Journal of Manufacturing and Materials Processing Article Laser Surface Structuring of Cemented Carbide for improving the Strength of Induction Brazed Joints Ammar Ahsan * , Igor Kryukov and Stefan Böhm Fachgebiet Trennende und Fügende Fertigungsverfahren (tff), Universität Kassel, Kurt-Wolters-Str. 3, 34125 Kassel, Germany; [email protected] (I.K.); [email protected] (S.B.) * Correspondence: [email protected]; Tel.: +49-561-804-7702 Received: 1 April 2019; Accepted: 29 May 2019; Published: 3 June 2019 Abstract: The effect of micro patterning of cemented carbide surface using nanosecond diode pumped solid-state pulsed laser on the strength of induction brazed carbide and steel joints has been investigated. Surface patterns increase the total surface area of the joint and, for an originally hydrophilic surface, increase the wettability of a liquid on a solid surface such that, instead of building droplets, the liquid spreads and flows on the surface. Microcomputed tomography (µ-CT) was used to observe the filler/carbide interface after brazing and to analyze the presence of porosity or remnant flux in the joint. Microstructures of the brazed joints with various surface patterns were analyzed using scanning electron microscopy. The strength of the joints was measured using shear tests. Results have shown that the groove pattern on the surface of carbide increases the joint strength by 70–80%, whereas, surface patterns of bi-directional grooves (grid) reduced the joint strength drastically. Dimples on the carbide surface did not show any improvement in the strength of the brazed joints compared to samples with no surface pattern. Keywords: brazing; carbide; surface structuring; laser ablation 1. Introduction Geometrically defined polycrystalline diamond (PCD) saw tips with a backing layer of cemented carbide (WC-Co) represent an alternative to sintered diamond blades with geometrically undefined cutting edges for sawing of natural stones like granite, marble etc. [1–3]. It has been shown that using a high negative rake angle on the saw tips, which is advantageous for reduction of tool wear, results in failure in the brazed joints of the saw tips [1]. The saw tips used particularly on the band saws, because of their small size, cannot be screw clamped to the tool body as in the case of a typical indexed milling cutter. Brazing is the most common process of joining carbide or carbide backed PCD tips to the body of the saw blades. Most of the sawing tools are still brazed in an air atmosphere with an oxy-acetylene flame or an induction coil. However, the strength of the brazed joints varies widely because of various factors such as filler metal composition [4], gap thickness [5], brazing time and temperature [6], surface roughness [7], surface chemical composition [8–10], cleanliness of the surface, incomplete removal of oxides and inclusions of flux, presence of porosity, and thermal cracks [11]. Tungsten carbide, especially the grades with high hardness and low binder content, shows poor wettability by most brazing alloy standards, especially when brazing in air [12,13]. The filler metal forms a metallic bond with the metal (binder) present on the surface and for carbide grades containing low binder content, the surface contains very little binder for the filler metal to form a bond with. The state-of-the-art solution is the galvanic coating of the carbide surface with a metal usually cobalt or nickel (metalizing). However, due to the health and safety issues the electrolytes used in the process are classified as hazardous substances by the European Union (EU) and their production and application will be gradually limited [14–16]. A novel approach for enhancing the wettability of the surfaces is J. Manuf. Mater. Process. 2019, 3, 44; doi:10.3390/jmmp3020044 www.mdpi.com/journal/jmmp J. Manuf. Mater. Process. 2019, 3, x FOR PEER REVIEW 2 of 11 J. Manuf. Mater. Process. 2019, 3, 44 2 of 11 production and application will be gradually limited [14–16]. 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However, of brazed this joint. technique However, has this not technique been tried has on tungstennot been tried carbide on/ steeltungsten brazed carbide/steel joints and specificallybrazed joints air and atmosphere specifically brazed air joints.atmosphere This work brazed is intended joints. This to enhance work is the intended strength ofto theenhance uncoated the tungstenstrength of carbide the uncoated/steel brazed tungsten joints carbide/steel by structuring brazed the surface joints by of thestructuring carbide usingthe surface a short-pulsed of the carbide laser. Furthermore,using a short‐ computerpulsed laser. tomography Furthermore, is used computer as a non-destructive tomography testing is used approach as a non to‐destructive assess the qualitytesting ofapproach the brazed to assess joints. the quality of the brazed joints. Pulsed Laser Ablation Laser ablation refers to the material removal by using short high-intensityhigh‐intensity laser pulses as the heating source. Pulsed Pulsed lasers lasers produce produce shorter shorter bursts bursts of of energy, energy, which result in much higher peak energy levels than a continuouscontinuous wave (CW) laser source [[21].21]. Pulse durations may be in micro-micro‐ or nanoseconds (short pulses) or in pico-pico‐ and femtoseconds (i.e.,(i.e., ultra-shortultra‐short pulses).pulses). For pulse durations above 10 picoseconds (ps), the ablation process consists of heat conduction, melting, evaporation and and plasma plasma formation formation as as shown shown in in Figure Figure 11 [22].[ 22 ].Pulse Pulse durations durations of ofless less than than 10 10ps ps(ultrashort (ultrashort pulses) pulses) are areshorter shorter than than the time the time required required for electrons, for electrons, which which absorb absorb the energy the energy of the ofphotons, the photons, to transfer to transfer their energy their energy to the tolattice. the lattice. Therefore, Therefore, ablation ablation processes processes with ultrashort with ultrashort pulses pulsesinvolve involve no melting no melting and virtually and virtually no heat no heataffected affected zone zone [23]. [ 23Although]. Although ultrashort ultrashort laser laser ablation ablation is isextremely extremely precise, precise, the the ablation ablation rate rate is is significantly significantly lower lower than than with with laser laser ablation ablation in in a nanosecond regime, which allows thethe highest ablationablation eefficiencyfficiency [[22].22]. Figure 1. Pulsed laser beam and matter interaction ( a) short pulse laser ( μµs, ns)—matter interaction; (b) ultrashort pulse (ps,(ps, fs)fs) beam-matterbeam‐matter interactioninteraction [[22].22]. 2. Materials and Methods 2. Materials and Methods For this work, tungsten carbide (WC-8%Co) type K-20F (ISO K20-K30) with a grain size of 0.7 µm For this work, tungsten carbide (WC‐8%Co) type K‐20F (ISO K20‐K30) with a grain size of from HHT-Hartmetall, Germany, was used in a cylindrical form with a 9 mm diameter and 10 mm 0.7 μm from HHT‐Hartmetall, Germany, was used in a cylindrical form with a 9 mm diameter and height. These were brazed on to 20 mm diameter and 15 mm high cylinders made of S235JR steel. 10 mm height. These were brazed on to 20 mm diameter and 15 mm high cylinders made of S235JR Both carbide and steel parts were sand blasted using 10 µm corundum particles and then cleaned in steel. Both carbide and steel parts were sand blasted using 10 μm corundum particles and then an ultrasonic bath of ethanol. Sand blasting was necessary as the carbide surface in an ‘as-delivered’ cleaned in an ultrasonic bath of ethanol. Sand blasting was necessary as the carbide surface in an condition was oxidized and had a pale-yellow tint most likely due to the presence of tungsten oxide ‘as‐delivered’ condition was oxidized and had a pale‐yellow tint most likely due to the presence of on the surface. The roughness of the blank surface after sand blasting and ultrasonic cleaning was tungsten oxide on the surface. The roughness of the blank surface after sand blasting and ultrasonic measured optically using white light interferometry and the roughness parameters were found to be J.