Laser cladding, also known as laser metal deposition, is an additive manufacturing technique for adding one type of material on to the substrate of the other, which falls in the category of directed energy deposition by the ASTM/ISO standards. Laser cladding involves the feeding of a stream of metallic powder into a melt pool that is usually generated by a laser beam, depositing a layer or layers coating of the targeted areas. Laser cladding technology allows materials to be deposited accurately, selectively and with minimal heat input into the underlying substrate. Therefore, laser-cladding of superalloys on the traditional metal substrate will effectively reduce the overall manufacturing cost of the component. Furthermore, with an appropriate post-processing technique, the superalloy cladded components will enhance their functionalities as well as improve their service life.
In this work, we explored a preliminary evaluation of INCONEL 625 (a nickel-based superalloy) laser-cladded on to AISI 4140 (a low alloy steel) as a potential solution for an effective cost-reduction and an improvement in the service life of the components without compromising their functionalities. The finding suggested when the INCONEL 625 powders are laser-cladded on to the AISI 4140 substrate, the components achieve surface functionalities similar to those of INCONEL 625 at a significantly lower cost compared to those made of wrought INCONEL 625. With a robotic hammer peening as the post-processing method, the specimens show higher surface hardness, improved corrosion resistance, elimination of surface tensile stress and introduction of compressive residual stress. And as a result, the components are expected to have a longer service life in fatigue, wear and corrosive applications.
AISI 4140, a low alloy steel, is commonly used in the marine & offshore industries due to its significant mechanical performance at a low base-material cost; however, in an application exposed to a harsh environment, it will lead to severe mechanical failure due to its poor wear and corrosion resistance. On the other hand, INCONEL 625, a nickel-based super alloy that is also widely used in marine & offshore industries, shows better performance in the similar harsh environment, owing to its chemical compositions with enhancing properties particularly in the wear and corrosion resistance. The base material price of INCONEL 625 is significantly higher compared to AISI 4140 although both have very similar mechanical properties. A balance between cost and performance will be an important factor for the engineers for their material selection.
Therefore, the laser cladding of INCONEL 625 on to AISI 4140 substrate will be a potential solution for engineers working in the marine & offshore applications as a cost-effective approach for improving the service life of the components without compromising their functionalities. Furthermore, the laser cladding process will provide a metallurgical bonding between the deposited layer of INCONEL 625 and the substrate of AISI 4140. This approach provides significant reductions in the manufacturing time and cost as well as an increase in the material utilization while providing the components with outstanding mechanical, wear and corrosion properties. However, the laser cladding of INCONEL 625 on to AISI 4140 substrate would have several drawbacks such as non-uniformity in surface microstructure, cladding process-induced surface and subsurface tensile residual stresses and surface microcracks. Therefore, an appropriate post-processing technique is a must to overcome these drawbacks.
In this work, we have explored a preliminary evaluation of INCONEL 625 laser-cladded on to AISI 4140 as a potential solution for an effective cost-reduction and a robotic hammer peening as a post processing technique to improve the service life of the components without compromising their functionalities. The effect of robotic hammer peening (RHP) process on the INCONEL 625 cladded on to AISI 4140 in terms of the introduction of compressive residual stress, improvement in the surface hardness and corrosion resistance and peening-induced grain refinement were studied.
Laser cladding of INCONEL 625 powders on to AISI 4140 flat plates (30 cm x 30 cm x 2 cm) was done using a laser beam size of 3.0 mm with a rated laser power of 1.62 kW. A deposition speed of 1200 mm/min was used. The powder flow rate was set at 15 g/min. A single layer of 0.8 mm thickness with a track width of 2.3 mm and overlap of 50% was used. Argon was selected as both carrier gas and shielding. The flow rates were set at 5 litre per min (at 1 bar) for carrier gas and 20 litre per min (at 1 bar) for shielding. Three layers of deposition was set at the nominal deposition thickness of 2.4 mm.
The INCONEL 625 laser cladded AISI4140 plate was then ground to the nominal deposition thickness of 1.5 mm. The cross-sectional optical microscopic images of three flat plates are shown in Figure 1. It can be seen from Figure 1 (a) that the final average thickness of 1.495 mm was achieved. Figure 1(b) shows the high magnification optical micrograph of cross-section of interface between INCONEL 625 cladded layer and AISI4140 substrate. There was no void nor delamination observed, so it can be concluded that a nominal thickness of 1.5 mm thick IN625 layer was successfully laser-cladded on to AISI4140 substrate.
To study the effects of the RHP process on laser-cladded coupons, we fixed the tool radius, wobble angle and frequency of the RHP and varied the impact energy and overlap percentage. The ECOPEEN Type A hammer peening tool developed by ECOROLL AG, Germany and mounted on an ABB 6-axis industrial robot was used. The detailed experiment matrix is shown in Table 1 below.
To completely characterize the residual stress depth profile, a Centre-Hole Drilling (CHD) technique, namely the PRISM (Precision Real-Time Instrument for Surface Measurement) developed by Stresstech, UK, was employed. PRISM provides very fast stress depth profile, and it requires little sample preparation. The system does not use strain gage which is one of the main advantages compared to the conventional deep hole drilling method. 
The surface hardness evaluation was conducted by a Vickers hardness tester on the cladded surfaces before and after the RHP treatment using a 0.5 kg test load. The basic principle of the Vickers test, similar to other hardness tests, is to observe a material's ability to resist plastic deformation from a standard source. The Vickers test can be used for all metals and has one of the widest scales among hardness tests. 
Electrochemical pitting corrosion evaluation  of the samples was carried out using a Metrohm Autolab Potentialstat under ambient conditions (~25 °C). The electrolyte used in this work was 3.5 wt% sodium chloride (NaCl) solution, prepared by mixing reagent grade NaCl powder and de-ionized water. The concentration of sodium chloride solution was kept at 3.5 % w/w because it is similar to that present in typical sea water. The electrochemical characterization was done by cyclic polarization test, and the potential cycle between −1 and +3 V vs. Ag/AgCl reference electrode was applied with a sweep rate of 1 mV/s.
The microstructural study of the samples was conducted by an Electron Backscatter Diffraction (EBSD), being an extended application of a scanning electron microscope (SEM) that gives crystallographic information about the microstructure of a sample. This analysis is a very powerful tool for microstructural characterisation. 
Results and discussion
The stress calculation algorithm is compatible with the requirements described in the strain-gage hole-drilling ASTM standard (ASTM E837).  Figure 2 shows the residual stress profiles of laser-cladded coupons before and after the RHP treatment. Laser-cladded coupons show process-induced tensile residual stress significantly. This is because of the thermal stresses induced during the laser cladding process. This thermal stress mainly comes from shrinkage of the clad layers during cooling process. Similar finding was reported on the system of INCONEL 625 cladded on AISI 4330 substrate.  After the RHP treatment, laser cladding-induced tensile residual stresses in the coupon completely vanished and a significant amount of RHP-induced compressive residual stress was observed. Furthermore, at all overlap percentages, higher impact energy level showed higher magnitude of compressive residual stresses. On the other hand, for the same impact energy, the higher amount of overlap would give the deeper compressive residual stresses.
The surface hardness evaluation was conducted using a Vickers hardness tester and the values are reported in Figure 3. The figure shows comparison between surface hardness of laser-cladded and wrought INCONEL 625 coupons before and after the RHP treatments at different overlap percentages and energy levels with both coupon types showing similar hardness range. The significant improvement in the surface hardness of all these coupons were observed after the RHP treatment. In general, the final surface hardness value of wrought INCONEL 625 coupons after the RHP treatment was slightly higher compared to the laser-cladded coupons.
The electrochemical corrosion evaluations were conducted at room temperature, with results summarised in Table 2. In electrochemical corrosion evaluation, the electrical potential at open circuit (Eocp) is considered as the coupon’s resistance to the pitting corrosion in a defined environment. The higher Eocp (shifting towards positive value) indicates that the test coupon requires higher energy to form pits in the electrolyte under a controlled environment; therefore, the higher the Eocp values, the higher the corrosion resistance.  From Table 2, the RHP treatment on the laser-cladded INCONEL 625 coupons could improve corrosion resistance, and at all parameters, the RHP-treated coupons showed significant improvement in corrosion resistance. This might be due to formation of passivation layer (formation of smaller grain layer) on the RHP-treated coupons. This finding agrees with several other research studies reporting that other nickel-based alloys used in hammer peening treatment were able to improve the corrosion resistance of the material by formation of smaller grain layer and higher dislocation intensity of the grains. [8, 9] Similar findings were reported by the other peening process. [10,11] Furthermore, the RHP treatment did not significantly change the corrosion current (Icorr), therefore it can be concluded that even though the RHP treatment could increase energy to form pits in the electrolyte under a controlled environment, it has no effect in reducing the corrosion rate. As a result, once corrosion pits are formed on the surface, the corrosion rate will be the same regardless of with or without the RHP treatment on the coupons.
The microstructure study was conducted by the Electron Backscatter Diffraction (EBSD). A significant amount of grain refinement and dislocation density was observed after the RHP treatment. After the RHP treatment, the grain sizes at the surface of the coupons were decreased. Furthermore, from the Kernel Average Misorientation (KAM) analysis, the RHP treatment could introduce significant amounts of dislocations near the surface as shown in Figure 4. The grain refinement and increase in the dislocation density are the reasons why the robotic hammer-peened coupons showed compressive residual stress on the surface, with higher surface hardness and better corrosion resistance compared to those of the un-peened coupons; however, there was no phase change observed in the robotic hammer-peened coupons.
In this work, it is observed that robotic hammer peening treatment could eliminate laser cladding-induced surface tensile stresses and induce compressive residual stresses in the INCONEL 625 cladded on AISI 4140 substrates. Furthermore, the microstructure study shows the RHP treatment could impart grain refinement and introduce a significant amount of dislocations near the surface of the coupons. As a result, there was a significant improvement in both surface hardness and corrosion resistance of the INCONEL 625-cladded AISI 4140 coupons. Therefore, our preliminary study shows when the INCONEL 625 powders were laser-cladded on to the AISI 4140 substrate, the components achieve surface functionalities similar to those of bulk INCONEL 625 and there would be potential to reduce their cost significantly compared to those made of wrought INCONEL 625. Additionally, with a robotic hammer peening treatment as the post-processing method, the specimens showed higher surface hardness, improved corrosion resistance, elimination of cladding-induced surface tensile stress and the introduction of peening-induced compressive residual stress, and as a result, the components are expected to have longer service life in fatigue, wear and corrosive applications.
Acknowledgement: The authors thank ECOROLL AG, Germany for providing the ECOPEEN Type A hammer peening tool used in this work as an in-kind contribution. The work was supported by A*STAR’s RIE2020 Industry Alignment Fund – Pre-Positioning (IAF-PP) grant (no. A20F9a0045)
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Henry Kuo Feng Cheng, PhD
Chan Wai Luen, BEng
Senior Development Engineer, Advanced Remanufacturing and Technology Centre (ARTC), Agency for Science, Technology and Research (A*STAR), 3 CleanTech Loop #01-01 CleanTech Two Singapore 637143
Wang Pei, PhD, Scientist
Institute of Material Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR), 2 Fusionopolis Way, Singapore 138634
Weng Fei, PhD, Scientist
Singapore Institute of Manufacturing Technology (SIMTech), Agency for Science, Technology and Research (A*STAR), 2 Fusionopolis Way, Singapore 138634