Low Laser Absorption Research Articles

Achieving low-porosity AlSi10Mg cladding layers for the additive repair of aluminum alloy parts by directed energy deposition

Directed Energy Deposition (DED) has broad application prospects for repairing damaged aluminum alloy parts such as piston aero-engine casing. However, due to the low laser absorptivity and the complex thermal cycle process, it is challenging to obtain aluminum alloy cladding layers with high-quality morphology and low-porosity by DED method. In this work, a series of individual stages of DED process are studied during single-tracks overlapping and layers stacking. Correspondingly, the within layer scanning strategy, inter-layer stacking strategy, and process parameters are examined carefully. Using the proposed DED processing flow that combines substrate preheating with intermittent process, the deposited layers without balling and collapse are successfully achieved. Furtherly, the bi-directional scanning strategy for single-tracks overlapping within layer is proposed to compensate the thickness nonuniformity caused by heat accumulation. The inter defects identified as “lack of fusion” and “gas pores” are observed in the cladding layers with uniform surface. By analyzing the pore formation mechanism and considering the energy distribution and re-melting, the multi-layer offset stacking strategy is proposed to inhibit the porosity to 0.25 %. Combined with the optimized process parameters, the porosity is reduced up to 0.09 %, ensuring the density of the DED deposition coating. This work provides new pathway and direct guidance for fabricating high-quality AlSi10Mg cladding layers during the DED repairing of valuable aluminum alloy parts.

Blue Laser Conduction Spot Welding of Pure Copper Hairpins

Joining rectangular copper hairpins significantly improves the performance of hairpin motors. Copper welding poses a significant challenge due to its low laser absorptivity when using conventional infrared lasers. Although employing keyhole welding with a high-power and small-spot infrared laser can increase the absorption rate through multireflections, it introduces instabilities caused by the multi-recoil forces within the keyhole and the absorptivity differences among solid, liquid, and keyhole states of copper. The proposed solution adopts a 450 nm blue laser, which exhibits a superior laser absorptivity toward copper and facilitates stable conduction mode spot welding with an enlarged laser spot. This research achieved successful blue laser conduction spot welding of copper hairpins with a gap and, for the first time, systematically analyzed the force and instability factors of a molten pool. The results demonstrated that with a laser power of 1950 W, laser spot diameter of 0.6 mm, and defocusing amount of –3 mm, only 0.7 s of welding time was required to join copper hairpins with a 0.6 mm gap. Notably, there were no noticeable defects in weld beads during blue laser welding. Meanwhile, the study identified recoil force as a significant uncertain factor inducing defects, particularly in the keyhole mode. Compared to infrared technology, the blue laser, characterized by a higher absorption rate and a larger spot diameter, effectively reduced uncertainty through conduction welding. This research showcased the significant potential for blue lasers in manufacturing copper hairpin motors.

Advancements and Perspectives in Additive Manufacturing of Tungsten Alloys and Composites: Challenges and Solutions

This review explores additive manufacturing (AM) for refractory tungsten (W) and its alloys, highlighting the primary challenges and determining factors in the AM of pure W, W alloys and composites. The challenges mainly arise from W’s high melting point, low laser absorptivity, high thermal conductivity, high melt viscosity, high oxygen affinity, high ductile-to-brittle transition temperature, and inherent embrittlement, which lead to defects and anomalies in AM-produced parts. This review focuses on both processes and alloying strategies to address the issues related to densification, micro-cracking, and the resultant properties in W-based components. Cracking in additively manufactured W remains a persistent issue due to thermal stress, embrittlement, and oxide formation. Powder characteristics, process parameters, and thermal management strategies are crucial for W densification. Throughout the review, existing knowledge and insights are organized into comprehensive tables, serving as valuable resources for researchers delving deeper into this topic. Future research in W-AM should focus on understanding the interaction between AM process parameters and microstructural and material design. Advances in atomic-level understanding, thermodynamic modeling, and data analytics have the potential to significantly enhance the precision, sustainability, and applicability of W-AM.

The surface morphologies and internal pores evolution of a CuSn10 alloy fabricated by laser powder bed fusion

The fabrication of high quality copper alloys using laser powder bed fusion (LPBF) technology remains challenging due to their low laser absorption and high thermal conductivity. LPBF often involves intrinsic defects, such as rough surface and internal pores, which largely deteriorate the performance of as-built copper alloy components. Here we studied the effect of scanning speed on the evolution of surface morphologies and internal pores of LPBF-fabricated CuSn10 alloy. The melt flow behaviors were studied by both experiments and in situ observations where combined a high-speed camera with an infrared thermal imager. With increasing the scanning speed, the morphology of melt track evolves from a continuous track to a discontinuous one, whereas the internal defects change from a great number of spherical pores to many irregular pores. A higher scanning speed decreases the peak temperature of the melt pool and causes less overlaps between neighboring melt tracks, thus deteriorating wetting behaviors. It is expected that the obtained results are beneficial for guiding the additive manufacturing of copper alloy parts with high metallurgical quality and thus promoting their industrial applications.

Study on Densification and Defects of Cu-Cr-Zr Alloy Formed by Selective Laser Melting

Abstract Selective laser melting (SLM) technology features high forming accuracy and a short preparation cycle, providing a significant advantage for shaping complex structural components. Copper-chromium-zirconium exhibits outstanding comprehensive performance and finds wide applications in rocket combustion chambers, automotive inductor coils, nuclear shielding materials, welding electrodes, and other areas. However, due to its low laser absorption rate and high thermal conductivity, the forming process of copper-chromium-zirconium is unstable and prone to forming defects. This study aims to address this issue by employing the response surface method to design process parameters and investigate the impact of these parameters on microstructure and properties, to obtain high-density Cu-Cr-Zr alloy.

Additive Manufacturing of Micro‐Architected Copper based on an Ion‐Exchangeable Hydrogel

AbstractAdditive manufacturing (AM) of copper through laser‐based processes poses challenges, primarily attributed to the high thermal conductivity and low laser absorptivity of copper powder or wire as the feedstock. Although the use of copper salts in vat photopolymerization‐based AM techniques has garnered recent attention, achieving micro‐architected copper with high conductivity and density has remained elusive. In this study, we present a facile and efficient process to create complex 3D micro‐architected copper structures with superior electrical conductivity and hardness. The process entails the formulation of an ion‐exchangeable photoresin, followed by the utilization of digital light processing (DLP) printing to sculpt 3D hydrogel scaffolds, which were transformed into Cu2+‐chelated polymer frameworks (Cu‐CPFs) with a high loading of Cu2+ ions through ion exchange, followed by debinding and sintering, results in the transformation of Cu‐CPFs into miniaturized copper architectures. This methodology represents an efficient pathway for the creation of intricate micro‐architected 3D metal structures.

Additive Manufacturing of Micro-Architected Copper based on an Ion-Exchangeable Hydrogel.

Additive manufacturing (AM) of copper through laser-based processes poses challenges, primarily attributed to the high thermal conductivity and low laser absorptivity of copper powder or wire as the feedstock. Although the use of copper salts in vat photopolymerization-based AM techniques has garnered recent attention, achieving micro-architected copper with high conductivity and density has remained elusive. In this study, we present a facile and efficient process to create complex 3D micro-architected copper structures with superior electrical conductivity and hardness. The process entails the formulation of an ion-exchangeable photoresin, followed by the utilization of digital light processing (DLP) printing to sculpt 3D hydrogel scaffolds, which were transformed into Cu2+-chelated polymer frameworks (Cu-CPFs) with a high loading of Cu2+ ions through ion exchange, followed by debinding and sintering, results in the transformation of Cu-CPFs into miniaturized copper architectures. This methodology represents an efficient pathway for the creation of intricate micro-architected 3D metal structures.

Blue Laser Conduction Welding of Dissimilar Cu and Al Sheets

Joining Cu-Al with 0.6 mm is important for the high-power density battery in the new energy field. Low laser absorptivity is a challenge in Cu-Al welding with conventional infrared laser at around 1000 nm wavelength, where keyhole welding is necessary. It is difficult to control welding qualities in keyhole welding due to the intense flow and violently changing absorption rate. At 450 nm wavelength, Cu and Al have high laser absorptizzvity which has the potential to implement stable conduction welding. Therefore, this work adopted a blue laser welding system, and for the first time, realized the conduction welding of 0.6 mm Cu and 0.6 mm Al sheets. The welding process, surface appearance, mechanical properties, and electrical properties were investigated. The results showed that high-power (1950 W) could effectively realize stable Cu-Al conduction welding without spatters, and the welding speed could reach 40 mm/s. Compared with an infrared laser, the blue laser could weld Cu-Al using the form of Cu on top and Al on bottom, which was beneficial for a wide process window and stable welding process. A larger bead width and more consistent intermetallic compound thickness resulting from the blue laser were conducive to performance improvement. In addition, the relationship between welding parameters, molten pool characteristics, and process qualities was built. It provided a possibility to control the welding quality under the influence of strong heat accumulation and high thermal conductivity. This work demonstrated that blue laser has great potential in joining Cu-Al for new energy applications.

On the Processability and Microstructural Evolution of CuCrZr in Multilayer Laser-Directed Energy Deposition Additive Manufacturing via Statistical and Experimental Methods

Laser-directed energy deposition (LDED) is a promising technology for coating, repairing, and building near-net-shape 3D structures. However, the processing of copper alloys, specifically, has presented a significant challenge due to their low laser absorptivity at the 1060 nm laser wavelength and high thermal conductivity. This study undertook a methodical examination by employing a 2 kW disk laser, operating at a wavelength of 1064 nm, and a coaxial nozzle head to comprehensively examine the processability of the highly conductive CuCrZr alloy for expanding the range of materials that can be successfully processed using LDED. The investigation focuses not only on optimizing the input process parameters that are the laser power, scanning speed, powder feed rate, and overlap ratio, but also on planning the toolpath trajectory, as these factors were found to exert a substantial influence on processability, geometrical accuracy, and the occurrence of defects such as lack of fusion. The optimal toolpath trajectory discovered involved implementing a zigzag strategy combined with a 90° rotation of the scanning direction. Additionally, a start point rotation was considered between each layer to even out the deposition of the layers. Moreover, a contour with a radial path at the corners was introduced to enhance the overall trajectory. Based on the hierarchal experimental study, the appropriate ranges for the key process parameters that leads to 99.99% relative density have been identified. They were found to be from 1100 up to 2000 W for the laser power (P), and from 0.003 up to 0.016 g/mm for the amount of powder that is fed to the melt pool distance (F/V). Regarding the influence of process parameters on the microstructure of the samples with equal deposition height, it was observed that varying combinations of process parameters within the optimal processing window resulted in variations in grain size ranging from 105 to 215 µm.

A novel hierarchical manufacturing method of the selective laser melted Al 7075 alloy

Manufacturing of the 7075 wrought aluminum alloys by selective laser melting technology (SLM) is extremely difficult owing to the inherent low laser absorptivity, poor weldability and cracking. To address these challenges and improve the mechanical properties of the SLM Al 7075 alloy, a novel hierarchical manufacturing method of the selective laser melted Al 7075 alloy from the nanometer scale to the millimeter scale was presented in this study. Nano-WC/Al 7075 sample with fine equiaxed grains was fabricated using SLM. Fewer cracks can be detected within the SLM nano-WC/Al 7075 alloy than the SLM Al 7075 alloy. A large quantity of nano-WC and sintered micro-WC particles were distributed within the equiaxed grains. A well-coherent interface with low lattice mismatch (i.e. 2.03%) is exhibited at the (111)α-Al/(001)WC planes. Al4W, Al2Cu and MgZn2 phases nucleated and grew on the surface of the nano-WC particles. A type of fine microstructure composed of the hierarchically reinforced micro/nano particles is produced in the SLM nano-WC/Al 7075 sample, resulting in an excellent micromechanical properties. Notably, wear rate of the SLM nano-WC/Al 7075 sample is about 40.4% lower than that of the SLM Al 7075 specimen, similarly with that of the SLM Ti6Al4V counterpart. These results highlight that the site-specific and quantitative modification of nano-WC particles of Al 7075 powder can be achieved using electrostatic self-assembly. Therefore, introducing nano-WC particles and generating multiple micro/nano reinforcement phases is an effective method to significantly improve their tribological properties.

Plume characteristics of polymeric material doped with different metal particles under pulsed laser irradiation

The polymer emerges as a promising propellant for laser propulsion, which is expected to achieve high specific impulse and momentum coupling coefficient. However, the low laser absorptivity of polymer deteriorates its further application. Metals can improve the laser absorption of laser energy, contributing to better propulsion performance. This paper investigated the influence of different alloy particles (Cu-Zn, Al-Si, Ni-Ti, Co-Cr) on the plume characteristics of polymeric material under laser irradiation. The plume images, length, area, and relative intensity distribution of the doped sample were analyzed and compared. It is found that each alloy particle could contribute to the stable and massive formation of the ablation plume. When laser irradiates these doped samples, an obvious jet was generated from the ablation surface, and there was an additional expansion cluster at the end of the jet. Compared to other samples, the target filled with Al-Si witnessed the maximal plume length of 13 cm, expansion velocity of 26 m/s, and plume area of 33 cm2. Besides, the higher doping ratio contributed to the larger plume length while the smaller plume area in general, but the doping ratio had little effect on the plume length and area for the short laser-material interaction process.

Numerical and experimental study of molten pool behaviour and defect formation in multi-material and functionally graded materials laser powder bed fusion

Laser powder bed fusion (LPBF) of multi-material and functionally graded materials (FGM) has attracted significant research interest due to its ability to fabricate components with superior performance compared with those manufactured with single powder material. However, the forming mechanisms of various defects remain unknown. In this paper, a DEM-CFD model was first established to obtain an in-depth understanding of this process. It was discovered that the defects including partially melted and un-melted Invar36 powder were embedded in the lower level of the powder layer; this was attributed to the low laser absorptivity, low melting point and high thermal conductivity of the Cu10Sn powder. Inter-layer defects were more likely to occur with an increased powder layer thickness. In addition, the scanned track width was found related to an equilibrium achieved among the thermal properties of the powder mixture. Process parameters were optimised to obtain FGM structures without defects in both horizontal and vertical directions. Invar36/Cu10Sn samples were fabricated with a multi-material LPBF system using different mixed powder contents and laser volumetric energy densities (VEDs). By increasing the VED, fewer defects were observed between the interface of two processed powder layers, which had a good agreement with the modelling results.

Microstructure and mechanical property of graphene oxide/AlSi10Mg composites fabricated by laser additive manufacturing

The low laser absorptivity and high thermal conductivity of Al are the foremost concerns when developing Al matrix composites (AMCs) through laser powder bed fusion (L-PBF) process. In this study, we demonstrated an example of improving the 3D-printability of AMCs by means of powder surface modification. Flexible graphene oxide (GO) sheets were carefully coated onto the surface of AlSi10Mg powders under electrostatic self-assembly via a hetero-agglomeration process. In addition to maintaining a shape and particle size similar to the initial metallic powders, the GO-coated AlSi10Mg powders exhibited enhanced laser absorptivity and decreased thermal conductivity, beneficial to their fusion. Under high-energy irradiation, the GO sheets were partially transformed to Al4C3 nanorods individually distributed in the matrix, while the un-reacted parts floated within molten pools under buoyancy, forming an in-situ carbon layer tightly deposited on the surface of the composite build. This work may provide significant guidance for the design and production of high-performance AMCs with advanced architectures for practical applications.

Laser powder bed fusion of crack-free TiN/Al7075 composites with enhanced mechanical properties

The manufacturing of 7xxx series wrought aluminum alloys using laser powder bed fusion (LPBF) is restricted due to the inherent low laser absorptivity and high crack sensitivity. In this work, Al7075 composite powders decorated with TiN nanoparticles are produced by an ultrasonic vibration dispersion technique. Results show that the laser reflectivity of the composite powders is notably reduced, leading to improved LPBF processability. Therefore, nearly dense and crack-free TiN/Al7075 composites are successfully fabricated by LPBF. The microstructure of as-built composites is significantly refined after adding TiN nanoparticles, thus inhibiting the crack initiation. The TiN/Al7075 composites exhibit remarkably improved microhardness and compressive properties compared to unreinforced Al7075 alloy. This paper provides a new path in fabricating high-strength aluminum alloys by LPBF.

Three-Dimensional Printing of Ceramics through "Carving" a Gel and "Filling in" the Precursor Polymer.

Achieving a viable process for three-dimensional (3D) printing of ceramics is a sought-after goal in a wide range of fields including electronics and sensors for harsh environments, microelectromechanical devices, energy storage materials, and structural materials, among others. Low laser absorption of ceramic powders renders available additive manufacturing (AM) technologies for metals not suitable for ceramics. Polymer solutions that can be converted to ceramics (preceramic polymers) offer a unique opportunity to 3D-print ceramics; however, due to the low viscosity of these polymers, so far, their 3D printing has only been possible by combining them with specialized light-sensitive agents and subsequently cross-linking them layer by layer by rastering an optical beam. The slow rate, lack of scalability to large specimens, and specialized chemistry requirements of this optical process are fundamental limitations. Here, we demonstrate 3D printing of ceramics enabled by dispensing the preceramic polymer at the tip of a moving nozzle into a gel that can reversibly switch between fluid and solid states, and subsequently thermally cross-linking the entire printed part "at-once" while still inside the same gel. The solid gel, which is composed of mineral oil and silica nanoparticles, converts to fluid at the tip of the moving nozzle, allows the polymer solution to be dispensed, and quickly returns to a solid state to maintain the geometry of the printed polymer both during printing and the subsequent high-temperature (160 °C) cross-linking. We retrieve the cross-linked part from the gel and convert it to ceramic by high-temperature pyrolysis. This scalable process opens up new opportunities for low-cost and high-speed production of complex three-dimensional ceramic parts and will be widely used for high temperature and corrosive environment applications, including electronics and sensors, microelectromechanical systems, energy and structural applications.

Tribological and Mechanical Characterization of Nickel Aluminium Bronze (NAB) Manufactured by Laser Powder-Bed Fusion (L-PBF)

To cope with the increasing demands of high-performance materials, efforts were made using Laser Powder Bed Fusion technique (L-PBF), taking advantage of its extremely short solidification time to fabricate copper alloy with highly refined microstructure. Copper and Copper alloys are reputed for their high reflectivity resulting in low laser absorption, which makes their 3D fabrication process very challenging. This study aims to provide high density samples in order to obtain high mechanical and tribological properties. This objective is realized through the optimum energy density value which is obtained by monitoring laser power P, scan speed v, hatch space h and layer thickness t. Upon mapping of optimum parameters, the best combination is adopted to manufacture the samples of this study. Mechanical and tribological characterization of nickel aluminium bronze alloy confirmed the efficiency of laser powder-bed fusion (L-PBF) in providing high performance materials, higher properties such as wear resistance, tensile strength and hardness were obtained compared to other manufacturing techniques.

Challenges and proposed solutions for aluminium in laser powder bed fusion

There is a growing interest for additive manufacturing of aluminium in the industry due to its favourable properties. There are, however, many challenges regarding selective laser melting of aluminium alloy powder that must be solved to make additive manufactured aluminium parts more reliable. Among the most highlighted issues are oxidation, high reflectivity and low laser absorption of the powder. In this paper, a brief literature review is conducted to analyse issues related to the named challenges and how these issues can be addressed. As the result, solutions and gaps for porosity, cracks, uneven grain growth and shrinkage effect for laser powder bed fusion of aluminium alloys are identified and presented. Cracking is found to be the most investigated issue among all presented, while shrinkage effect is not addressed in the current state-of-the-art.

Evolution of carbon nanotubes and their metallurgical reactions in Al-based composites in response to laser irradiation during selective laser melting

Aluminium-based composites reinforced with carbon nanotubes are widely sought for their outstanding metallurgical and structural properties that largely depend on the manufacturing route. In this work, the process-structure-property relationship for a composite made from high-energy-ball-milled pure Al and multi-walled carbon nanotubes (MWCNT) processed by laser powder-bed-fusion additive manufacturing was investigated. The strong good bond between MWCNT (high laser absorptivity) and Al (low laser absorptivity) allowed the energy transfer from the former to the latter and maintained the heat for longer allowing for better melt efficiency hence improved processability. The response of MWCNT to laser irradiation and their interfacial reactions with Al were probed in a holistic investigation. X-ray diffraction confirmed the partial transformation of C into Al-carbides in addition to the presence of some nano-crystalline graphitic materials. Microscopy revealed evidence of carbides segregation at the melt pool boundaries as well as migration along the build direction. Raman spectroscopy showed that laser irradiation promoted re-graphitisation in MWCNT, reducing the amount of defects introduced by milling. Two types of Al4C3 formed as a result of the metallurgical reaction between Al and MWCNT. These were needle-like and hexagonal Al4C3 and their mechanisms of formation, direct precipitation and dissolution-precipitation, respectively, were explained in light of the thermal profile experienced by the material during melting and solidification. Large scale electron backscatter diffraction showed that there is no distinctive texture developing during melting and solidification. Micro- and nano-indentation testing showed uniform mechanical properties.

Reutilization of a reflected laser beam as an effective approach for machining metallic materials with low laser absorptivity.

Laser micromachining technology is a method of precision manufacturing that is experiencing growth with wide applications. Due to the increasingly tense energy situation, it is an appropriate time to consider the efficiency and economic issues of growth in precision manufacturing. The reutilization of a reflected laser beam (RRLB) for modifying metallic materials with low laser absorption is proposed in this article to reduce the energy and time consumption of the laser micromachining process. The novel laser machining approach, using an RRLB, which combined a nanosecond laser with an RRLB optical system of fiber laser, was applied on 6061 aluminum to validate its superior characteristics. The characteristics of the RRLB were clarified by the experiment on 6061 aluminum. Compared with normal laser machining (NLM), the processing efficiency of the RRLB can be greatly improved. Owing to the reutilization of the reflected laser beam energy without a thermal relaxation time, a higher-intensity laser-metal interaction can be realized for the RRLB; thus, the incoming energy can be utilized more effectively by ablating more material instead of diffusing it into the sample. Moreover, practical examples by using the RRLB, such as surface darkening on 6061 aluminum, surface polishing on additive manufactured Al alloy, and surface colorization on titanium and stainless steel, also demonstrated the excellent versatility and superiority of the RRLB approach.

New Aluminum Alloys Specifically Designed for Laser Powder Bed Fusion: A Review.

Aluminum alloys are key materials in additive manufacturing (AM) technologies thanks to their low density that, coupled with the possibility to create complex geometries of these innovative processes, can be exploited for several applications in aerospace and automotive fields. The AM process of these alloys had to face many challenges because, due to their low laser absorption, high thermal conductivity and reduced powder flowability, they are characterized by poor processability. Nowadays mainly Al-Si alloys are processed, however, in recent years many efforts have been carried out in developing new compositions specifically designed for laser based powder bed AM processes. This paper reviews the state of the art of the aluminum alloys used in the laser powder bed fusion process, together with the microstructural and mechanical characterizations.