The Effectiveness of Grain Refinement by Machine Hammer Peening in High Deposition Rate Wire-Arc AM Ti-6Al-4V

Abstract

Surface deformation, applied in-process by machine hammer peening (MHP), has the potential to refine the coarse columnar b-grain structures normally found in high deposition rate Wire-Arc Additive Manufacturing (WAAM) processes with Ti alloys like Ti-6Al-4V. Effective refinement, as well as a reduction in texture strength, has been achieved in relatively thick sections and to a depth that is greater than that expected from the surface deformation induced by MHP. By application of MHP to each deposition track, the average b-grain size could be reduced from cm's to less than 0.5 mm. Systematic experiments have been performed to investigate the origin of this interesting effect, which included 'stop-action' trials and separation of the strain and temperature gradients induced by the two process steps. The maximum depth of the plastic deformation from MHP required to generate new b-grain orientations was determined by electron backscatter diffraction local average misorientation analysis to be < 0.5 mm, which was less than the melt pool depth in the WAAM process. Nevertheless, new b-grain orientations were observed to form within the peened layer ahead of the approaching heat source as the peak temperature rose above the b transus, which then grew into the less deformed core of the wall as the temperature rose. This allowed the new grain orientations to penetrate deeper than the melt pool depth and survive to act as substrates for epitaxial growth at the fusion boundary during solidification, resulting in significant grain refinement.

Strength and Ductility Improvement through Thermomechanical Treatment of Wire-Feed Electron Beam Additive Manufactured Low Stacking Fault Energy (SFE) Aluminum Bronze

“…The same is true with the shot peening on an as-printed aluminum alloy AlSi10Mg [40]. These functionally flexible methods [36][37][38][39][40] were almost as effective as rolling.…”

“…Hammer peening is applied to modify the subsurface structures by severe plastic deformation with subdivision and misorientation in the previously grown grains, and eliminate the growth textures and large columnar grains [36,37]. Surface modification processes such as cavitation peening, shot peening and laser peening methods were applied on Ti-6Al-4V samples fabricated with the use of direct metal laser sintering and electron beam melting [38], achieving 70-97% improvement in fatigue strength.…”

Post-Processing of Cold Sprayed Ti-6Al-4V Coatings by Mechanical Peening

“…CHP is an impact based process in which the impact energy is provided by the kinetic energy of an oscillating tool. Hönnige et al [29] and Kumar et al [30] have shown that shock waves from CHP produces grain refinement in the component inducing microstresses near the surface. The CHP treatment resulted in a much higher degree of compaction (i.e., percentage reduction of the coating thickness after the treatment), as shown in Figure 10, of the sprayed coating (in comparison to DCR).…”

The potential for grain refinement of Wire-Arc Additive Manufactured (WAAM) Ti-6Al-4V by ZrN and TiN inoculation

“…Inter-pass deformation can be implemented by using techniques such as rolling [18], or peening [19], to deform the surface of the deposit before the next layer/melt track is applied. In this approach the stored energy from the inter-pass deformation, combined with the heat input from subsequent deposition passes, results in β recrystallization during reheating through the β transus [20]. This method is attractive as there is no change in the alloy composition; however, it adds an additional processing step which slows production time.…”