Fraunhofer USA Center for Laser Applications

Metal matrix composites (MMCs) have many industry applications in different sectors, e.g.: automobile and aerospace. However, due to hard ceramic reinforcing components in MMCs, difficulties can arise when machining via conventional manufacturing processes. Heavy tool wear is especially problematic. The Laser Assisted Machining (LAM) method could potentially improve the processing of these materials. In laser assisted cutting, the workpiece area is heated directly by a laser beam before the cutting edge. The work reported here centres on improving Al/SiC composites machinability by laser assisted machining when compared with conventional turning process. Influence of laser during laser assisted turning on cutting force, tool wear and machined surfaces roughness was investigated. This research was carry out for polycrystalline diamond inserts (PCD) and sintered carbide inserts with coated and uncoated. The results obtained with the laser assisted machining were compared with results obtain in conventional turning.

The laser cladding process has achieved a wide level of acceptance in recent years from various sectors of industry due to its minimal heat input into the part and its inherent ability to achieve low dilution rates. However, the deposition rates of the laser cladding process are typically lower when compared to competitive cladding technologies. This research paper evaluates Induction Assisted Laser Cladding using Fraunhofer’s new COAXpowerline nozzle system and documents the improvements in clad travel speeds and powder feed rates which can be achieved using Inconel 625 and 60-40 Tungsten Carbide-Nickel base powder.

Laser forming offers the industrial promise of controlled shaping of metallic and non-metallic components for prototyping, the correction of design shape or distortion and precision adjustment applications. The potential process advantages include precise incremental adjustment, flexibility of application and no mechanical ‘spring-back’ effect. To date there has been a considerable amount of work carried out on two-dimensional laser forming, using multi-pass straight line scan strategies to produce a reasonably controlled bend angle in a number of materials, including aerospace alloys. A key area, however, where there is a limited understanding, is the variation in bend angle per pass during multi-pass laser forming along a single irradiation track, in particular the decrease in bend angle per pass after many irradiations for a given set of process parameters. Understanding of this is essential if the process is to be fully controlled for a manufacturing environment. The research presented in this paper through empirical data offers a coherent picture of the key influencing factors and at which point in the bend evolution each is active.

X ray collimator optics for space application require an array of high aspect ratio holes of 60:1 with a minimal tantalum (Ta) thickness of ≥2 mm and a very high open area fraction (hole versus wall fraction) of 70% to achieve high collimator efficiency. Each collimator with a drilled area of 110 mm × 70 mm contains several million holes and need a fast drilling process. Laser percussion drilling was performed using an IR pulsed disk laser in a 1 and 2 mm thick Ta plate. A tightly spaced hexagonal closed packed pattern was used to maximize open area fraction with hole-to-hole spacing as small as 80 μm. However, this resulted in a high concentration of debris and a thick recast layer on the remaining walls between the holes. Different process gases were investigated to minimize debris formation and reduce the recast layer thickness. Ramping of pulse energy during the drill cycle was investigated to minimize the adhesion between the substrate and recast layer. Chemical etching was used to remove the debris and recast from the top surface and the inside of the laser-drilled holes. Hole cross sections showed that a high aspect ratio was achieved with a hole diameter of about Ø50 μm in 2 mm thick Ta. To achieve the shortest drilling time of 200 ms per hole, the process parameters were optimized and a hybrid nozzle, with both horizontal and vertical gas flow, was developed and implemented.

Precise positioning of the laser beam on the work piece is crucial for high quality laser welds; e.g. for butt welding the focal point of the laser beam with respect to the joint must be maintained within an accuracy better than 20µm - 150µm, depending on the focused beam radius. These stringent accuracy requirements call for high precision robots, a repeatable work piece profile and precise clamping. To compensate for insufficient repeatability of work piece or clamping, seam-tracking devices are used. A sensor measures the joint position and computes a correction vector to follow the actual joint trajectory. The deviation is compensated either by robot trajectory adjustment or by an additional tracking axis. Disadvantages of this approach are complex installation of the devices due to interfacing with the robot control, the need of teaching and calibrating the sensor and principle based accuracy restriction that limit the usability in more complex 2d contours and with low accuracy robots. We recently introduced a more flexible and precise approach that utilizes an advanced camera-based sensor that is capable of measuring seam position, relative displacement between work piece and sensor and melt pool of the process with one single device. This paper describes a realized ‘self guided’ welding head, which uses this approach in combination with an integrated high power scanner. The result is a welding head that follows a curved or linear butt weld with high precision and independent of the actual robot trajectory; without the need of calibration, robot interfacing and alignment.

Numerous research centres all over the world are involved in the studying laser surface modification. By controlling laser parameters such as the amount of power, scanning speed, and pulse duration, it is possible to form coatings with different surface properties. Theses include surface geometry, microhardness, stress states or resistance to wear. One of the cheapest and common use techniques is electro spark alloying (ESA) or electro spark deposition (ESD), which, like laser treatment, requires a focused stream of energy. Laser surface texturing technology has significant advantages as sliding friction decrease. It gives direct wear improvement for tribilogical pairs seen in many industry applications. The work reported here centres on empirical Cu-Mo coatings studies generated by using Nd: YAG laser system. The coatings were first deposited as an ELFA-541 on C45 carbon steel and than laser eroded within various process parameters. The analysis included macrogeometry and microhardness measurement of selected areas after laser treatment.

Electro spark alloying (ESA) technique is a well established method. However, new lasers are becoming available on the market that can make this technique more attractive for various industry applications, e.g.: surface texturing technology, which has significant advantages for the wear decreasing into sliding friction. The work reported here concentrates on electro-spark coatings properties under laser treatment. The quality of coatings was proved via microstructure analysis, microgeometry measurement, hardness and tribological tests. These tests were carried out on WC-Co coating (the anode) obtained by electro spark deposition over carbon steel 45 (the cathode) and molten via laser beam (Nd:YAG). It was noticed that electro-spark deposited WC-Co coatings after laser treatment are characterized by lower microhardness, friction force and higher seizure resistance. Also, the laser treatment post-process produced homogeneous chemical composition, refinement of structure, healed microcracks and pores of the electro-spark deposited coatings. Direct application of WC-Co electro-spark coatings modified via laser treatment can be used in sliding friction pairs. The wear resistance protection should significant extend the final product durability, which will be investigated in future tests.

2-Dimensional laser forming can currently control bend angle, with reasonably accurate results, in various materials including aerospace alloys. However, this is a different situation for 3-Dimensional laser forming. To advance this process further for realistic forming applications and for straightening and aligning operations in a manufacturing industry it is necessary to consider larger scale controlled 3D laser forming. The work presented in this paper uses a predictive and adaptive approach to control the laser forming of mild steel and aluminium sheet into a desired surface. Key to the control of the process was the development of a predictive model, detailed in the paper, to give scan strategies based on a required geometry and the surface error. The forming rate and distribution of the magnitude of forming across the surface were controlled in the closed loop by the process speed. When the geometry is not formed within one pass, an incremental adaptive approach is used for subsequent passes, utilising the error between the current and desired geometry to give a new scan strategy, thus any unwanted distortion due to material variability can be accounted for and distortion control and removal is possible.

Laser color marking on metal surface is a one step process which has high potential for product marking and decoration. Lasers are used for creating permanent colors without using any coatings or chemicals. Product differentiation and customization is achieved with programmable multi-color designs where design can be changed easily in marking software. Highly intricate design patterns can be generated using this technology. The principle of generating colors using lasers is formation of thin oxide layers by heating metal surface uniformly and reflection of white light from different thickness oxide layers which is perceived as different colors by the viewer. Although the laser color marking process is simple and flexible, one challenge that hinders its wider adoption is way to monitor generated color for consistency over marked area and repeatability from one surface to the another. This paper discusses laser color marking using pulsed IR disc laser. Colors across a broad spectrum has been successfully generated such as violet, blue, green, red, orange, brown, metallic colors such as gold and silver and shades of gray. An integrated camera based color monitoring system has been developed that can provide information on laser marked color consistency and repeatability using image processing techniques and its results will be presented.

The Direct Laser Deposition (DLD) method is a relatively new technology which has significantly matured in the last few years. Although this technique has been well known from the mid ’80s, process control is still under investigation. The major application of DLD is in the repair or modification of existing parts or the manufacture of completely new 2-D or 3-D parts. This technology is also commonly used for the addition of special surface layers on a base material, e.g. hard surface wear resistance like Tungsten Carbide (W-C) or diamond cladding layers. These special wear resistance layers can be applied on the top of drill bits or used as a ‘guard layer’ for working machines and various automotive parts, e.g. excavator bucket tips. The real-time spectrographic analysis of the melt pool composition is one from the main factors to be improved. So far compositional monitoring in-situ is not in practical use and still requires development. It was noticed that metal vapor phase occurred in DLD and this can give significant information about the composition created in the melt pool. It is also an indication of the range of temperatures involved in the process. The work reported here centres on studies performed with various laser cladding nozzle configurations and investigates the feasibility of applying real-time spectroscopy and fast digital imaging to the DLD process. Also several metal compositions have already been analyzed in-situ. This research was carried out with conventional Nd: YAG, solid state, CW laser system.

Steel plates used in shipbuilding are generally protected from corrosion by painting. However, when the plates are welded or cut, any paint near the process is vaporized, and has to be reapplied. In this study, a technique was developed that combines applying a protective corrosion resistant layer via laser cladding together with painting in a single process. By doing this, it is only necessary to go back over a weld with one pass to provide enhanced corrosion protection.

Micro-joining and hermetic sealing of dissimilar and biocompatible materials is a critical issue for a broad spectrum of products such as micro-electronical, micro-optical and biomedical products and devices. Novel implantable microsystems currently under development will include functions such as localized sensing of temperature and pressure, electrical stimulation of neural tissue and the delivery of drugs. These devices are designed to be long-term implants that are remotely powered and controlled. The development of new, biocompatible materials and manufacturing processes that ensure long-lasting functionality and reliability are critical challenges. Important factors in the assembly of such systems are the small size of the features, the heat sensitivity of integrated electronics and media, the precision alignment required to hold small tolerances, and the type of materials and material combinations to be hermetically sealed. Laser micromachining has emerged as a compelling solution to address these manufacturing challenges. This paper will describe the latest achievements in microjoining of non-metallic materials. The focus is on glass, metal and polymers that have been joined using CO2, Nd:YAG and diode lasers. Results in joining similar and dissimilar materials in different joint configurations are presented, as well as requirements for sample preparation and fixturing. The potential for applications in the biomedical sector will be demonstrated.

High-speed laser scanning systems have been known from laser marking applications for several years. The beam deflection is done by small galvo-driven scanning mirrors, which were capable of transmitting several hundred watts of laser power. Innovative developments have resulted in scanning systems capable of transmitting several thousand watts of CO2 and Nd:YAG laser power. In conjunction with new processing optics and intelligent sensor systems, this technology opens up new perspectives in high precision, high-speed laser welding of complex shaped and small parts. The paper will introduce a welding application in which an integrated optical sensor provides exact positioning of the laser beam prior to welding. The tilting motion of the mirrors guides the laser beam rapidly and precisely on a well-defined path on the part contour with both the welding head and the part fixed. Another example will describe a hardening application. Here the scanning head is utilized to optimize the shape, size and intensity distribution of the laser spot on a tool surface. A pyrometer based process control is used to monitor the surface temperature and change the process parameters accordingly.

High rate laser pitting technique for solar cell texturing Efficiency of crystalline silicon solar cells can be improved by creating a texture on the surface to increase optical absorption. Different techniques have been developed for texturing, with the current state-of-the-art (SOA) being wet chemical etching. The process has poor optical performance, produces surfaces that are difficult to passivate or contact and is relatively expensive due to the use of hazardous chemicals. This project shall develop an alternative process for texturing mc-Si using laser micromachining. It will have the following features compared to the current SOA texturing process: -Superior optical surfaces for reduced front-surface reflection and enhanced optical absorption in thin mc-Si substrates -Improved surface passivation -More easily integrated into advanced back-contact cell concepts -Reduced use of hazardous chemicals and waste treatment -Similar or lower cost The process is based on laser pitting. The objective is to develop and demonstrate a high rate laser pitting process which will exceed the rate of former laser texturing processes by a factor of ten. The laser and scanning technologies will be demonstrated on a laboratory scale, but will use inherently technologies that can easily be scaled to production rates. The drastic increase in process velocity is required for the process to be implemented as an in-line process in PV manufacturing. The project includes laser process development, development of advanced optical systems for beam manipulation and cell reflectivity and efficiency testing. An improvement of over 0.5% absolute in efficiency is anticipated after laser-based texturing. The surface textures will be characterized optically, and solar cells will be fabricated with the new laser texturing to ensure that the new process is compatible with high-efficiency cell processing. The result will be demonstration of a prototype process that is suitable for scale-up to a production tool and process. The developed technique will have an reducing impact on product pricing. As efficiency has a substantial impact on the economics of solar cell production due to the high material cost content; in essence, improved efficiency through cost-effective texturing reduces the material cost component since the product is priced in terms of $/W. The project is a collaboration between Fraunhofer USA, Inc. and a c-Si PV manufacturer.