility in diameter and welding depth. Now with 2 kW available power in the green, the welding performance for high quality welds can be increased compared to the already available 1 kW green laser system. The higher power leads to a larger melt volume and higher process efficiency. It is also possible to weld other materials with high quality results, for example stainless steel or aluminum. A particular advantage compared to the common IR lasers is the ability of the green laser for heat conduction mode because of the higher absorption. But due to the high beam quality also high-quality deep penetration welding is applicable, with the comfort to use scanner optics with a big field of work. This offers a whole range of new applications, e.g. remotedeburring of copper parts or producing a shiny copper surface by using shielding gas in addition. A further significant benefit of the green wavelength is that the surface condition does not influence the welding process, which allows to skip many expensive preprocessing steps such as tin-coating. Especially for e-mobility tasks the green 2 kW laser source is a suitable option for increasing productivity and quality: One example are copper foil stacks for battery production, which can be welded with higher speed and reduced spatter formation without ultrasonic pre-welding.

Bead-on-plate welding of pure copper sheet with a thickness of 200 μm were carried out with a 200 W blue diode laser. Output power was varied from 150 to 200 W. The spot diameter was set the 50,100 μm constant. The maximum laser intensity was 1×10⁷ W/cm². The threshold of the output power required for full penetration welding to 200 μm thick pure copper was obtained. Next, at each laser output power, the welding speed was changed to control the input energy, and the welding speed required for full penetration welding was determined. The maximum welding speed 35 mm/s was obtained with a laser intensity of 1×10⁷ W/cm². To investigate welding quality the bead appearance and crosssectional observation were performed.

Joining aluminum and steel today is still challenging. The large difference of melting points of Al and Fe and the rapid formation of fragile intermetallic compounds (IM) make the welding of Al-based and Fe-based alloys at most very difficult if not impossible. Nonetheless, the need of such joints is growing since one tends to incorporate more and more aluminum parts as replacement of “low stressed” steel parts in order to achieve lightweight structures, especially in transportation industry. Here the idea is to depose a thick (<<mm) layer of aluminum on the steel using a powder additive manufacturing process: the cold spray. Aluminum powder is blown toward the steel using pressured gas at rather low temperature (around 80°C). The aluminum part is then welded to the additive manufactured aluminum layer on the steel using conventional laser welding. In the second part of this presentation copper deposition using cold spray is used on thin PCB conductors to allow laser welding of connectors.

Carbon fibre reinforced plastics (CFRP), particularly with a thermoplastic matrix material, have increasingly been used in the last decades. This is especially true in industrial sectors with a strong focus on lightweight applications such as the aviation industry. During the production of CFRP parts imperfections can occur resulting in the need of rework. Furthermore, a damage can occur in service time. In both cases, a large amount of carbon fibres and matrix material has to be mechanically removed, which comes along with high tool wear. Afterwards, the area has to be refilled. This is done by adhesive bonding of CFRP patches. Normal adhesives have long curing times of several hours. To enhance the repair process of thermoplastic CFRP, a two step laser based process was developed. In the first step, CFRP is removed by laser ablation, which allows a high reproducibility and accuracy. Goal is to generate a flat surface with a defined matrix amount. In the second step, laser heat conduction welding is used to refill the removed area with thermoplastic patches. This study was conducted with a carbon fiber fabric within a polyphenylene sulfide matrix. In order to develop a high quality heat conduction process, the ablation process was optimized to generate a defined surface. For the evaluation of the welding process, lap shear samples were welded with different setups. These samples were tested and fraction pattern evaluated.

Multi-kilowatt Laser Beam Welding processes are facing new challenges: reducing the final parts weight and improving reliability to decrease the amount of discarded parts. Appropriate beam shaping enables those improvements by decreasing the process defects and by allowing welding of new types of materials and of thinner parts. We describe here the design and the process test results of a fully reflective beam shaper laser head compatible with high-power lasers demands integrated on a robot. The high efficiency cooling permitted by a reflective design reduces focus shift. A mm-wide annular shape onto the processed part enables melt pool size control.

Adapted laser beam and intensity shapes realized by fiber design offer many opportunities, especially for laser beam welding. An adjustable ring mode laser beam source was used for the investigations presented. Continuous and pulsed mode are possible, so many combinations and temporal overlays of inner and outer ring power are feasible with a maximum laser beam power of 6 kW. This work shows investigations for several metallic materials and combinations. For laser beam welding of similar lap joints made of aluminum-silicon coated hot forming manganese-boron steel the influence of different intensity profiles is analyzed with regard to weld geometries. Dissimilar lap joints made of aluminum and copper are tested regarding the mixing ratio and the homogeneity of the mixing in the weld metal as well as the formation of intermetallic phases. Furthermore, a short digression is made regarding the removal of zinc layers on micro-alloyed, fine-grain structural steel sheets for subsequent reliable wetting for laser beam brazing or for the prevention of spatters for laser beam welding.

Laser beam welding is a necessary and helpful tool in modern production technology. It provides low and located heat input, narrow weld widths, high welding speeds and weld depths. Nevertheless, in the weld metal and the surrounding area the microstructure and the mechanical characteristics can be changed afterwards. A decrease of strength and fatigue life is a possible result. To realize a manipulation or control of the weld metal’s microstructure during the welding process is a great challenge. Improving the strength as well as the homogeneity of mechanical properties and chemical composition are the aims of this approach. With indirect introduced ultrasonic amplitudes, the weld pool dynamics and the solidification are affected. The investigation focusses on the effects in the microstructure of high power (8 kW) laser beam welded stainless steel (AISI 304) with weld depths up to 15 mm. For two different amplitudes (3 and 6 μm) and three different positions of the weld pool in the vibration distribution (antinode, centered and node position) the weld metal is evaluated with metallographic cross sections. The types and the amount of microstructures are analyzed. The solidification of the weld metal is influenced by the vibration. Thus, the orientation, size and growth of the grains as well as the growth direction are changed. Furthermore, the weld characteristics (weld depth, weld width, weld area) are compared to the previously considered aspects.

We successfully prepared reflection-type diffractive optical elements (R-DOEs) using SiC and Si wafers for application to high-power laser material processing. SiC and Si, whose thermal conductivity are 200 W/mꞏK, are preferred materials to be cooled by water to prevent thermal lens effect. Our calculation revealed that these R-DOEs achieved 30-kW laser processing by using water cooling on the backside of these R-DOEs. Measured energy conversion efficiencies were 75% using a single-mode laser with a wavelength of 1.064 μm. Moreover, the integrated intensities of a reconstructed image measured using both single-mode and multi-mode lasers were almost the same. We also succeeded in designing a simple cooling unit. Distortion of these R-DOEs caused by water pressure was also measured to prevent any change in focusing length and distortion of the shaped beam. The measured curvature radius was 100 mm, in which there was a -0.3-mm change in focusing length. The measured reconstructed image was not distorted. We experimentally confirmed that the laser irradiation tolerance of the combined R-DOE and cooling unit was more than 10-kW. These results corresponded well with our theoretical estimation. These results suggest reflection-type DOEs are a good beam shaper for high power laser processing using more-than-10-kW laser sources.

Deep ultra-violet (DUV) laser and short pulse lasers are used for laser processing, because they can decrease the heat effect for process materials. We are developing a hybrid ArF excimer laser that is consists of a solid-state laser, multi wavelength conversion and ArF excimer amplifier. This laser can generate DUV light of 193 nm wavelength short pulse width. In this research, we demonstrated laser drilling on ultra-high temperature structural material that is silicon carbide ceramic matrix composites (SiC-CMC) using high peak power DUV laser. The removal rate was 150 nm/shot with 460 ps pulse. This rate was more than 4 times higher than ArF excimer laser (20 ns pulse width). The HAZ was also reduced by using high peak power DUV light source.

“CaliBend” proposes a direct diode laser approach for thermal assisted processing of steel sheets. Our module aims at enabling bending processes of high-strength steels (HSS) and with minimal bending radii without inducing damage to the metal part. In this work we present a laser source integrated on an industrial servo press for continuous die processing. The metal sheet is heated up locally by laser radiation before the bending stroke. By reaching power densities close to 40 W/mm² , the bending line of the metal sheet reaches the hot forming temperature regime in under two seconds allowing a faultless while increasing the typically low forming limits of HSS.

This PDF file contains the front matter associated with SPIE Proceedings Volume 11273 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.