David J Nolan

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.

Diffusible Hydrogen Content in Rutile Flux-Cored Arc Welds as a Function of the Welding Parameters

The objective of the current work was to establish the effect of flux-cored arc welding (FCAW) parameters, such as welding current, contact-tip to workpiece distance (CTWD) and shielding gas type, on diffusible hydrogen content for single run, horizontal position, bead-on-plate welds using seamed and seamless rutile consumable wires. The work included an investigation of arc characteristics under typical welding conditions, using high-speed digital imaging and laser backlighting, in order to provide information on metal transfer and arc length. The work has shown that under most conditions investigated, the hydrogen levels for the seamed rutile wire were above the 10 ml/100 g specified by the consumable’s classification (H10). In contrast, the seamless wire met the requirements of the H5 classification for all welding conditions investigated. In general, lower diffusible hydrogen levels were achieved when using CO2 shielding gas, although the effect is less significant with the H5 seamless rutile wire. It is further concluded that the time that the wire resides in the resistive heating zone, tRHZ, and the arc length are important factors in determining diffusible hydrogen content.

Diffusible Hydrogen Testing in Australia

Diffusible hydrogen content in weld metals is a determining factor in the development of hydrogen-assisted cold cracking. In view of the importance of accurate and reliable quantification of diffusible hydrogen content, a number of industry standards have been developed to provide guidance on standardisation of methods for measurement of diffusible hydrogen content. A variety of test methods are currently used by industry and research organisations for the measurement and qualification of consumables and these test methods are generally accepted to be valid and comparable. However, the maintenance and calibration of the various hydrogen test methods is a matter of self regulation and, for this reason, the reliability and reproducibility of results across various test methods and laboratories is currently a matter of interest for Australian industry. Further, a more advanced and rapid measurement system is being utilised and a broad-based, independent and closely controlled trial is required to compare against traditional test methods and validate its application. In order to address these interests, the current work was commissioned by the Welding Technology Institute of Australia (WTIA) Technical Panel 2 — Working Group on Welding Metallurgy, primarily to assess the comparative performance of the various test methods (mercury displacement, gas chromatography, hot extraction) across a number of Australian laboratories with a uniform sample set produced under controlled welding conditions. A secondary aim was to determine the validity of the rapid test method known as Hot Extraction at 400°C.

Susceptibility of Low Strength Rutile Flux-Cored Weld Metal to Hydrogen Assisted Cold Cracking

This paper presents the results of an investigation into hydrogen assisted cold cracking (HACC) susceptibility of seamless and seamed low strength rutile flux cored wires, with nominal diffusible hydrogen (HD) levels of 5 and 10 ml/100 g, respectively. The objective was to assess the influence of key welding parameters on susceptibility of weld metal to cold cracking. Parameters investigated were the welding current, the contact-tip to work-piece distance (CTWD), the shielding gas and the preheat temperature. The gapped bead-on-plate (G-BOP) test was used to examine the effects of these parameters on the extent of weld metal transverse cracking at a range of preheat temperatures. The overall results indicate that the weld metal susceptibility to cold cracking correlates with diffusible hydrogen content, HD. It was found that, without preheat, the seamless wire weld deposits (< 5 ml/100 g) did not crack, whereas all those weld metals produced using the seamed wire (> 5 ml/100 g) exhibited cold cracking. Weld metal deposited using 75Ar-25CO2 shielding gas resulted in a higher HD levels than for CO2 shielding gas and, consequently, a higher susceptibility to cold cracking for no or low temperature preheat conditions. Preheating was found to have a strong effect on crack susceptibility, substantially decreasing the amount of cold cracking in the seamed wire welds.

Laser Cladding of Stainless Steel Substrates with Stellite 6

Stellite 6 coatings were produced using laser cladding of two different steel substrates (martensitic and austenitic stainless steels). The chemical composition and microstructure of these coatings were characterized by atomic absorption spectroscopy, optical microscopy and scanning electron microscopy. The microhardness of the coatings was measured and the wear mechanism of the coatings was examined using a pin-on-plate (reciprocating) wear testing machine. The results showed less cracking and pore development for Stellite 6 coatings applied to the martensitic stainless steel (SS) substrate. The wear test results showed that the weight loss for the coating on martensitic SS was significantly lower than for the austenitic SS substrate. It is concluded that the higher hardness of the coating on the martensitic SS, together with the harder and more rigid substrate increase the wear resistance of the Stellite 6 coating.

Laser Cladding of Wear Resistant Stellite 6 Coating on P22 Steel Substrate with Two Different Energy Inputs

Stellite 6 was deposited by laser cladding on a P22 steel substrate with energy inputs of 1 kW (P22-1) and 1.8 kW (P22-1.8). The chemical compositions and microstructures of these coatings were characterized by atomic absorption spectroscopy, optical microscopy and scanning electron microscopy. The microhardness of the coatings was measured and the wear mechanism of the coatings was examined using a pin-on-plate (reciprocating) wear testing machine. The results showed less cracking and pore development for Stellite 6 coatings applied to the P22 steel substrate with the lower heat input (P22-1). Further, the Stellite coating for P22-1 was significantly harder than that obtained for P22-1.8. The wear test results showed that the weight loss for P22-1 was much lower than for P22-1.8. It is concluded that the lower hardness of the coating for P22-1.8, markedly reduced the wear resistance of the Stellite 6 coating.

Laser resurfacing of Al-Zn-Mg-Si coatings

The feasibility of refinement of Al–Zn–Mg–Si hot dip coatings by diode laser resurfacing has been demonstrated. The microstructure of the laser resurfaced coatings was characterised mainly by transmission electron microscopy and also by scanning electron microscopy. The variance of heat input results in different cooling rates and varying degrees of refinement; that is, the higher the power density, the lower the cooling rate and the larger the secondary dendrite arm spacing of α-Al dendrites. In addition, the inherent variation of chemical composition in coatings also affects the microstructure as well as phase distribution. This laser resurfacing technique has not only reduced the secondary dendrite arm spacing/α-Al dendrites but also changed the solidification path and produce different phases and microstructure in the interdendritic areas.

Microstructural Development in a Laser-Remelted Al-Zn-Si-Mg Coating

In the last five decades, there has been intense development in the field of Zn-Al galvanic coating modification. Recently, Mg was added to improve corrosion properties. Further improvements to the coating are possible with additional laser surface treatment. In this article, we focus on remelting the Al-Zn-Mg-Si layer, using a diode laser with a wide-beam format, concentrating on the microstructure development during extreme cooling rates. Laser remelting of the Al-Zn-Mg-Si coating and rapid self-quenching produces a finer grain size, and a microstructure that is substantially refined and homogenized with respect to the phase distribution. Using EBSD results, we are able to understand microstructure modification. The laser modified coating has some porosity and intergranular cracking which are difficult to avoid, however this does not seem to be detrimental to mechanical properties, such as ductility on bending. The newly developed technology has a high potential for improved corrosion performance due to highly refined microstructure.

Study of laser cladding of stellite 6 on nickel superalloy substrate with two different energy inputs

Stellite 6 was deposited by laser cladding on a nickel superalloy substrate (NIS) with energy inputs of 1 kW (NIS 1) and 1.8 kW (NIS 1.8). The chemical compositions and microstructures of these coatings were characterized by atomic absorption spectroscopy, optical microscopy and scanning electron microscopy. The microhardness of the coatings was measured and the wear mechanism of the coatings was examined using a pin-on-plate (reciprocating) wear testing machine. The results showed less cracking and pore development for Stellite 6 coatings applied to the nickel superalloy substrate with the lower heat input (NIS 1). Further, the Stellite coating for NIS 1 was significantly harder than that obtained for NIS 1.8. The wear test results showed that the weight loss for NIS 1 was much lower than for NIS 1.8. It is concluded that the lower hardness of the coating for NIS 1.8, together with the softer underlying substrate structure, markedly reduced the wear resistance of the Stellite 6 coating.

Thermal modification of zinc-aluminum coated steel using lasers

Zn-55%Al-1.6%Si coated steel products combine the barrier protection of aluminium with the sacrificial protection of zinc, giving the advantages of both metals. The localised corrosion resistance can be further improved by modifying the coating microstructure, and this can be achieved using laser surface re-melting and rapid solidification. However, one challenge with laser processing of such coatings is their high reflectivity. An understanding of the influence of laser parameters and coating material properties is required to ensure efficient coupling of the laser source and optimum energy absorption. This paper presents some results of the effects of variations in topography, surface microstructure and surface composition on the surface melting characteristics of the coated steel. Scanning electron microscopy, electron dispersive X-ray spectroscopy and surface profilometry are used to examine the effects.Zn-55%Al-1.6%Si coated steel products combine the barrier protection of aluminium with the sacrificial protection of zinc, giving the advantages of both metals. The localised corrosion resistance can be further improved by modifying the coating microstructure, and this can be achieved using laser surface re-melting and rapid solidification. However, one challenge with laser processing of such coatings is their high reflectivity. An understanding of the influence of laser parameters and coating material properties is required to ensure efficient coupling of the laser source and optimum energy absorption. This paper presents some results of the effects of variations in topography, surface microstructure and surface composition on the surface melting characteristics of the coated steel. Scanning electron microscopy, electron dispersive X-ray spectroscopy and surface profilometry are used to examine the effects.