in Vol. 20 - November Issue - Year 2019
Parametric Study of Fixtured Vibropeening
Figure 1: TFM58/32VP Vibratory Trough
Figure 2: Residual stress profile of IN718 AT -130mm in transverse direction
Figure 3: Residual stress profile of IN718 AT -130mm in longitudinal direction
Figure 4: FWHM distribution of IN718 AT -130mm in transverse direction
Figure 5: FWHM distribution of IN718 AT -130MM in longitudinal direction
VIBROPEENING is a new surface treatment process compared to existing shot peening plus polishing technology. Vibrostrengthening, also referred to as fixtured vibratory finishing was proposed by Sangid et al. (2011) to impart beneficial compressive residual stresses (CRS) on workpieces using steel shots.  This form of fixturing the workpiece has shown to increase the efficiency of vibratory processes by reducing the time taken to achieve the intended output. ,  This technology has been studied to be potentially deployed in aerospace industry to introduce compressive residual stresses without large amount of accompanying cold work and surface roughness , . Minimizing the cold work in the material by surface enhancement was postulated to provide substantial benefit in terms of the fatigue performance at elevated temperature application. 
The main control parameters of fixtured vibropeening are motor frequency and amplitude resulting from flyweight setting , . Flyweight setting is defined as the degree of difference between the two unbalanced weights beneath the trough which causes the trough to vibrate. However, there are more parameters that can influence the performance and effectiveness of vibropeening process which have been selected from previous operators’ experience . For instance, the use of compound, media mass, immersion depth, treatment process time and setup of fixture are other process parameters. In vibratory process specifically vibropolishing, the use of compound or lubricant and type of media are key parameters mentioned by Gour et al. (2018) to achieve desired surface properties. Furthermore, the effect of the use of compound has been confirmed by Ciampini et al. (2008) that the Almen intensity is higher with the addition of water or compound compared to dry process without use of compound .
In this report focus will be mainly on the effect of process time on the residual stress and coldwork on an IN 718 coupon.
Experimental Setup and Measurement Protocol
The experiments were conducted on a vibratory trough manufactured by the company of Walther Trowal, TFM58/32VP (Figure 1) with trough size of 580 mm x 320 mm x 360 mm. The trough is equipped with a PU lining container and motor which can drive the trough to vibrate at controlled frequency and amplitude.
Trials with IN718 test pieces were conducted to assess residual stress. The as-received condition of the coupons is similar as high pressure compressors (HPC) blisk condition, which is degreased, heat treated and anodized. The residual stress was measured using XSTRESS Robotic X-Ray Diffraction (XRD) measurement system with layer removal using electropolishing steps.
3. Results And Discussions
The changes in Almen deflection always follows exponential increment, which is increase in value and subsequently reaching a plateau at saturation point. Hence, the change in residual stress with treatment time is assumed to be the same. There is always a question of how much the component need to be peened to achieve desired residual stress profile. However, this depends on the properties of each material, such as elastic modulus, hardness and ductility to name a few. Therefore, it is important to determine the saturation point of the treating material to avoid over-peening, where the residual stress profile will not have any improvement further or even deteriorates .
Besides, process time or treatment time for vibropeening process is normally selected based on the saturation time from Almen intensity test. The correspondence relationship between Almen intensity and residual stress have been proven in shot peening process ,  . However, in vibropeening process this is may not be the case due to different mechanisms of the two processes. Study by Canals et al. proved that the increase of peening time to twice the Almen strips saturation time showed increase in compressive residual stress profiles of materials Ti-6Al-4V and E-16NiCrMo13, but no further increment with thrice the saturation time . Hence, in this study, the saturation time of vibropeening process with fixed set of experimental parameters is determined using IN718 flat test coupons. This can provide the suitable range of process time and helps to identify the maximum treatment capacity of vibropeening process. Almen intensity test has been conducted prior to the flat coupon’s trials and the time for studies is chosen based on the saturation time from the intensity tests. The flat coupons used in this study has coupon dimension is 30mm x 40mm x 4.5mm.
The residual stress profiles of the IN 718 test coupon and the corresponding FWHM distributions also plotted below.
From Figure 2 and 3, high compressive surface residual stress can be observed on the untreated coupons, which is due to the manufacturing process. However, the high stress state only occurs at near surface depths. Stress free state can be observed at depths beyond 20 microns. The flat coupon after treatment time of 4 hours showed higher surface and maximum compressive residual stress, and greater depth of influence compared to that of 2 hours in both transverse and longitudinal directions. When the treatment time increased from 1 hour to 2 hours, the residual stress profile becomes better in transverse direction. This result is aligned with the hypothesis which the residual stress state is better with longer treatment time. In transverse direction, the surface and maximum compressive residual stress after 8 hours treatment time is similar to that of 4 hours, but the depth of influence after 8 hours is shallower. This indicates that the impact of vibropeening with 8 hours treatment time have reduced in intensity or energy, or the imparted stress has possibly been relaxed after 8 hours. On the other hand, in longitudinal direction, the residual stress profile after 4 hours treatment time is better than 8 hours, with higher maximum compressive residual stress and greater depth of influence. This proved that there could be stress relaxation with longer treatment time which could negate the positive impact of vibropeening on the components . This is essential to provide the understanding of optimal process time for vibropeening.
In addition, full-width-half-max (FWHM) is obtained from the XRD measurements and represents the peak broadening under plastic deformation and could be analogous to plastic strain. It is known that the values of FWHM are closely related to those of percentage cold work introduced by surface enhancement process into a workpiece. , ,  From Figure 4 and 5, FWHM distributions showed that the plastic deformation on IN718 coupons after 2 hours treatment time is higher compared to 1, 4 and 8 hours in both transverse and longitudinal directions. This showed that higher plastic strain and dense dislocation network is generated after 2 hours, and the recovery of plastic strain and dislocation annihilation occurs after 4 hours and 8 hours of treatment time. The lower FWHM of the flat coupon after 1-hour treatment time showed that the plastic deformation is not saturated yet, and it increased when increase treatment time to 2 hours. The plastic deformation at depths beyond 120 microns after 4 and 8 hours have been annihilated to the same as that of untreated condition. Similar FWHM distributions can be observed in the study by Kumar et al. after thermal relaxation on Udimet®720Li . This implies that after 2 hours of the process, the cold work has been reduced due to stress relaxation. Hence, the optimal process time of vibropeening treatment on IN718 material should be between 2 to 4 hours.
With these stress profiles, it is very likely that the fatigue life of treated component can be increased by peening at optimal time. High cycle fatigue (HCF) is linked to the compressive residual stress profiles generated on the material.  Studies by Feldmann et al. (2014) demonstrated that high cycle fatigue life can be improved by vibropeening process by 35%, whereas shot peening process increased by 61%.  It is unclear about the treatment time in the study, but with the current findings there is a potential that HCF of component can be further optimized at the optimal process time to be similar or better than shot peening.
The longer the process time, the better the compressive residual stress profile on the treated component. However, there is no further beneficial impact of vibropeening after the optimal treatment time is achieved, whereby after this the stress state of coupons will be saturated. The imparted stress will not increase with further peening and might even experience relaxation due to over-peening. This also showed that there is a maximum introducible compressive residual stress state for a given material by vibropeening process.
5. Future Steps
Based on the current findings on the study of these two parameters in fixtured vibropeening, there are several questions and theories that would need more detailed experiments to further prove the statements. The variation of the resultant forces in the trough during the vibropeening process has been reasonably explained based on visual observations and results of Almen strips. However, future investigations can be focused on the measurement of impact force from the media using sensors to quantify the change in forces at every region in the trough. Besides, more vibropeening experiments can be conducted on flat coupons with more time settings between the optimal process time range and collect the residual stress data to identify the best process time.
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The contents of this report have been published in Metals 2019, 9(8), 910; https://doi.org/10.3390/met9080910. The author aims to bring a technical result out of the paper to the attention of the MFN readers.
Chan Wai Luen 1,2, Kunal Ahluwalia 2, Abhay Gopinath 1
1Rolls-Royce Singapore, Central Technology Group, 1 Seletar Aerospace Crescent, Singapore 797565, Singapore.
2Advanced Remanufacturing and Technology Centre
3Cleantech Loop #01-01, Singapore 637143, Singapore