Florian Scherm

Laser welding of Mg alloy MgAl3Zn1 (AZ31) to Al alloy AlMg3 (AA5754) using ZnAl filler material

Abstract Welding of magnesium with aluminium alloys is challenging due to the formation of brittle intermetallic phases. In this study laser welding of an Mg alloy with an Al alloy, using a Nd∶YAG laser with a two focus optics and ZnAl filler material, was investigated. Mechanical tests, as well as microstructure investigations by light optical microscopy, scanning electron microscopy and energy dispersive X-ray microanalysis were carried out in order to characterise the hybrid joint. The strength of the joints is significantly affected by the aluminium content in the filler wire. Strengths of the joint were comparable to those obtained by other joining techniques such as diffusion or adhesive bonding using welding speeds of 1·75 m min−1.

Development of Novel Functionally Graded Al2O3-Lanthanum Hexaaluminate Ceramics for Thermal Barrier Coatings

Lanthanum hexaaluminate (LHA) has superior thermo-chemical stability at temperatures higher than 1000 °C and is a promising competitor to Y-ZrO2-based thermal barrier coatings (TBCs). The yet unresolved problem is control of microstructure of high LHA content ceramics and adjustment of porosity upon, to arrive at a material exhibiting low thermal conductivity at high temperature combined with structural reliability. Therefore, a functionally graded alumina/lanthanum hexaaluminate (FGLHA) with a gradient in composition was developed. The thermal diffusivity and thermal conductivity of FGLHA were compared with the one of the monolithic composite ceramics. The alumina-rich composites showed excellent mechanical properties whereas the LHA-rich composites presented lower thermal conductivity.

Direct metal deposition of abrasive tracks—Potentials concerning geometry and materials

Diamond tools are essential in various industries. The high hardness of diamond enables the machining of a wide range of different materials. The materials processed with diamond cutting tools vary from ceramic or metal components to natural materials like stone. In most cases, the abrasive diamonds of such cutting tools are embedded in a metal matrix. Depending on the application, this metal matrix has to fulfill certain requirements of wear resistance in order to achieve a sufficient tool life in combination with optimal cutting results. Laser cladding offers the opportunity to produce metal bond diamond tools with different matrix materials. In this process, the matrix is applied as metal powder. Together with the diamond particles, the metal powder is blown into the focus of the laser beam. By moving the laser over the workpiece, an abrasive line trace, consisting of diamonds embedded in a metal matrix, is built. In this way, arbitrary geometries (only limited by the handling system) can be generated. For example, dot patterns or spiral tracks can easily be processed. The use of an Yb-fiber laser with high beam quality (1.05 mm mrad) enables us to build up structures with dimensions less than 1 mm. In contrast to our process, the established ways of manufacturing (electroplating or brazing) require huge efforts to produce abrasive coatings with complex shapes. In order to rate the potential of this laser cladding method, especially for small structures, 1 mm and below, the ability and the influence of different matrix materials are of great interest. Therefore, this work examines the evaluation of three different matrix materials: Co-based matrix, Fe-based matrix, and Ti-based matrix. The abrasive clads were analyzed by light optical microscopy and scanning electron microscopy (SEM); in some cases, preparation took place by focused ion beam etching. Moreover, the diamonds were chemically leached out of the metal matrix and analyzed by SEM and Raman microscopy in order to understand the interfacial reactions between the diamond and the matrix melt. The diamond strength, after the laser cladding process, was measured by mechanical tests.Diamond tools are essential in various industries. The high hardness of diamond enables the machining of a wide range of different materials. The materials processed with diamond cutting tools vary from ceramic or metal components to natural materials like stone. In most cases, the abrasive diamonds of such cutting tools are embedded in a metal matrix. Depending on the application, this metal matrix has to fulfill certain requirements of wear resistance in order to achieve a sufficient tool life in combination with optimal cutting results. Laser cladding offers the opportunity to produce metal bond diamond tools with different matrix materials. In this process, the matrix is applied as metal powder. Together with the diamond particles, the metal powder is blown into the focus of the laser beam. By moving the laser over the workpiece, an abrasive line tra...

An Enhanced Three-Step Oxidation Process to Improve Oxide Adhesion on Zirconium Alloys

Oxidation of zirconium-based alloys results in a thermally-grown oxide scale with excellent corrosion resistance, and good wear and friction properties, which make them interesting for tribological applications. Nevertheless, adhesion of the oxide layer to the substrate must be enhanced. A new three-step oxidation process was introduced in order to achieve an improvement. Following an initial oxidation step (1st step), a heat treatment was carried out in vacuum during which the oxide dissolves and diffuses into the metallic zirconium substrate (2nd step). These two steps resulted in an oxygen dissolution layer with increased hardness formed in-between the oxide and the substrate, which serves as a bonding layer with increased thickness. In a 3rd step a new oxide layer was obtained. The improved oxide layer adhesion was characterized by indentation tests on three different groups of oxidized samples of the newly developed alloy.

Laser Cladding of Ultra-Thin Nickel-Based Superalloy Sheets

Laser cladding is a well-established process to apply coatings on metals. However, on substrates considerably thinner than 1 mm it is only rarely described in the literature. In this work 200 µm thin sheets of nickel-based superalloy 718 are coated with a powder of a cobalt-based alloy, Co–28Cr–9W–1.5Si, by laser cladding. The process window is very narrow, therefore, a precisely controlled Yb fiber laser was used. To minimize the input of energy into the substrate, lines were deposited by setting single overlapping points. In a design of experiments (DoE) study, the process parameters of laser power, laser spot area, step size, exposure time, and solidification time were varied and optimized by examining the clad width, weld penetration, and alloying depth. The microstructure of the samples was investigated by optical microscope (OM) and scanning electron microscopy (SEM), combined with electron backscatter diffraction (EBSD) and energy dispersive X-ray spectroscopy (EDX). Similarly to laser cladding of thicker substrates, the laser power shows the highest influence on the resulting clad. With a higher laser power, the clad width and alloying depth increase, and with a larger laser spot area the weld penetration decreases. If the process parameters are controlled precisely, laser cladding of such thin sheets is manageable.