Milan Brandt

Abrasive wear performance and microstructure of laser clad WC/Ni layers

Abstract WC/Ni clad layers were produced on H13 tool steel substrates with a pulsed Nd:YAG laser and optical fibres using the injected powder technique. The effects of parameters, such as laser beam profile, WC powder shape and substrate pre-heat temperature on the clad layer microstructure and wear properties were investigated. The microhardness of the clad layers was measured using a Vickers microhardness tester. The microstructure of the clad layers was assessed by optical and scanning electron microscopy. The results show that up to 1xa0mm thick, fully dense and crack-free of WC/Ni clad layers can be formed on H13 substrates. The results further show that the higher is the microhardness of the clad layers, the higher is its wear resistance. Average surface microhardness values of the clad layers were as high as 650xa0HV. WC particle shape influences the microstructure and wear resistance of clad layer with crushed particles producing more wear resistant surfaces. The clad layers’ abrasive wear resistance was a factor of 5–10 times higher than that of unclad H13 tool steel.

Tribological behaviour and microstructure of TiCxN(1−x) coatings deposited by filtered arc

Abstract A series of macroparticle-free TiN, TiC x N (1− x ) and TiC coatings were deposited on 316 austenitic stainless steel using a titanium target in a filtered arc deposition system (FADS) and reactive mixtures of N 2 and/or CH 4 gases. The surface topography, chemical composition and microstructure of these coatings were characterised by optical microscopy (OM), atomic force microscopy (AFM), X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). The microhardness has been measured and the adhesion of the coatings has been evaluated. Further, the wear and friction behaviour of the coatings were assessed under controlled test conditions in a pin-on-disc tribometer. The results show a significant increase in surface roughness, microhardness and wear resistance as the CH 4 :N 2 gas flow rate ratio is increased. The composition of the coatings was strongly dependent on reactive gas flow rate during deposition. Surface particles were observed on high carbon content coatings and subsequently determined to be carbonaceous particles by using OM, AFM and EDS. At lowest load (10xa0N), all coatings exhibited low friction and wear. At loads of 15 and 25xa0N, the higher carbon content TiCN and TiC coatings showed a much lower friction and wear compared to TiN and low carbon TiCN.

Experimental investigation of cutting forces and tool wear during laser-assisted milling of Ti-6Al-4V alloy

The improvement of machinability during laser-assisted milling of Ti-6Al-4V alloy was investigated. The effects of laser processing and milling parameters on cutting forces and tool wear have been examined. It is found that local heating and softening of the workpiece by the laser beam in front of the cutting tool significantly reduced the cutting forces, especially the force in the feed direction during up-cut milling. Laser power, tool–beam distance, depth of cut and cutting speed are the parameters influencing the change of feed force during laser-assisted milling. Analysis of the workpiece temperature rise due to laser beam heating shows that the feed force is strongly dependent on the workpiece temperature in front of the cutting zone; significant reduction of feed force occurred when the temperature in front of thecutting zone was in the range 200–450°C. Edge chipping is found to be the tool failure mode for both conventional milling and laser-assisted milling. A significant improvement in tool life during laser-assisted milling was obtained when the workpiece temperature in front of the cutting zone was at an optimum value. Compressed air was used to remove the chip from the cutting tool, which made the milling process more effective. The optimum workpiece temperature in front of the cutting zone with compressed air delivered through the spindle is about 350°C, higher than that with compressed air delivered through a stationary nozzle (about 230°C). The maximum tool life in the former case is much longer than that in the latter case.

Pre-placed WC/Ni clad layers produced with a pulsed Nd:YAG laser via optical fibres

Laser clad WC/Ni layers were produced on H13 tool steel substrates with a pulsed Nd:YAG laser and optical fibres using the pre-placed powder technique. The effects of parameter variation, such as laser pulse energy, beam profile, traverse speed and volume fraction of the reinforced WC particles, on clad layer formation and its properties were investigated. The microhardness of the clad layers was measured and the microstructure was characterised by optical and scanning electron microscopy and X-ray diffraction. The results show that relatively thick (>0.5 mm), fully dense and crack-free clad layers of WC/Ni can be formed on H13 substrates without any pre-heating. The results further show that the volume fraction of the reinforced WC particles is the dominant factor affecting most clad layer properties such as its porosity, microhardness and wear resistance. The greater the volume fraction of WC particles, the lower the porosity, the higher the microhardness and the higher the wear resistance of the clad layer. Average microhardness values of the matrix were as high as 800 HV and the pin-on-plate (reciprocating) wear tests showed the weight loss of the clad layers is substantially lower than that for the unclad substrate.

Laser cladding repair of turbine blades in power plants: from research to commercialisation

AbstractReliable and efficient power generation is a major global issue due to both political and environmental concerns. Nevertheless, many critical components, particularly the blades of the low pressure (LP) side of power generating steam turbines, are subject to failure due to severe erosion at the leading edges. Since taking machines offline for maintenance and removal of damaged blade for repair is extremely expensive, increasing the service life of these critical components offers significant economic and political benefits. Conventional techniques to increase service life include brazing of an erosion shield at the leading edge of the turbine blade, open arc hardfacing, and cladding with erosion resistant materials using gas tungsten, manual metal or plasma transferred arc welding. The authors have been investigating since 2001 the use of laser cladding technology to deposit a high quality and erosion resistant protection shield on the leading edge of LP blades. The project has demonstrated the fe...

Evaluation of duplex coatings produced with a pulsed Nd:YAG laser and filtered arc

Duplex coatings combining a physical vapour deposition (PVD) thin film and laser-clad WC/Ni layer were produced on an austenitic steel substrate. PVD TiN, TiCN and TiC thin films were deposited using a filtered arc deposition system. Laser-clad WC/Ni layers were produced using a pulsed Nd:YAG laser and optical fibres. Mechanical properties (microhardness and adhesion) of the coatings were determined using a Ultra-Micro Indentation System (UMIS-2000) and adhesion scratch tester, respectively. The wear resistance of the duplex coatings was evaluated by a pin-on-disc wear tester. The results showed that increasing the concentration of carbon in the PVD thin films increases its hardness, reduces its adhesion to the clad layer and increases its wear resistance. All coatings withstood the applied loads (25 and 40 N) and no sign of breakthrough was observed on any of them. This indicated that the duplex coatings exhibited a significant increase in load-bearing capacity, confirming the pivotal role of the laser-clad layer in supporting the applied load.

Effect of laser beam on machining of titanium alloys

Laser assisted machining of titanium alloys has been investigated in comparison with conventional machining. The effects of laser beam on the cutting forces, chip formation, machined surface and tool vibration have been examined.It is found that the cutting forces were reduced with the assistance of laser beam. The reduction of cutting forces was primarily dependent on the cutting speed, tool-beam distance and laser lens-workpiece distance. With increasing cutting speed, the chip formation changed from the sharp segmented chip through continuous chip to segmented chip which is the typical chip formation in conventional machining of titanium alloys.Laser assisted machining of titanium alloys has been investigated in comparison with conventional machining. The effects of laser beam on the cutting forces, chip formation, machined surface and tool vibration have been examined.It is found that the cutting forces were reduced with the assistance of laser beam. The reduction of cutting forces was primarily dependent on the cutting speed, tool-beam distance and laser lens-workpiece distance. With increasing cutting speed, the chip formation changed from the sharp segmented chip through continuous chip to segmented chip which is the typical chip formation in conventional machining of titanium alloys.

Microstructure of Laser Treated ZE41A-T5 Magnesium Alloy

Modification of the microstructure of ZE41A-T5 magnesium alloy substrates was investigated by laser surface re-melting and solidification using a 2.5 kW Nd:YAG laser. The effects of laser power, high scan rate and beam configuration were examined. The microstructure of laser treated ZE41 consisted of small precipitates dispersed in a fine dendritic α-magnesium matrix at high scan rates. The redistribution of chemical elements depended mainly on the dwell time in the liquid stage. At high scan rates, long dwell times were achieved by splitting the laser beam into two spots trailing in the scan direction which resulted in a more homogenous distribution of Mg, Zn and Zr. Cracking due to thermal shrinkage during solidification was prevented by reducing the temperature of the melt pool. This was achieved by lowering the laser power, increasing the scan rate and laser spot size. Increasing the laser spot size in the scan direction was conducive to producing homogeneous microstructures without cracks.

Investigation of capillary waves excited by a Nd : YAG laser using pyrometry

A mechanism for excitation of capillary waves on a weldpool formed by a laser is shown to be due to coupling of the weldpool height with the laser beam profile. This leads to an additional low frequency mode, which has been observed and has a frequency of about an order of magnitude lower than the classically expected capillary waves. This mechanism for wave excitation on the weldpool can only happen if the focal point of the laser beam is located above the surface of the weldpool; otherwise the waves are damped.

Effect of increment and single-track geometry on the formation of multi-track laser cladding

The variation of track geometry during multi-track laser cladding of stellite 6 on mild steel starting with different geometry profiles and levels of dilution in the single-track clad was examined. In transverse cross-section of the multi-track clad, the total area in each track includes the areas of melted powder (clad area), remelted previous track (remelted area) and melted substrate. Both clad area and total area increase with track number and finally reach constant values, but the increase of total area is much greater than that of clad area. The remelted area of previous track increases with the level of dilution of the single-track clad and reaches its maximum value when the dilution of single-track clad is over 20%. The percentage of the maximum remelted area of the previous track equals the percentage of the track overlap. The inter-track porosity will appear when the difference of the total area and the remelted area of the previous track is closer to or smaller than the clad area because there is not enough laser energy to melt the powder captured by the melt pool.The variation of track geometry during multi-track laser cladding of stellite 6 on mild steel starting with different geometry profiles and levels of dilution in the single-track clad was examined. In transverse cross-section of the multi-track clad, the total area in each track includes the areas of melted powder (clad area), remelted previous track (remelted area) and melted substrate. Both clad area and total area increase with track number and finally reach constant values, but the increase of total area is much greater than that of clad area. The remelted area of previous track increases with the level of dilution of the single-track clad and reaches its maximum value when the dilution of single-track clad is over 20%. The percentage of the maximum remelted area of the previous track equals the percentage of the track overlap. The inter-track porosity will appear when the difference of the total area and the remelted area of the previous track is closer to or smaller than the clad area because there ...