VOL. 20 November ISSUE YEAR 2019
in Vol. 20 - November Issue - Year 2019
Vibratory Peening – A Viable Alternative Technique
Conventionally-peened set (left) and vibratory-peened set (right)
Brian McGillivray, President of VibraFinish in Mississauga, Ontario (Canada) has been designing and manufacturing vibratory finishing equipment for a process commonly referred to as 'mass finishing' for over four decades now. In addition to manufacturing, VibraFinish also provides finishing services for their automotive, aerospace and customers in other advanced manufacturing sectors. A well-maintained, low profile entity in their main factory, within a 10-minute drive from Toronto’s Pearson International Airport, is a lab that carries out research and developmental activities on an innovative process that they have termed VibraMax. VibraMax combines the benefits of residual stress generated through shot peening with the superior surface finish that the industry is well familiar with in mass finishing. Historically, we are aware that a component when being blast cleaned also gets ‘peened’ to a certain degree as part of the cleaning process. Though unmeasured and largely uncontrolled, this is also how conventional shot peening in its current form has evolved. However, vibratory peening is a distinct process from plain mass finishing. Where most vibratory finishing is carried out with ceramic abrasive, vibratory peening is carried out with steel balls, in a better-controlled environment. Also, instead of the workpiece being allowed to freely move in a vibratory finishing tub amongst vibrating media, the part to be peened is fixtured on to the tub, thereby providing ample opportunity for the abrasive to repeatedly impact all surfaces of the part.
Why Vibratory Peening?
In order to understand vibratory peening, it will help to review the features of vibratory or mass finishing. Vibratory finishing involves a tub filled with ceramic media that by constant and uniform vibration, act as thousands of ‘files’ that scrub the parts also loaded in the tub. A liquid compound is added to the media to assist with finishing, lubrication, and at times also serves other purposes like alkaline cleaning, acidic burnishing, washing and preventing rust formation. It is significant to note that this process is not reliant on operator skill such as alternative processes like buffing, belting etc. are subject to. Given the potential for repeatability, vibratory finishing is also employed for deburring, burnishing and to finish parts prior to painting, plating, heat treating, anodizing, or simply to obtain a final finish.
The peened or 'dented' appearance in conventional peening, though visually demonstrative of a peened surface, is also considered to potentially present local stress risers in certain components. Therefore, a relatively smooth finish is a factor that is being demanded in several shot peening applications, in addition to Almen intensity value, percentage coverage, and in some cases, measures of residual compressive stress. Unless of course the conventional peening process can provide a fine surface finish post-peening, the user is typically required to perform an additional operation to meet the finishing requirements. Needless to elaborate, an additional process translates to extra cost, time, space and all those variables that directly impact the viability and profitability of an operation.
The above factors, combined with controllability achievable with vibratory bowls, make it worthy of consideration and further exploration. Variable factors include the amplitude and frequency of vibration. In a batch-type bowl, the angle between the weights can be varied to control the ratio between forward and vibratory motions. In other words, a combined process of peening and finishing holds much attraction to the discerning shot-peening operation.
Research in collaboration with academia
Dr. Hong-yan Miao and Prof. Martin Levesque from Polytechnique Montreal studied the fatigue life improvements of a certain alloy type with conventional peening and compared it to vibratory peening. The parameters and results of their testing are as listed in the table.
All process parameters in vibratory peening, much like conventional shot peening, are variable. Alterations to vibration amplitude, media size mix, positioning depth and plane can bring about exponential changes in magnitude of intensity and residual stress. Such alterations that result in residual stress values comparable to that from shot peening are noticed to extend over a greater depth with vibratory peening.
The concept of 'mix of media sizes' in vibratory peening is anomalous when compared to conventional peening where we follow a strict regimen of constant media size and maintain it using an inline size classifier. Though the reasoning behind the mix of sizes is proprietary in nature, a logical guess would be that media movement in the bowl due to a mix of different sizes is a contributing factor to the benefits of adopting this technique.
Results from roughness measurements (as machined, shot peened and vibratory peened) reveal the results in Table 1:
In addition to the above, the study also tested fatigue lives from both processes. The results displayed better consistency with shot peening (lower variation). However, given the vulnerability to failure due to a rougher surface profile, it would be interesting to compare the fatigue life measures at different levels of surface roughness.
Fatigue tests performed as part of this study generated similar average fatigue lives for both processes. However, the study did find that the values from shot peening had significantly less standard deviation (minimal variation). The study concluded that rather than comparing similar Almen intensity values, future studies should compare the fatigue life measures for similar residual stress profiles, at different levels of roughness. Ultimately, the measure of all such processes is the extent to which fatigue life can been impacted, in the positive direction.
Yet another study carried out by M Sangid, J Stori and P Ferriera (Process Characterization of vibro-strengthening and application to fatigue enhancement of aluminum aerospace components–part I: "Experimental study of process parameters"), published in the International Journal of Advanced Manufacturing Technology in August 2010 revealed interesting findings.
The study used ceramic media and observed that excessive media wear led to deposits of ceramic powder at the bottom of the tub, forming a slurry that affected the process. Fatigue life improvements dropped with media wear.
Experiments carried out on a combined (shot peening and vibratory peening) process concluded that it was inferior to using just vibratory peening.
The study recommended conducting vibratory peening at high frequencies albeit the rougher surface achieved. The residual stress produced outweighed the resultant surface roughness as part of the process.
Though the magnitude of residual stress produced was greater with shot peening, the dramatic improvement in surface finish with vibratory peening contributed to longer fatigue life.
Though the study encourages use of vibratory peening for complex geometries, it places a gap limitation of 25 mm between fatigue critical surfaces for the process to be effective.
Common industrial components and vibratory peening
The next logical step was to identify application suitability and establish process boundaries/limitations. The first set of tests was geared to using it to process commonly-used components. The two components chosen were based on their metallurgy and significantly different geometry. When shot peening a component with relatively closed geometry in an airblast machine, it is entire possible to aim and manipulate nozzles, and dwell the blast stream etc. to reach intricate areas. In other words, the dexterity available with a blast nozzle is quite high. On the contrary, when a part is submerged in a ton of media, as in a vibratory bowl, the process is heavily reliant on media movement that isn’t directly controlled. The parts chosen for this test were a transmission gear and turbine blade from a land-based turbine, the latter with a fairly open geometry in comparison to the former.
Gear: Surface finish improvements on all surfaces of the gear, root, tooth faces and top of tooth were not significant with vibratory peening from initial conditions. Two identifiable reasons for this are (a) hardness of the gear and (b) size of steel balls compared to the radius of the tooth. The teeth geometry allowed for less than a 25 mm gap, validating test results in research referred to earlier. On the other hand, with conventional shot peening, size of shot (S-110), combined with nozzle dexterity and directional capability of the blast stream contributed to better coverage on the fatigue-critical root area of the gear. Additionally, the media in the shot-peening machine would have had to create a deeper dent than the 3mm diameter balls, also leading to a rougher surface than vibratory peening.
X-ray diffraction was carried out on two identical samples of gears. The results pointed towards greater surface residual stress in the shot-peened sample as compared to that achieved with vibratory peening. However, the relaxation of this stress towards the depth of the material was much more controlled with vibratory peening. The lower surface stress with vibratory peening follows the trend of all previous test results. On the other hand, the steep drop noticed in the shot-peened component could possibly be due to the surface roughness.
Blade: Open geometry of the blade allowed for greater media access in both processes. The blade was finished to an appropriate smoothness as expected in vibratory peening, and to a significantly greater degree of roughness with conventional peening.
The airfoil section of the blade was targeted for stress measurements. Interestingly, the compressive stress generated at the surface was greater with vibratory peening when compared with the shot-peened sample. The open geometry of the part and softer material likely contributed to this result. As with the gear, the relaxation of residual stress when going deeper into the component was drastic with the shot-peened part and followed a gradual decline with the vibro-peened component. In both cases, an obvious inference is that the geometry of the part determined the magnitude of residual stress generated.
Conclusions and commercialization
Consider a scenario where a standard golf iron when processed through a vibratory peening machine adds extra distance and angle to the golfer’s game. Or a critical medical implant is manufactured with strength and finish in a single cycle, or a flight-critical engine component that benefits from vibratory peening as compared to the stress loss drawbacks of a combined process of conventional shot peening and polishing. All these possibilities are very real, and vibratory peening holds much potential for industrial applications as well as commonly used components outside the industrial world. The benefits of residual stress generation along with a smooth profile in a single step are tremendous. Though still at a relatively early stage, the process has demonstrated enough promise to continue exploration with different part types and to learn more about the possibilities of expanding its reach to benefit a greater variety of components.