single track deposition study of biodegradable mg rare earth alloy by micro laser metal wire deposition

Abstract The high reactivity of the material is one of the key issues in powder-based additive manufacturing of biodegradable Mg-alloys. In fact, the powder feedstock is highly inflammable and difficult to manage in laser powder-bed fusion and metal deposition. On the other hand, wire feedstock can provide an intrinsically safer solution for the purpose. However, the process should be developed for maintaining geometrical precision and deposition quality. In this work, micro laser metal wire deposition (μLMWD) of a biodegradable Mg-alloy with Dy as the main alloying element (Resoloy) is demonstrated. A flash-pumped Nd:YAG laser was used along with a custom-built wire feeder. Single-tracks were deposited and analysed for their geometrical attributes, microhardness, and deposition efficiency. The results were evaluated to determine the processability of the heat sensitive Mg-alloy with respect to an austenitic stainless steel with known processability. Despite a narrow processability window, single-track deposits were successfully produced. Micrometric feature resolution was maintained with crack and pore free deposits

Remote cutting of Li-ion battery electrodes with infrared and green ns-pulsed fibre lasers

"Thin sheet anode and cathode materials made in composite structures constitute some of the most important components of a Li-ion battery. These materials are currently cut by punching technology, which shows degrading behaviour as the tool wears out. A viable option for Li-ion battery electrode manufacturing is the use of remote laser cutting. However, the operation requires fulfilling both productivity and quality aspects to substitute the conventional production method. One of the most critical aspects in quality is the clearance width, which is defined as the extent of the exposed middle layer of the sandwich at the laser cut kerf. This work investigates the quality aspects of laser cutting of Li-ion electrodes when a green fibre laser source (λ=532 nm, τ= 1 ns) is used rather than the more traditional infrared (IR) fibre laser source (λ=1,064 nm, τ=250 ns). The processing conditions were investigated to reveal the technological feasibility zones. Clearance width was studied within the technological feasibility zones for all the material-laser combinations. Results showed that high productivity criterion is met by the IR system, since cutting speed could reach 30 m/min with 54Waverage laser power on both anode and cathode. On the other hand, the green laser provided clearance width below 20 μm. In the best case, the clearance on anode could be eliminated with the green laser system. Although the maximum cutting speed was 4.5 m/min, upscaling of green laser power can provide required productivity.

Microcutting of multi-layer foils with IR and green ns-pulsed fibre lasers for Li-Ion batteries

open2noLi-Ion batteries are crucial components in mobile devices that range from cellular phones to electrical vehicles. With the increasing demand in the market for these devices, manufacturers are required to reduce production cycle times. The main components of the Li-Ion batteries are anode and cathode foils, which are cut in required forms by punching. These materials consist of Cu sheets sandwiched between graphite layers for anode, and Al sheets sandwiched between Li metal oxide layers for cathode. In punching, the process quality degrades in time due to tool wear. This eventually causes machine down times for tool repair or change, which can increase the whole process cycle time drastically. Laser remote cutting based on ablation can be adequate solution to substitute the current technology, if the cutting edge quality and productivity can be matched to punching. This paper investigates laser microcutting of Li-Ion battery anode and cathode thin foils with ns-pulsed fibre lasers. These laser sources are cost effective and provide industrially robust operation. Two systems operating with 1 μm and 0.5 μm wavelength and 250 ns and 1 ns pulse durations respectively were compared. The cut kerfs were evaluated in terms of clearance, which is defined as the extent of the exposed middle layer of the sandwich (i.e. Cu or Al) at the laser cut kerf.Demir, Ali Gökhan; Previtali, BarbaraDemir, ALI GOKHAN; Previtali, Barbar

Probing multipulse laser ablation by means of self-mixing interferometry

In this work, self-mixing interferometry (SMI) is implemented inline to a laser microdrilling system to monitor the machining process by probing the ablation-induced plume. An analytical model based on the Sedov-Taylor blast wave equation is developed for the expansion of the process plume under multiple-pulse laser percussion drilling conditions. Signals were acquired during laser microdrilling of blind holes on stainless steel, copper alloy, pure titanium, and titanium nitride ceramic coating. The maximum optical path difference was measured from the signals to estimate the refractive index changes. An amplitude coefficient was derived by fitting the analytical model to the measured optical path differences. The morphology of the drilled holes was investigated in terms of maximum hole depth and dross height. The results indicate that the SMI signal rises when the ablation process is dominated by vaporization, changing the refractive index of the processing zone significantly. Such ablation conditions correspond to limited formation of dross. The results imply that SMI can be used as a nonintrusive tool in laser micromachining applications for monitoring the process quality in an indirect way

Comparative study of CW, nanosecond- and femtosecond-pulsed laser microcutting of AZ31 magnesium alloy stents

Magnesium alloys constitute an interesting solution for cardiovascular stents due to their biocompatibility and biodegradability in human body. Laser microcutting is the industrially accepted method for stent manufacturing. However, the laser-material interaction should be well investigated to control the quality characteristics of the microcutting process that concern the surface roughness, chemical composition, and microstructure of the final device. Despite the recent developments in industrial laser systems, a universal laser source that can be manipulated flexibly in terms of process parameters is far from reality. Therefore, comparative studies are required to demonstrate processing capabilities. In particular, the laser pulse duration is a key factor determining the processing regime. This work approaches the laser microcutting of AZ31 Mg alloy from the perspective of a comparative study to evaluate the machining capabilities in continuous wave (CW), ns- and fs-pulsed regimes. Three industrial grade machining systems were compared to reach a benchmark in machining quality, productivity, and ease of postprocessing. The results confirmed that moving toward the ultrashort pulse domain the machining quality increases, but the need for postprocessing remains. The real advantage of ultrashort pulsed machining was the ease in postprocessing and maintaining geometrical integrity of the stent mesh after chemical etching. Resultantly, the overall production cycle time was shortest for fs-pulsed laser system, despite the fact that CW laser system provided highest cutting speed

Fibre Laser Cutting and Chemical Etching of AZ31 for Manufacturing Biodegradable Stents

The use of magnesium-alloy stents shows promise as a less intrusive solution for the treatment of cardiovascular pathologies as a result of the high biocompatibility of the material and its intrinsic dissolution in body fluids. However, in addition to requiring innovative solutions in material choice and design, these stents also require a greater understanding of the manufacturing process to achieve the desired quality with improved productivity. The present study demonstrates the manufacturing steps for the realisation of biodegradable stents in AZ31 magnesium alloy. These steps include laser microcutting with a Q-switched fibre laser for the generation of the stent mesh and subsequent chemical etching for the cleaning of kerf and surface finish. Specifically, for the laser microcutting step, inert and reactive gas cutting conditions were compared. The effect of chemical etching on the reduction in material thickness, as well as on spatter removal, was also evaluated. Prototype stents were produced, and the material composition and surface quality were characterised. The potentialities of combining nanosecond laser microcutting and chemical etching are shown and discussed

Laser micromachining of TiN coatings with variable pulse durations and shapes in ns regime

"The micro-structuring of thin surface coatings has become increasingly popular following the discovery of improved performance, especially in terms of the resulting tribological properties. The directwriting of microstructures via laser ablation offers flexibility, extending the applicability ofmicro-structuring to variousmaterials and machined geometries. However, the laser ablation of coatings requires better comprehension to provide sufficientmachining quality with improved productivity to render such processes more viable for industrial applications. This paper presents the processing conditions for the ablation of approximately 4-μm-thick coatings of TiN in the ns pulse regime, which is generally characterised by higher productivity with low machining quality. A range of pulse durations between 12 ns and 200 ns was employed. The effect of pulse duration on ablation threshold fluence and irradiance was investigated. The pulse shape was decomposed into peak and tail regions to investigate their respective effects on the ablation process. The opportune regulation of pulse properties allowed for themaintenance of high productivity and high-quality laser micromachining under delicate processing conditions, in the case of ceramic TiN surface coatings with limited thickness.

Use of Sheet Material for Rapid Prototyping of Cardiovascular Stents

Manufacturing of cardiovascular stents most commonly involve the use of tubular precursors and laser microcutting of the stent mesh, followed by chemical and electrochemical surface treatments. For mass manufacturing purposes, this production route is well-established, while for small batch or prototype production it proves to be cumbersome. Especially concerning newly developed alloys based, the production of microtubes is time consuming and highly costly. On the other hand, production of these new alloys in sheet metal form is a simpler approach, since the process uses non-dedicated tools and is easier as opposed to extrusion and tube drawing. Accordingly, in this work, the use of sheet material as precursor for rapid prototyping of cardiovascular stents is proposed. In particular, a ns-pulsed fiber laser is used for cutting permanent AISI 316L. Laser microcutting conditions are investigated in terms of generated spatter and kerf geometry. Chemical etching is employed to clean the dross generated around the cut kerf. A novel stent geometry allowing for transforming the sheet material to a tubular form is employed to produce prototype stents

Interplay between powder catchment efficiency and layer height in self-stabilized laser metal deposition

In laser metal deposition (LMD) the height of the deposited track can vary within and between layers, causing significant deviations during the process evolution. Previous works have shown that in certain conditions a self-stabilizing mechanism occurs, maintaining a regular height growth and a constant standoff distance between the part and the deposition nozzle. Here we analyze the link between the powder catchment efficiency and the deposition height stability. To this purpose, a monitoring system was developed to study the deposition in different process conditions, using inline measurements of the specimen weight in combination with the layer height information obtained with coaxial optical triangulation. An analytical model was used to estimate the deposition efficiency in real-time from the height monitoring and the process parameters, which was verified by the direct mass measurements. The results show that the track height stabilization is associated to a reduction of the powder catchment efficiency, which is governed by the melt pool relative position with respect to the powder cone and the laser beam. For a given set of parameters, the standoff distance can be estimated to achieve the highest powder catchment efficiency and a regular height through the build direction

Laser surface structuring of AZ31 Mg alloy for controlled wettability

Structured surfaces exhibit functional properties that can enhance the performance of a bioimplant in terms of biocompatibility, adhesion, or corrosion behavior. In order to tailor the surface property, chemical and physical methods can be used in a sequence of many steps. On the other hand, laser surface processing can provide a single step solution to achieve the designated surface function with the use of simpler equipment and high repeatability. This work provides the details on the surface structuring of AZ31, a biocompatible and biodegradable Mg alloy, by a single-step laser surface structuring based on remelting. The surfaces are characterized in terms of topography, chemistry, and physical integrity, as well as the effective change in the surface wetting behavior is demonstrated. The results imply a great potential in local or complete surface structuring of medical implants for functionalization by the flexible positioning of the laser beam