Influence of various process conditions on surface finishes induced by the direct metal deposition laser technique on a Ti–6Al–4V alloy

The direct metal deposition (DMD) with laser is a free-form metal deposition process for manufacturing dense pieces, which allows generating a prototype or small series of near net-shape structures. One of the most critical issues is that produced pieces have a deleterious surface finish which systematically requires post machining steps. This problem has never been fully addressed before.The present work describes investigations on the DMD process, using an Yb-YAG disk laser, and a widely used titanium alloy (Ti–6Al–4V) to understand the influence of the main process parameters on the surface finish quality. The focus of our work was: (1) to understand the physical mechanisms responsible for deleterious surface finishes, (2) to propose different experimental solutions for improving surface finish.In order to understand the physical mechanisms responsible for deleterious surface finishes, we have carried out: (1) a precise characterization of the laser beam and the powder stream; (2) a large number of multi-layered walls using different process parameters (P(W), V(m/min), D_m (g/min), Gaussian or uniform beam distribution); (3) a real time fast camera analysis of melt pool dynamics and melt-pool – powder stream coupling; (4) a characterization of wall morphologies versus process parameters using 2D and 3D profilometry.The results confirm that surface degradation depends on two distinct aspects: the sticking of non-melted or partially melted particles on the free surfaces, and the formation of menisci with more or less pronounced curvature radii. Among other aspects, a reduction of layer thickness and an increase of melt-pool volumes to favor re-melting processes are shown to have a beneficial effect on roughness parameters. Last, a simple analytical model was proposed to correlate melt-pool geometries to resulting surface finishes.     

Porous structures fabrication by continuous and pulsed laser metal deposition for biomedical applications; modelling and experimental investigation

The use of porous surface structures is gaining popularity in biomedical implant manufacture due to its ability to promote increased osseointegration and cell proliferation. Laser direct metal deposition (LDMD) is a rapid manufacturing technique capable of producing such a structure. In this work LDMD with a diode laser in continuous mode and with a CO₂ laser in pulsed modes are used to produce multi-layer porous structures. Gas-atomized Ti-6Al-4V and 316L stainless steel powders are used as the deposition material. The porous structures are compared with respect to their internal geometry, pore size, and part density using a range of techniques including micro-tomography. Results show that the two methods produce radically different internal structures, but in both cases a range of part densities can be produced by varying process parameters such as laser power and powder mass flow rate. Prudent selection of these parameters allows the interconnected pores that are considered most suitable for promoting osseointegration to be obtained. Analytical models of the processes are also developed by using Wolfram Mathematica software to solve interacting, transient heat, temperature and mass flow models. Measured and modelled results are compared and show good agreement.     

Assessment of controlled short-circuiting application in filling passes with MIG/MAG double-wire process

In this work, a technical–economic assessment of the double-wire MIG/MAG welding process was carried out for flat-position welding of filler passes, employing controlled short-circuiting (CSC) operating mode in the trailing wire, as a way of improving control of the molten pool and reducing splashes. The assessment was based on a comparative study between the conventional double-wire MIG/MAG technique (using pulsed current in both wires) and a combination of pulsed current in the leading wire and CSC in the trailing wire. The following were kept constant: the values of average current for each wire, the volume of material deposited per unit length of weld, the type and size of the bevel and the wire, the composition of the shielding gas and the CTWD. The surface finish and geometry of the weld beads, deposition rate and deposition efficiency and the maximum welding speeds resulting from the use of each technique were assessed. The results show that the use of the CSC mode in the trailing arc increases the production capacity of the process slightly, leads to less splashing, improves the finish of the bead, produces smaller fusion zone and smaller HAZ and yet maintains a similar operational envelope (with respect to the range of useful welding speeds), which are positive factors. On the other hand, as a disadvantage, the use of CSC led to beads that are more convex and have less penetration.     

UHV arc for high quality film deposition

The vacuum arc is a well-known technique for producing coatings with enhanced adhesion and film density. Many cathodic arc deposition systems are actually in use in industry and research. They all work under (high) vacuum conditions in which water vapor pressure is an important source of film contamination, especially in the pulsed arc mode of operation. Here we present a cathodic arc system working under ultra-high vacuum conditions (UHVCA). We have used for arc ignition a Nd-YAG pulsed laser focused on the cathode surface, which provides a reliable system and allows eliminating all possible sources of contaminants. We have proven that the arc technique produces bulk-like films suitable for superconducting applications. UHVCA has been used to produce ultra-pure niobium films with excellent structural and electrical properties at a deposition temperature lower than 100 °C. The UHVCA technique therefore opens up new perspectives for all applications requiring pure films and low deposition temperatures.     

Hybrid additive manufacturing of metal laminated forming tools

Deep drawing dies are manufactured using metal sheets. Laser metal deposition is used for bonding the sheets and smoothening the edges. The strength and surface finish of the dies are the key challenges. Milling, roller burnishing, and laser treatment are applied as post-processing for improving the surface finish. A semi-analytical model is developed for selecting the sheet combination for sufficient strength. The new rapid prototyping process offers high flexibility for complex die geometries. The evaluation by deep drawing experiments using DC06 and high-strength HC380LA blanks revealed the feasibility of the new manufacturing routes regarding deep drawability and surface finish.     

激光表面处理Al0.3CoCrFeNi高熵合金组织特征

通过4种不同功率的脉冲激光（150,250,350,450 W）对铸态Al_0.3CoCrFeNi高熵合金进行处理。按相对于激光源的位置,合金组织分为3个部分：内部具有细胞状结构的中心影响区,具有长条结构的边缘影响区,以及未受影响的原始晶粒区域。由于激光引起的快速熔化和凝固,经不同功率处理的样品截面中均观察到细胞结构;另外,在激光影响区边缘发现了长条细胞结构。之后,将激光处理的样品分别在660和800 ℃下退火2 h,可以观察到沿细胞结构边界有沉淀相析出。通过硬度测试,表明激光表面处理和后续热处理相结合的方法显著提升了合金的硬度。     

Particle-in-flight monitoring in thermal spray processes

A diagnostic system based on non-intensified CCD image sensor is applied for particle-in-flight monitoring of different deposition processes: cold gas dynamic spray (CGDS), computer-controlled detonation spray (CCDS) and direct metal deposition (DMD). An additional illumination source for measuring particle velocity in CGDS and DMD processes is used. Particle velocity measurements are carried out aiming optimization of a Cold Spray nozzle with two zones of powder injection for spaying Al powder. In a pulsed-periodic process like detonation spraying, particle-in-flight visualization and velocity measurements are done by synchronizing detonation pulses with the CCD-camera-based diagnostic tool. A significant variation of particles velocity along the detonation plume is observed. In DMD process, dependence between the carrier gas flow rate and particle velocity is found.     

Numerical simulation and experimental investigation of gas–powder flow from radially symmetrical nozzles in laser-based direct metal deposition

Laser-based direct metal deposition (DMD) is a solid freeform fabrication process capable of fabricating fully dense and metallurgically sound parts. The process has been greatly enhanced toward multi-directional deposition by the use of discontinuous radially symmetrical powder nozzles to supply the build material. Since many operational parameters depend on the gas–powder stream characteristics between the nozzles and the deposition point, an extensive understanding of the gas–powder flow is necessary. Three-dimensional (3D) multi-phase gas–powder flow structures of radially symmetrical nozzles are modeled using computational fluid dynamics methods. The obtained results are in good agreement with the experimental ones, and they provide a good insight into the process phenomena.     

Pulsed laser deposition of functionally gradient diamond-like carbon (DLC) films using a 355 nm picosecond laser

In this study, functionally gradient diamond-like carbon (FGDLC) films are fabricated using a novel pulsed laser deposition technique to enhance adhesion strength. A 355 nm picosecond laser beam is split into two beams, and the power of each split beam is changed individually by a motorized beam attenuator as a function of time. In this way, two laser beams with customized time-varying powers are available for ablating two different target materials. Two beams are irradiated on graphite and 316L stainless steel targets, respectively, in a vacuum chamber, and the produced dissimilar plasmas are mixed in space before they are deposited on a stainless steel 316L substrate. Using this method, we have built FGDLC films with a thickness of ～510 nm, where the composition changes gradually from stainless steel to DLC in the direction of deposition. We have confirmed that FGDLC films show much higher adhesion strength than normal DLC films.     

Characteristics of laser aided direct metal/material deposition process for tool steel

Laser aided direct metal/material deposition (DMD) process builds metallic parts layer-by-layer directly from the CAD representation. In general, the process uses powdered metal/materials fed into a melt-pool, creating fully dense parts. Success of this technology in the die and tool industry depends on the parts quality to be achieved. To obtain designed geometric dimensions and material properties, delicate control of the parameters such as laser power, spot diameter, traverse speed and powder mass flow rate is critical. In this paper, the dimensional and material characteristics of directed deposited H13 tool steel by CO₂ laser are investigated for the DMD process with a feedback height control system. The relationships between DMD process variables and the product characteristics are analyzed using statistical techniques. The performance of the DMD process is examined with the material characteristics of hardness, porosity, microstructure, and composition.     

Deposition of antimicrobial coatings on microstereolithography-fabricated microneedles

Microneedles are small-scale needle-like projections that may be used for transdermal delivery of pharmacologic agents, including protein-containing and nucleic acid-containing agents. Commercial translation of polymeric microneedles would benefit from the use of facile and cost effective fabrication methods. In this study, visible light dynamic mask microstereolithography, a rapid prototyping technique that utilizes digital light projection for selective polymerization of a liquid resin, was used for fabrication of solid microneedle array structures out of an acrylate-based polymer. Pulsed laser deposition was used to deposit silver and zinc oxide coatings on the surfaces of the visible light dynamic mask microstereolithography-fabricated microneedle array structures. Agar diffusion studies were used to demonstrate the antimicrobial activity of the coated microneedle array structures. This study indicates that light-based technologies, including visible light dynamic mask microstereolithography and pulsed laser deposition, may be used to fabricate microneedles with antimicrobial properties for treatment of local skin infections.     

钛合金的激光表面处理研究进展

介绍了钛合金的各种激光表面改性技术及其研究进展,讨论了激光表面改性处理工艺参数对钛合金性能的影响。并介绍了其应用前景。利用激光表面处理可克服钛合金硬度低、易发生粘着磨损等缺点,同时可提高其耐蚀性及抗高温氧化等性能。此外,利用脉冲激光沉积技术在改善钛合金的生物活性等方面也取得了好的结果。     

Self-organized gold nanostructures on laser patterned PET

Irradiation of polyethyleneterephtalate (PET) with linearly polarized light from a pulsed 248 nm KrF laser with 6000 pulses per area and fluences between 2.0 and 8.4 mJ·cm^? 2 resulted in formation of coherent ripple patterns with a lateral periodicity in the order of laser light wavelength and structure depth of several 10 nm. The structure period could be controlled by variation of the incidence angle of the laser beam. Subsequently, thin gold layers were deposited onto the laser patterned PET surface either by sputtering and evaporation. The resulting structures were analyzed by atomic force microscopy (AFM) and focused ion beam cuts in combination with scanning electron microscopy (FIB-SEM). Evaporation leads to a continuous metal coverage copying the nanostructured polymer surface, while sputtering leads to formation of isolated nano-wires on the ridge of the laser-induced ripples. The width of the gold nano-wires could be tailored by the ripple periodicity formed before the deposition process.     

激光复合加工制备超疏水金属表面的研究进展

随着我国工业化进程的不断推进,金属材料已经广泛应用到生产生活的各个领域.仿生超疏水金属表面不仅能够延长金属材料在各种环境下的使用寿命,而且还能赋予材料表面自清洁、减阻、油水分离等新的性能.目前,研究人员已采用多种工艺在金属基体上制备出超疏水表面,超疏水金属表面的制备已经成为仿生学研究中的一个热点.首先介绍了润湿理论的发展,引出了制备超疏水金属表面的各种工艺方法,进一步归纳总结了激光加工制备超疏水金属表面的优势、特点和表面微结构.在此基础上,重点论述了近年来将激光加工工艺与化学刻蚀工艺、沉积工艺、离子注入工艺、涂层工艺和氧化工艺相结合的激光复合加工工艺,以及运用激光复合加工工艺制备的超疏水金属表面的结构和特点.激光复合加工不仅能够在金属表面形成更加丰富的微/纳米复合结构,而且能够使金属表面更快地获得超疏水性能,从而制备出稳定持久的超疏水金属表面.此外,复合加工能够降低对单一制备工艺的依耐性,扩大加工范围,降低生产成本.激光复合加工制备超疏水金属表面在实际应用中具有巨大的潜力.     

纯铝脉冲电子束处理后喷射型火山坑形貌成因

为了深入了解强流脉冲电子柬轰击处理过的材料表面产生的火山坑形貌成因及其形成过程.本文利用俄罗斯Nadezhda.2强流脉冲电子束发生装置对实验样品进行不同脉冲次数的电子束轰击处理.通过扫描电子显微镜进行观察分析.发现在材料表而产生了大量的类似火山喷发后形成的火山坑形貌,在喷发口周边密布着大量纳米级的球形颗粒.通过熔化潜热温度补偿的数值模拟结果和实验结果的对比.揭示了亚表层率先升温及熔化.随着内部熔化体积膨胀力的增大,熔体从内向外喷发,最终形成喷射型的火山坑形貌.     

Effect of laser power and deposition environment on the microstructure and properties of direct laser metal-deposited 12CrNi2 steel

Direct laser metal deposition was used for preparing blocks of steel 12CrNi2 using four different laser powers under two different deposition environments including atmospheric environment and Ar-protected chamber.The results showed that microstructures and mechanical properties were significantly affected by different laser powers.Increasing laser power and deposition in Ar chamber will lead to a decrease in the quantity and size of the voids,which brings more elongation to the samples.Bainitic microstructure was replaced by Widmanstatten ferrite and pearlite,and the amount of proeutectoid ferrite increased with increasing laser power.Moreover,microstructures of previous layers were completely altered in high laser power.Excessive heat accumulation by using high heat input can produce equiaxed ferritic grains with the pearlites in previously deposited layers.Hardness of deposited samples increased from the bottom layer toward the top layer.By using a diode laser with a spot diameter size of 2 mm,the 900-W laser power is suitable for producing crack-and void-free samples.However,post-deposition heat treatment is necessary for obtaining homogeneous desired microstructure and grain size in the manufactured samples.     

Macrostructural and microstructural features of 1 000 MPa grade TRIP steel joint by CO2 laser welding

Bead-on-plate CO2 laser welding of 1 000 MPa grade transformation induced plasticity (TRIP) steel was conducted under different welding powers, welding speeds and shield gases. The macrostructural and microstructural features of the welded joint were investigated. The increase of welding speed reduced the width of the weld bead and the porosities in the weld bead resulting from the different flow mode of melted metal in weld pool. The decrease of welding power or use of shield gas of helium also contributed to the reduction of porosity in the weld bead due to the alleviation of induced plasma formation, thus stabilizing the keyhole. The porosity formation intimately correlated with the evaporation of alloy element Mn in the base metal. The laser welded metal had same martensite microstructure as that of water-quenched base metal. The welding parameters which increased cooling rate all led to fine microstructures of the weld bead.     

Metal droplet deposition: review of innovative joining technique

Abstract

With pulsed gas metal arc (GMA) welding, many metal droplets per second are pinched off from the welding wire. The resultant continuous flow of metal droplets forms the weldment together with the parts to be joined. Each droplet has a certain heat content that it passes on to its surroundings. The droplet deposition welding process, instead of forming many droplets per second, is able to form only one droplet at any desired moment. This single metal droplet can then be used to make a welded joint. The relatively new welding process of droplet deposition welding can be applied, for example, as a replacement joining technique for brazing and lead-tin soldering. The present paper reviews the history of this welding process and its industrial application.     

激光预处理硬质合金衬底沉积金刚石薄膜研究

研究了准分子激光辐照对ＷＣ－６ｗｔ％Ｃｏ（ＹＧ６）硬质合金工具衬底沉积金刚石膜的影响。证实准分子激光辐照可产生钴选择性蒸发和表面粗糙化；从而有效地降低表面钴含量，消除钴对金刚石膜沉积的有害影响。硬质合金ＹＧ６工具衬底沉积金刚石薄膜工艺参数范围较窄，采用阶段沉积法可以得到质量良好，附着力强的金刚石薄膜。