Additive manufacturing (AM) of metal alloys has been extensively developed in past decades for industrial applications in advanced manufacturing and remanufacturing, targeting at customized, complex, and/or critical structural components with increased cost-efficiency in both material and time consumptions. Compared to traditional manufacturing from bulk materials, remanufacturing based on spoiled components can save the cost by up to 85%, depending on materials and AM techniques. By optimizing the feedstocks, the processing parameters and the post-process treatments, high-quality AM alloys, e.g., Ni-based superalloys, low-density high-strength Ti-based alloys, stainless steels (SS), etc., can now be fabricated by direct energy deposition (DED) methods, including laser-cladding (LC), wire-arc AM (WAAM), etc., with desired specifications comparable to or even better than their counterparts manufactured by traditional methods.
Nevertheless, as-built AM components usually have limitations due to the intrinsic rapid heating and fast cooling during typical DED processes, e.g., rough surface, certain level of porosity and residual tensile stress (RTS), which tend to degrade the surface integrity and mechanical performance. In this regard, post-AM processes, e.g., heat-treatment, machining, cold-working, etc., which have been widely developed in traditional manufacturing, have important consequences in remanufacturing of metallic structural components by AM methods. However, challenges might exist in post-AM processes, e.g., heat-treatment, to release RTS and improve the structural properties, especially during onsite remanufacturing of complex structural components. Therefore, monolithic AM methods, e.g., in-situ process integrations between DED and surface integrity enhancement by cold-working, have attracted increasing interest in recent years. Here, we present a glimpse on surface integrity enhancement of AM alloys and its applications in the monolithic AM process, addressing typical strategies through case studies.
Methods and Strategies
The schematic in Fig. 1(a) presents a typical integration between robotic hammer peening (RHP) and LC while that in Fig. 1(b) is an integration between rolling and WAAM. Both can be realized in industrial robot and CNC (computer numerical control) platforms thanks to the rapid developments of automation and digital control technologies in recent decades. The surface integrity enhancement process, e.g., RHP or rolling, can be applied simply after completing the whole component or immediately after the deposition of individual layers, i.e., interpass.
Case Studies and Discussion
Inconel 718 coatings have been deposited by LC on low-alloy steel (LAS) substrates (AISI 4140). The in-plane dimensions of the flat coupons were 50 mm x 50 mm in square while the thickness of the coupons was ~8.0 mm. The LC process was carried out on an industrial robotic platform [Dura-Metal (S) Pte. Ltd, Singapore] using Inconel 718 powder feedstocks. Figure 2(a) schematically shows the scan pattern of LC on the front surface of the LAS coupon. After the LC process, the Inconel 718 coating was ground to a surface roughness of ~0.67 µm with remaining thickness of ~2.0 mm on the LAS substrate. RHP was processed using a 5.0 mm diameter WC hammer head on the Inconel 718 coating with the stroke aligned along the normal direction of the surface. The stroke length was 2.0 mm, and the stroke frequency was 220 Hz.
The surface height profiles were measured from the rear surface of the coupons along the longitudinal and transverse directions before and after the RHP process, from which the curvature radiuses were derived and presented in Fig. 2(b). It is seen that curvature was induced into the initial flat substrate and the curvature radius is smaller along the longitudinal direction than that along the transverse direction of the LC-scanning pass. This observation indicates that RTS has been induced in the as-built Inconel 718 coating and the RTS along the longitudinal direction is larger than that along the transverse direction. This is consistent with the laser-treatment-induced RTS in the near-surface regions of SS420 coupons (with the same dimensions) at relatively fast laser-scanning rates . Also seen in Fig. 2(b) is that the curvature radius was significantly increased in both directions by the RHP process, indicating that the RTS of the Inconel 718 coating has been remarkably balanced by the RHP-induced residual compressive stress (RCS) in its near-surface regions. It should be mentioned that the substrate deformation might be negligible in a practical LC process as well as in a practical RHP process due to increased dimensions of the workpiece; H
however, the RHP-induced RCS is desired for enhancing the fatigue life of structural components.
Figures 2(c) and 2(d) present cross-sectional images recorded by scanning-electron microscopy (SEM) at the near-surface regions of the Inconel 718 coatings before and after the RHP process. Pore structures, i.e., the dark dots, observed in the coatings are most likely due to the unoptimized laser power employed in the LC process. However, the comparisons of Figs. 2(c) and 2(d) revealed that both the density and size of the pore structures have been significantly reduced by the RHP process, and the reductions are much more significant at the near-surface regions than at deeper regions [see Fig. 2(d)]. These observations suggest that the RHP process could close the pore structures, especially near the surface of the LC-deposited Inconel 718 coatings. Likewise, microhardness measured from the surface of the Inconel 718 coatings significantly increased after the RHP process, i.e., from 260 to 410 HV. The reduced RTS, the increased surface hardness, and the reduced density and size of the pore structures provide evidence that the RHP process significantly enhanced the surface integrity of the LC-deposited Inconel 718 coatings.
Microstructural studies on the cross-section of the surface integrity enhancement revealed that the effective depth of a cold-working process, e.g., RHP, is typical less than 1.0 mm while the severe plastic deformed depth is even shallower. To enhance the process effectiveness, interpass treatment has been proposed. For example, Honnige et al. have studied the effect of interpass rolling on the deposition of Inconel 718 walls by WAAM . The Inconel 718 wire feedstock had a diameter of 1.2 mm, and the substrate was an Inconel 718 plate of 200 mm (L) x 64 mm (W) x 6 mm (H). The nominal wall length was 180 mm, the scanning rate of the deposition head was 3.6 cm/min, and the wire feeding rate was 7 m/min, with the load of the interpass rolling being 50 kN. It was found that too-heavy-a load, especially at increased wall height, could cause onset of fracturing at the interface between the wall and the substrate. Figure 3(a) schematically presents the building up of the Inconel 718 walls by WAAM in a layer-by-layer pattern and curvature occurred after releasing the built wall from the fixture. Two samples, consisting of 9- and 11-layer WAAM claddings, were compared without (22.6 mm in height) and with (20.9 mm in height) introducing the interpass rolling, respectively .
Figure 3(b) presents the curvature radiuses of the WAAM-deposited Inconel 718 walls with and without introducing the interpass rolling. It is seen that the curvature radius was significantly increased from ~4.5 to ~16.7 m by introducing the interpass rolling. This comparison indicates that the RTS built in the Inconel 718 wall was significantly released by introducing the interpass rolling. Presented in Figures 3(c) and 3(d) are the microphotographs recorded from the cross-section perpendicular to the wall length direction before and after introducing the interpass rolling, respectively. They showed that the wall thickness was increased from ~6.9 to ~8.0 mm by introducing the interpass rolling. Likewise, the interpass rolling reduced the single-layer height from ~2.5 to ~1.9 mm . Also found was that the pore structures, which were observed in the WAAM-deposited Inconel 718 wall as indicated by the dark arrows in Fig. 3(c), disappeared after introducing the interpass rolling [Fig. 3(d)].
A similar interpass rolling process was introduced by Kan et al. into LC of SS 316L, where they compared an LC process of 10 mm high SS316L walls (60 mm x 8 mm), using powder feedstocks, with and without interlayer rolling . Figure 4 presents typical microstructures recorded by optical microscopy (OM) and electron backscatter diffraction (EBSD) from the cross-section of the LC-SS316L walls with and without interlayer rolling . An apparent contrast in Figs. 4(a) and 4(b) is that the pore structures were closed by the interlayer rolling. Likewise, the comparison in Figs. 4(c) and 4(d) shows that the grain structures of SS316L have been significantly refined by the interpass rolling of the LC process. Such structural changes increased the hardness, yield strength, and tensile strength by ~23%, ~ 29%, and ~ 8% respectively, for the LC-deposited SS316 L.
Instead of rolling, interpass RHP has also been introduced for integrity enhancement of LC process. For example, Neto et al. have introduced interpass RHP into LC of Ti-6Al-4V alloys. They found that the interlayer epitaxial growth could be interrupted, and specifically, the β-grain epitaxy was significantly reduced from centimeters to 1~2 mm . Such structural changes induced by interpass RHP tend to improve the mechanical performance and fatigue life of the LC-deposited Ti-6Al-4V alloys.
Integration between surface integrity enhancement through cold-working and DED-based AM of metallic alloys has been discussed through a few case studies. Making use of dynamic surface impacts, plastic deformations can be introduced in the near-surface regions of the workpiece, which give rise to porosity closure, grain size refinement, structural strengthening, and hardening. Introducing interpass cold-working into a DED-based AM process can effectively enhance the integrity of entire AM components without degrading their thermodynamic and chemical stabilities, which can, in turn, reduce the necessity to some extent for additional post-AM treatments.
This work is supported by A*STAR RIE2020 advanced manufacturing and engineering (AME) programmatic grant through the structural metal alloys program (SMAP, Grant no. A18B1b0061). The authors would also like to express their appreciation for technical support from Dura-Metal (S) Pte. Ltd. Dr. Henry K. F. Cheng is acknowledged for his help in surface enhancement.
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 J. Honnige, et al., “Study of residual stress and microstructural evolution in as-deposited and inter-pass rolled wire plus arc additively manufactured Inconel 718 alloy after ageing treatment” Mater. Sci. Eng. A Vol. 801, pp. 140368, 2021.
 W. H. Kan, et al., “Effect of in-situ layer-by-layer rolling on the microstructure, mechanical properties, and corrosion resistance of a directed energy deposited 316L stainless steel” Adv. Manufact. Vol. 55, pp. 102863, 2022.
 L. Neto, et al., “Mechanical properties enhancement of additive manufactured Ti-6Al-4V by machine hammer peening” in Proceedings of the first International Conference on Advanced Surface Enhancement (INCASE 2019), Singapore, Itoh, S.; Shukla, S., Eds. Springer Singapore: Singapore, 2019; pp 121-132.
(Ph. D. in Physics)
Chee Kiang Ivan Tan
(Ph. D. in Eng.)
Senior Scientist,Department Head
Institute of Materials Research and Engineering
(Agency for Science, Technology and Research), 2 Fusionopolis
Way, Singapore 138634, Singapore
(Ph. D. in Eng.)
Wei Luen Chan
Advanced Remanufacturing and Technology Centre (ARTC),
A*STAR (Agency for Science, Technology and Research), 3 Cleantech Loop,
Singapore 637143, Singapore