In industries such as aerospace, petrochemistry and automobile, many parts of different machines are under environment which shows high temperature and high pressure, and have their proneness to wear and corrosion. Therefore, the wear resistibility and stability under high temperature need to be further improved. Nowadays, Laser cladding (LC) is widely used in machine parts repairing and functional coating due to its advantages such as lower dilution rate, small heat-affected zone and good metallurgical bonding between coating and substrate. In this paper, LC is introduced in detail from aspects of process simulation, monitoring and parameter optimization. At the same time, the paper gives a comprehensive review over LC material system as high entropy alloys (HEAs), amorphous alloy and single crystal alloy have been gradually showing their advantages over traditional metal materials in LC. In addition, the applications of LC in functional coatings and in maintenance of machine parts are also outlined. Also, the existing problems and the development trend of LC is discussed then.

Recent research and development status of laser cladding: A review

Additive manufacturing is an emerging technology that challenges traditional manufacturing methods. However, the corrosion behaviour of additively manufactured parts must be considered if additive techniques are to find widespread application. In this paper, we review relationships between the unique microstructures and the corresponding corrosion behaviour of several metallic alloys fabricated by selective laser melting, one of the most popular powder-bed additive technologies for metals and alloys. Common issues related to corrosion in selective laser melted parts, such as pores, molten pool boundaries, surface roughness and anisotropy, are discussed. Widely printed alloys, including Ti-based, Al-based and Fe-based alloys, are selected to illustrate these relationships, and the corrosion properties of alloys produced by selective laser melting are summarised and compared to their conventionally processed counterparts.

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Corrosion of metallic materials fabricated by selective laser melting

A systematic research into the cladding of stellite 6 on stainless steel by pulsed Nd:YAG laser has been carried out. The effects of pulse energy, pulse frequency, powder mass flow rate and spot overlap on the clad layer height, dilution and heat-affected zone (HAZ) have been examined. It was found that both the clad height and penetration into the substrate increase with the pulse energy, spot overlap and pulse frequency, but the effects of these parameters on dilution are complex. The dilution reaches the lowest value (4%) at the incident energy of 18 and 25 J/ pulse, spot overlap of 89% and pulse frequency of 40 Hz. The powder mass flow rate of 22 g/min (for energy of 25 J/pulse and spot overlap of 83%) produces thick clad layer with low dilution but results in the formation of defects. The hardness of the clad layer decreases linearly with increasing dilution. No cracks have been found in single-track clad layers at a spot overlap of 89%, however, cracks occurred at lower spot overlap. These cracks were eliminated by the multi-track cladding when the track increment is less than 1/3 of the width of track, which is believed to be due to the remelting or heat treatment of the previous clad track by the subsequent track. The track bands in multi-track clad show coarser structure, higher element segregation and lower hardness.

Pulsed Nd:YAG laser cladding of stellite 6 on stainless steel

Effect of TiC content on the mechanical and corrosion properties of Inconel 718 alloy fabricated by a high-throughput dual-feed laser metal deposition system

The marine, aerospace, and power machinery industries show progression in the application of titanium alloy components due to their good properties. However, the alloy exhibits poor thermal stability, low hardness, and poor tribological properties; as a result, the use of Ti6Al4V in various industries is restricted. Consequently, a search for surface improvement of Ti6Al4V alloy arose with the intention of enhancing its endurance. The use of laser metal deposition method by integrating chemical barrier coatings is considered as advantageous; therefore, an investigation aimed at surface improvement of Ti6Al4V by incorporation of Ti-Co coatings developed. To fabricate the coatings, a 3-kW continuous wave ytterbium laser system (YLS) was used, and to control the movement of the cladding process, a KUKA robot was attached to the system. The microstructure, corrosion, and mechanical properties of the titanium alloy-cladded surfaces were studied at different laser process parameters. To analyze the microstructure of the cross section, optical and scanning electron microscopy were employed. A laser power of 750 W and scanning rate of 1.2 m/min were found to be the optimum process conditions for a 60Ti-40Co alloy. When comparing the mechanical properties of the alloy and bare substrate, the alloy exhibited a significant increase in terms of the hardness. It was found to have 719 HV as compared to 301 HV which is that of the substrate, this indicates to an increase of 58.14% in the hardness. Lower laser scanning rates result in a larger fraction of hard-intermetallic phases which in turn lead to coatings with enhanced hardness levels. Furthermore, the yield strength and tensile strength of the coatings increased to maxima of 2.30 and1.66 GPa, respectively in comparison to the substrate, due to the addition of Co. Additionally, the corrosion rates of all the coated specimens were reduced as a result of the oxide films formed on the laser-coated Ti6Al4V alloy samples.

Evaluation of microstructure, microhardness, and electrochemical properties of laser-deposited Ti-Co coatings on Ti-6Al-4V Alloy

The significance of additive manufacturing has been felt in aerospace industry, but the full implementation of this technique has not been adopted yet due to drawbacks in terms of quality and surface finishing. Quality and surface finishing need to be addressed for the full impact of additive manufacturing to be utilized in many industries, which in turn will impact on the economic aspect of nations. Additive manufacturing reliability must be addressed and research on reliability must be continuous in order to fully utilized all the advantages and benefits of this process in medical and aerospace industries for wide applications. The experiment of quartenary titanium alloy of Ti-Al-Si-Cu was carried out with cladding machine of 3000 Watts (CW) Ytterbium Laser System (YLS-2000-TR). This machine is situated at the National Laser Centre in the Council of Scientific and Industrial Research (NLC-CSIR). The characterization was done using the standardization ASTM E3-11 procedure. Optical images of the samples were taking via the cross-sectional areas of the samples using the standardization procedure ASTM E2228-10 standard with BX51M Olympus microscope. The microstructural evolution was carried out using the TESCAN machine with an X-MAX instrument with ASTM E766-14e1 standardization procedure.The metallurgical bond formed as a result of the melting between the base metal and the reinforcement powders was done by a reduced laser energy input in the range of 27 to 22.5 J/mm2 at samples fabricated at 900 W with increased scanning velocity. While samples fabricated at 1000 W showed decrease in laser energy input between 30 to 25 J/mm2 at increased scanning velocity. Narrow deposit width is achieved at higher scanning velocity due to small amount of reinforcement powders used during the laser material interaction. There is sharp reduction of 20.7% in clad height with 11% of copper to 12.1% in clad height reduction as the weight percent of copper is increased to 12% and further reduction to 10% in clad height as the weight percent of copper is increased to 13% with increased velocity between 1.0 to 1.2 m/min at lower laser power of 900 W. A slight reduction of 14.14 % was shown by specimen Ti-Al-9Si-3Cu. Different result was observed when the specimen was fabricated at 1000 W. The clad height reduction was in the range of 14.14 to 3.85 %.

Analysis of Geometrical Characteristics and Microstructural Evolution of Laser Deposited Titanium Alloy Based Composite Coatings

The Impact of Zn and Cu Reinforcements on Microstructural Enhancement and Performance of Laser Cladded Ti-Zn-Cu/Ti-6Al-4V Composite Coatings.

The effects of processing parameters were investigated namely laser intensity and speed of laser scanning of Laser Metal Deposition (LMD) process had on the microstructure, metallurgical evolution, porosity generated in the coating, the geometrical property of the coating and the sizes of the grains in the coating. The processing parameters were applied in combinations in order to find the optimized settings of the process that least affects the metallurgical properties of the Ti-6Al-4V alloy substrate cladded with reinforced aluminium based powder. The temperature gradient and the rate of solidification of reinforcing the Ti-6Al-4V substrate with the aluminium based power were also focused on in terms of how they were influenced by the laser intensity and the scanning speed used in the building process. The inherent material properties were dependent on the process input parameters. The characterized performances considered in the investigation was influenced significantly by the laser processing intensity. The results of the investigation showed that the density had increased in proportion to the increase of the processing laser power coupled with the reduction of the speed of the laser scan. Moreover, for a laser power equivalent to and exceeding 950 W, the density became less susceptible to the laser power. The increased temperature field led to changes in geometry of the coatings as a results of more absorbed laser energy. The materials properties were influenced by the Fe-intermetallic compounds. The molten pool had columnar grains which were fully developed, along its volume as determined in the examination of the microstructure. In the direction of processing, the sample processed at 1.0-1.2 m/min had a reduction in width of the coatings from 1.643 to 1.293 mm and along the height it reduced to 0.375 at 2% Fe and 1% Mn. Increase in the percentage of both Fe and Mn increased the width of the coating to 1.833 mm at 1.0 m/min while the height continued to reduce to minimum of 0.272 mm at 1.2 m/min respectively.

Olawale S. Fatoba

Papers

Analysis of Geometrical Characteristics and Microstructural Evolution of Laser Deposited Titanium Alloy Based Composite Coatings

The Impact of Zn and Cu Reinforcements on Microstructural Enhancement and Performance of Laser Cladded Ti-Zn-Cu/Ti-6Al-4V Composite Coatings.

Analysis of the Influence of Laser Parameters on the Microstructure and Surface Properties of Laser Deposited Aluminum Based Coatings