Laser cladding: The alternative for field thermite welds life extension
Materials & Design, Volume 111, 5 December 2016, Pages 165-173
F.C. Robles Hernández, A.O. Okonkwo, V. Kadekar, T. Metz, N. Badi
Abstract:Thermite field welds are the link of rails; however, they experience plastic deformation (batter) along the area above the heat affected zone (HAZ). Here we present the benefits of using laser cladding to minimize the plastic deformation on the HAZ. Our test consisted of a laboratory and a full scale approach under the most extreme revenue service conditions known in North America. The laser cladding coatings were used in the past for rails but its life was terminated in their infancy (after approximately 3–4 MGT) due to the presence of un-tempered martensite that negatively affected the rail toughness. The new tested laser cladding design is capable of producing a sound coating with tunable hardness (from 360–430 HBN). This is possible through a new heat treatment procedure carried using a propane torch or induction heating at temperatures between 400 and 500 °C and cooling rates below 3 °C/s. The microstructure shows that the rail after laser cladding is fully pearlitic. The laser cladded welds sustained up to 42 million gross tons in heavy haul traffic that is between 1000 and 1400% improvement when compared to other previous works.
Online monitoring of thermo-cycles and its correlation with microstructure in laser cladding of nickel based super alloy
Optics and Lasers in Engineering, Volume 88, January 2017, Pages 139-152
Gopinath Muvvala, Debapriya Patra Karmakar, Ashish Kumar Nath
Abstract:Laser cladding, basically a weld deposition technique, is finding applications in many areas including surface coatings, refurbishment of worn out components and generation of functionally graded components owing to its various advantages over conventional methods like TIG, PTA etc. One of the essential requirements to adopt this technique in industrial manufacturing is to fulfil the increasing demand on product quality which could be controlled through online process monitoring and correlating the signals with the mechanical and metallurgical properties. Rapid thermo-cycle i.e. the fast heating and cooling rates involved in this process affect above properties of the deposited layer to a great extent. Therefore, the current study aims to monitor the thermo-cycles online, understand its variation with process parameters and its effect on different quality aspects of the clad layer, like microstructure, elemental segregations and mechanical properties. The effect of process parameters on clad track geometry is also studied which helps in their judicious selection to deposit a predefined thickness of coating. In this study Inconel 718, a nickel based super alloy is used as a clad material and AISI 304 austenitic steel as a substrate material. The thermo-cycles during the cladding process were recorded using a single spot monochromatic pyrometer. The heating and cooling rates were estimated from the recorded thermo-cycles and its effects on microstructures were characterised using SEM and XRD analyses. Slow thermo-cycles resulted in severe elemental segregations favouring Laves phase formation and increased γ matrix size which is found to be detrimental to the mechanical properties. Slow cooling also resulted in termination of epitaxial growth, forming equiaxed grains near the surface, which is not preferred for single crystal growth. Heat treatment is carried out and the effect of slow cooling and the increased γ matrix size on dissolution of segregated elements in metal matrix is studied.
Microstructure and wear resistance of Stellite-6/WC MMC coatings produced by laser cladding using Yb:YAG disk laser
International Journal of Refractory Metals and Hard Materials, Volume 58, August 2016, Pages 157-164
Dariusz Bartkowski, Grzegorz Kinal
Abstract:The paper focuses on the study results of Stellite-6/WC metal matrix composite coatings (MMC coatings) produced by laser cladding technology using a 1 kW continuous wave Yb: YAG disk laser with powder feeding system. Specimens were preparation using CNC machining center equipped with a laser nozzle. Powder mixtures containing 60% tungsten carbides particles and 40% commercial Stellite-6 powder were used. In this study, three different values of laser beam power (400 W, 550 W and 700 W) and three different powder feed rate (5.12 g/min, 10.24 g/min and 15.36 g/min) were used. For all specimens, the same scanning speed of laser beam were applied. Changes in roughness, microstructure as well as wear resistance were investigated. It was found that increasing laser beam power caused a decrease in wear resistance of coating. Furthermore in described process appeared the best value of the powder feed rate which potentially resulting in better wear resistance. Exceeding this value influence on more intensive wear of coating. Special attention was given to the wear mechanism of MMC coatings.
Quantitative fractography and modelling of fatigue crack propagation in high strength AerMet®100 steel repaired with a laser cladding process
International Journal of Fatigue, In Press, Corrected Proof, Available online 12 July 2016
K.F. Walker, J.M. Lourenço, S. Sun, M. Brandt, C.H. Wang
Abstract: Ultra-high strength steels employed in safety-critical applications, such as AerMet®100 used in aircraft landing gear structures, are managed on very conservative rejection criteria for small defects and repair options are limited. A novel repair technique using laser cladding has recently been developed. In the present paper we report a study of the fatigue endurance of AerMet®100 steel components repaired by the laser cladding process, and a fracture mechanics based model to predict the fatigue endurance of repaired components. Three different types of samples were tested; baseline AerMet®100 sample with a small electro-discharge machining notch to initiate a crack, as-clad repaired, and as-clad repaired followed by heat treatment to relieve residual stresses. The specimens were subjected to cyclic loading under a special sequence consisting of constant amplitude segments at two different stress-ratios (ratio of minimum to maximum cyclic stress). The test results showed that the crack propagation lives from a common initial depth of 0.25 mm for the as-clad samples were significantly longer than the baseline samples by a factor of three to four. The longer life is attributed to the beneficial compressive residual stresses resulting from the repair process. The model predictions are found to correlate well with the results of quantitative fractography measurements from samples tested under variable amplitude cyclic loads.
Effect of rare earth elements and their oxides on tribo-mechanical performance of laser claddings: A review
Journal of Rare Earths, Volume 34, Issue 6, June 2016, Pages 549-564
M.M. Quazi, M.A. Fazal, A.S.M.A. Haseeb, Farazila Yusof, H.H. Masjuki, A. Arslan
Abstract:Laser cladding is a promising photon-based surface engineering technique broadly utilized for fabricating harder and wear resistant composite coatings. In spite of excellent properties, the practical applications of laser claddings are relatively restricted when compared with well-established coating techniques because of their inherent defects identified as cracks, pores and inclusions. Substantial evidence suggests that the incorporation of an appropriate amount of rare earth in laser claddings can remarkably prevent these defects. Additionally, the presence of rare earth in laser claddings can notably enhance tribo-mechanical properties such as surface hardness, modulus of elasticity, fracture toughness, friction coefficient and wear rate. In this literature review, the effect of rare earth in reducing dilution and cracks susceptibility of laser claddings in addition to microstructural refinement attained was examined. Mechanical and tribological properties of these claddings along with their underlying mechanism were discussed in detail. Finally, this article summarizes current applications of laser claddings based on rare earth and was concluded with future research directions.
Nd:YAG laser cladding of Co–Cr–Mo alloy on γ-TiAl substrate
Optics & Laser Technology, Volume 80, June 2016, Pages 145-152
Masoud Barekat, Reza Shoja Razavi, Ali Ghasemi
Abstract:In this work, Co–Cr–Mo powder is used to form laser clads on a γ-TiAl substrate. The single-track geometrical characteristics such as width, height, penetration depth, dilution and wetting angle play the important role to control the characteristics of laser clad coatings formed by overlap of individual tracks. This paper is investigated the relations between the main coaxial laser cladding parameters (laser power P, laser beam scanning speed S and powder feeding rate F) and geometrical characteristics of single tracks by linear regression analysis. The results show that the clad height, H, depends linearly on the FS−5/4 parameter with the laser power having a minimal effect. Similarly, the cladding width W is controlled by the PS−2/3 parameter. The penetration depth b and dilution, D are proportional to P2S−1/4F−1/4 and P2/3S1/2F−1/2 respectively and wetting angle is controlled by the P1/4S1/2F−1/2 parameter. These empirical dependencies are observed with high values of the correlation coefficient (R>0.9). Finally, based on these relations, a laser cladd processing map was designed to use as a guideline for the selection of proper processing parameters for a required coating.
