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Characterization of the Size and Shape of Shot Media for Standards DevelopmentAndrew Babiuk-Murray, Torie Lichti, Nikole McPheron, Michael ThoenenFaculty Advisors: Paul Mort, Mark Gruninger, Siavash GhanbariIndustrial Sponsors: Jack Champaigne, Tod Canty, Joe McGreal, Kumar Balan, Shota Watanabe, Larry Catanzarite, Paul Leplay, David Bouttes, Frederic Wiss, Anne-Laure Beaudonnet, Jeff Girman

Project Background

Experimental Procedure

Results

Recommendations

Discussion

MSE 430-440: Materials Processing and Design

Size ParameterMinimum Feret diameter (xFmin) is the shortest distancebetween two parallel tangents of a 2D projection. Sievingis compared to minimum Feret. Graphs below show theimage analysis data with sieve data overlain (red dots).

Shape Parameter

This work is sponsored by Electronics Incorporated, Mishawaka, IN, JMCanty, Buffalo, NY, Ervin Industries, Ann Arbor, MI, Toyo Seiko NorthAmerica, South Bend, IN, and Saint Gobain, Courbevoie, France.

• Shot peening is commonly used in the automotiveand aerospace industries, and uses small,spherical shot material bombarded at the surface ofa metallic part to impart compressive surfacestresses to improve fatigue resistance, preventstress corrosion cracking, and can improve thelifetime of the parts.

• Current standards for shot characterization involvethe use of sieves, only providing information on thesample’s size distribution.

• Image analysis is a method that can be used tocharacterize size and shape; using a camera andthousands of particle measurements. The largesample size allows for development of meaningfulstatistics to describe full distributions.

• To fully characterize media, minimum Feretdiameter is chosen to quantify size, while acombination of Elliptical Form Factor, EFF =βπA/P2, and Aspect Ratio, AR = XFmin/XLF arechosen to quantify shape.

Sieve analysis the current characterization method for shot media across the shot peening industry.Using sieves limits characterization to size and is unable to place specification limits on shape. Thespread of sieve mesh sizes is large, limiting resolution of particle size measurements. There is interestin using image analysis as a supplement or replacement for sieve analysis which allows for accuratesize and shape measurements of thousands of particles. The improved resolution of particle datashould allow for improved consistency and less variation among shot media. In order to implement thischange, the process must be adaptable and feasible across the industry for use with multiple mediatypes.

Canty SolidSizerA Canty SolidSizer supplied by JMCanty is used for image analysischaracterization of cast, cut-wire,and ceramic shot media. A samplesize of 50 mL of shot is placed intoa hopper above the vibratory feedtray. Vibrations cause media to fallover the edge in front of theparticle sizing camera. Thecamera and software included withthe SolidSizer calculate size andshape parameter values for eachparticle imaged.

SievingSieve analysis is performedusing sieves ranging from 2.0mm at the top to 63 μm at thebottom. Approximately 500grams of media is placed ontothe top (coarsest) sieve and thestack is shaken for threeminutes until all the media hassettled. The remaining mass ofmedia on each sieve ismeasured.

Ervin Test MachineAn in-use proxy test isperformed using an ErvinTester supplied by ErvinIndustries to characterizebreakdown of media duringa peening cycle. To test,100g of fresh shot is addedto the test drum and theTester is run for 500 cyclesat 7100 RPM. After each

Particle Technology Labs

Ervin Test Machine, Ervin Industries, Ann Arbor, MI.

round, the sample is sieved using a 40-mesh control sieve.The amount retained is weighed and fresh shot is added tomaintain a mass of 100g. This procedure is followed until totalmass loss equals 50% of initial mass. Samples arecharacterized between rounds using the SolidSizer.

Breakage

Aspect Ratio (AR) and Elliptical Form Factor (EFF) areparameters that account for both the angularity andelongation of a particle. For cut-wire shot it can be seenthat as conditioning increases both parametersapproach a value of 1. For cast shot, some particlescontain “satellites” or agglomerates attached to theparticle due to the processing techniques. Plots showthe basic archetypal shapes, potential satellite size andresulting ratios.

Image of particle taken usingimage analysis showing various measurement methods.

Graph depicting the ideal thresholds for shot and zirconia shot material. Note the 190 is a reference threshold found in between the two.

Graph depicting the AR and EFF for as cut cut-wire shot. Note that plot contours are for archetypal shapes.

Graph depicting the AR and EFF for cast shot. Note the tiny extra particles called “satellites”.

Hardened steel shot, regular steel shot and as cut cut-wire shot are tested in the Ervin Tester. The criticalimpact number, I*, for each sample was different.Hardened steel shot had the lowest I* value, indicatingfast breakdown due to its hardened state. As-cut CWshot had the largest I* value indicating slow breakdownand higher durability.

Size ParameterMinimum Feret diameterprovides reasonable correlationto sieving. It is also well definedin current ISO specificationsand when compared to otherpossibilities, i.e. max inscribeddiameter (Dimax), it requiresless computational overhead.Video analysis is utilized tocompare various thresholds on

Graph showing the image analysis data of xFmin with sieve data (dots) overlain for cast shot (ASR 230).

identical particles. The composite model predicts the xFminat various thresholds using a probit function, allowing forlinear regression. The equation for the model can berearranged to solve for the threshold, thus allowing for the“ideal threshold” to be found for each material type: 214 forzirconia, 173 for steel. Based on this information areference threshold of 190 is suggested. This is due to 190being a good middle ground between the two withouthaving to account for optical/material properties.

A composite model was created to predict the xFmin atvarious thresholds. Based on the graph below it is seenthat the model provides a good correlation. Using themodel, the ideal threshold can be found for eachmaterial type. Information from the sieve analysis andthe image analysis are used to calculate the threshold.The threshold affects what is included as a part of theparticle during image analysis.

Graph showing the predicted xFmin versus the measured xFmin. Note the grouping along the 45-degree line indicates a good fit model. Shape Parameter

Breakage

Sample I* (50% replacement)

ASH 230 1000

ASR 230 2000

AWCR28-AC 3000

Table displaying the differentcritical impact numbers, I*, foreach sample tested. The criticalimpact number represents thenumber of impacts to replace50% of the original mass.

Form

Fac

tor

Impacts / I*

Minimum Feret is the best size parameter for correlation withsieving. Two factors are needed to adequately describe theshape of particles. A reference threshold of 190 isrecommended for image analysis. It could be of interest tofurther investigate the breakage using shot from an actualpeening operation as well as investigate the effectiveness ofindustry used tools like the spiralator that are not in currentspecifications.

Area

-eq

Dia

, xA,

um

Hardened and regular steel shot had similar trends forsize (xA) and shape (FF). The hardened media exhibitsmore breakdown of the low quantile tails seen with thespread of red circles on both plots. The as-cut cut wireshot experienced significant rounding in the first 500cycles and had minimal size change during testing.

Multivariate plots of xA and FF at quantile, Q, normalized to critical impact number. Black lines represent averages for each sample.

Increasing angularity

Incr

easi

ng e

long

atio

n

Canty SolidSizer, JM Canty, Buffalo, NY.

Images of various particles exemplifying values for the shape parameters.