( 99 4% ) and steel O.45%C )

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1 ..~. 98 Proc. 1st Japan International SAMP Symposium, Nov. 28- Dec SURFAC AND AFFCTD LAYR OF SHOT PND TITANIUM KISUK IIDA, KATSUJI TOSHA Meiji UniversityjDepartment of Mechanical ngineering Higashi-mita., Tama-ku., Kawasaki" J apan Abstract The basic properties on the peened surface and surface layer were studied experimentally on titanium (99.4%) and steel (0.45%C). The surface and the affected layer consist of dent, surface roughness, work hardened layer and residual stress. ~The centrifugal peening machine and steel shot was used. The ratio of work hardening and the other influence produced by shot peening for titanium are larger than steel. 1. INTRODUCTION Titanium is a new material for automobile and aerospace in- surface, involving work hardening and work softening zone and the gradients of residual stress and half width. Shot peening is importa!1:t process automobile and the for the aerospace industries, because the shot peened parts are improved the strength on fatigue, corrosion and wear, (1) (2) (3) but reports are comparatively few on the titanium as to change of mechanical properties after shot peening. As the first step, shot peening was performed for titanium ( 99 4% ) and steel O.45%C ) under various peening conditions dustries. In these fields machine such as coverage, shot size and parts are required trustful velocity, thickness of specimen. properties for fatigue, stress Obtained data are diameter of corrosion, wear, etc. dent, surface roughness, hardness Shot peening produces the distribution and surface residual affected layer under peened stress.

2 K. rida, K. Tosha: Surface and Surface Layer of shot Peened'Titanium 2. PNING CONDITIONS AND PROCDUR The experimental conditions of shot peening and specimen are shown in Table 1. for measurement of dent, The equipments surface roughness, hardness and surface residual stress are shown in was performed until full coverage time. Diameter of dent was measured crosswise and averaged. Three surface profiles of one specimen were recorded without cut-off and Rmax was calculated from the re~ords. Table 2. Vickers hardness distribution Area coverage, defined from the ratio of the area of the dents to the whole peening area, initially increases rapidly and then slowly increases. Shot peening Table 1 Peening machine centrifugal type material cast steel HV: 800 Shot size D mm 0.32, 0.55, 1. 1, 2.2 velocity V m/s 17.5, 25, 30, 35 Peening time T s 1 - full coverage time Specimen. Residual stress material xperimental conditions commersially pure titanium 99.4%(HV158) annealed carbon steel 0.45 %C(HV180) size W:25, L:2;, t:10 X-ray di fraction sin 2 \V, method Ti: ( 3,0 2 )plain steel: C )plain was obtained from perpendicular section of peened surface, and averaged at the same depth data on the three positions. Surface residual stress was calculated formula tometer. (JR= - using dze 2(1+11) cot eo dsin21j where :113 GPa, v:o.321 for titanium, and : for steel, 206 GPa, v:0.28 angle, 2 e :diffracted angle, w: inlet angle of X-ray. Microstructure of titanium is shown in Fig.1. from diffracthe X-ray following 80 :standard Brag Table 2 quipments of measurement Measurement equipment Dent tool microscope magnification: )(]O Hardness micro vickers tester load: 100, 200 g Surface profile- recorder roughness magnification: X 2000 X-ray diffractometer Residual Cu ( Ti ) stress target Cr ( steel ) 30kV, 35mA Fig.1 Microstructure of titanium ( 99.4 % ) 100

3 Proceedings of the First Japan International SAMP Symposium, Nov. 28- Dec. 1, XPRIMNTAL RSULTS The peened surface is consisted from a number of dent. 3.1 Diameter of dent (d mm) Dent is a basic factor determining the surface roughness and the affected layer. (4) The relation of diameter of to dent velocity and size of shot are shown in Fig.2. dent, next equation From diameter of velocity and size of shot, is obtained. xponents of D and V are the same on steel and its coefficient kd is similar to the steels. (5) 3.2 Surface roughness (Rmax pm) Surface roughness is a harmful factor in shot peening for fatigue strength of steel. (6) The Influences of variables on surface roughness are shown in Fig.]. By the same way as the d = kd D V V2 1 o mm 1.0 d =kd-:v T./ 2.2 -c ea" 0.5 /././ 1.1 ~ 00.0 Q).+-J 0.3 0/ -I-J c: ".. (]) Q) A~ 0.55 f1:i 0.2 (1j J.: ri ~ A.././ Q a /' D;t:i ~/t::: ( a ) /' Velocity V 'ni'~s-1 of shot ( 1 ) Rmax =krj V D mm.-/ 2.2 / /- / 1.1 /e o,cp / 0.55 P / ~ / ::./ ~ ~ tf / ( a ) / ~ Velocity V m s- 1 of shot -a ~ 0.1 +J ~ 0.0'5 M CJ ri Q) 0.02 a (b ) C) 0.01 '---""--'Io.~~"""'--_--'--~ Shot size 0 mm 1.+J C Q) M 0.6 U ~ 0.4 ~ (l) o u Shot size D mm Fig.2 The relation of diameter of dent, velocity and size of shot ( Ti ) Fig.] The relation of surface roughness, velocity and size of shot ( Ti ) 101

4 K. Iida, K. Tosha: Surface and Surface Layer of shot Peened Titanium relations on dent, equation is obtained. Rmax = k R D V xponents of D and V the same on steel. (7) the following are 3.3 Hardness distribution Hardness (2) also distribution induced by shot peening is classified types; three work-hardening, work-nonhardening and work-softening. (8) As shown in Fig.4, the type of hardness distributions are workhardening. Hardness ' a....../.--- o~./ co I /,,/.. 0/- I I 0.4 8\ /0/ / I ~ oe u ',,/ ~ OJ..c: eu 0.8 +J~ ~J.,.{ o U1 I, HV eli o mm V m.5 ~ 2.2,35 (]) =' , J7.5 I oe ( Fig.4 Hardness distribution ( Ti ) -1 is larger than that of steel from 15 % to 25 %. The depth of work hardened layer of titanium is also larger than that of steel about 25 %. Hardness Hmax s ( a ) titanium e - 60 _e--- _--0-- : e _ ::r :c steel I 20 ( b) co 0.2 ( c ) 0.1 ~--,-----o.---""""'-1""o---~ Kinetic energy g.(m.s,)2 of a shot Fig.5 Maximum hardness and depth of work hardened layer versus kinetic energy of a shot 3.4 Surface residual stress As mentioned above, area coverage increases with peening time and surface residual stress also Defining the maximum hardness changes with peening time. The (Hmax) and depth of work hardened influence of peening time on layer ( c5) as shown in Fig.5(a), surface residual stress is shown the influences of the kinetic in Fig.6. In early stage, surface energy of shot on them are shown residual stress increases in Fig.5(b) and (c). The more the rapidly', and then saturates kinetic energy of shot, the more before full coverage. The the maximum hardness and the saturate time of surface residual depth of work hardened layer. The stress of titanium is shorter work hardening ratio of titanium than that of steel.

5 ~ 400 a. : to I 100 Fig.6 D:2.2mm, V:35m-s 1. steel /' o,~_ _. /. I / titanium 9 II I o "-----J. --L. --J.. --'-_--' o Peening time T 5 Influence of peening time on surface residual stress The influences of peening variables on surface residual are shown in Fig.? stress The influence of diameter of shot is larger than that of velocity of shot. In the case of shot peening, surface residual stress shows size effect for the thickness of steel specimen. (9) Critical thickness tc shown in Fig.8 means the minimum thickness to induce constant surface residual stress in the same shot peening conditions, and tc of titanium is larger than that of steel. The ratio of depth of work harthickness dened layer to of specimen is 5 as the same as steel. When the thickness of specimen equals to the depth of work hardened layer, surface residual stress is not induced by shot peening- ~300 I 200 -e ( a ) Velocity of shot V m.~ ~ ~.' (b) :400~ ~~~... ~---~~ V m.i l a: 300~ ~!---35 '0 "'" I ~ L..~ 1.'.J..'.L I_-" '_I Fig.? o 1 2 Shot size 0 mm Surface residual stress versus velocity and size of shot ( Ti ) co a. 400 : ~, 100 o Fig.8 0: 2.2 mm, V: 35 m s 1 tc steei 6_ /._e_ /...-- titaniu d./ I./ /ti tc /0 o 4 8 Thickness of specimen Influence of thickness of specimen on surface residual stress 4. CONCLUSIONS mm 1) The formula obtained from titanium is similar to steel and shows next relation between shot 103

6 K. Tida, K. Tosha: Surface and Surface Layer of shot Peened Titanium size, velocity and diameter of dent: d = kd D V 1./2 2) The relation between shot size and velocity and surface roughness is also similar to steel. Rmax = k A D V 3) Type of hardness distribution is work-hardening. The ratio of maximum hardness to the matrix is larger than that of steel from 15% to 25%. The depth of work hardened layer of titanium is from 0.2 to 1.2 mm, and larger than that of steel about 25 %. 4) Surface residual stress of titanium is compressive from 250 to 420 MFa similar to steel. 5) The critical thickness of titanium is large~. than steel about 25 %. The ratio of thickness of specimen to the critical thickness is the same as steel. 5. RFRNCS (1) J. C. Straub: Special Performance of Transmission Parts by Shot Peening, SA730800,1, (1973) (2) W. H. Friske & J. P. Page: Shot Peening to Prevent the Corrosion Cracking of Austenitic Stainless Steels, ASM, J. Material for nergy Systems, -1, June, 20 (1979) (3) Shot Peening, 5th d., Wheelablator Corp., 24 (1965) (4) K. Iida: Dent and Affected Layer Produced by Shot Peening, Proc. of 1st International Conf. JSP, 51, 8, 1569 (1985) (7) K. Iida & K. Tosha: The Hardon Shot Peening, May, 283 (1984) (5 ) K. Iida & K.Tosha: On the Basic Properties of Shot Peening, J. of JSP, 39, J, 287 (1973) (6) K. Iida & K. Tosha: On the Relation between Peening Condition and Fatigue Strength, J. of ness distribution and the Work Softening under Peened Surface, J. of JSP, 41, 8, 796 (1975) (8) K. Iida & K. Tosha: Fatigue Strength of Work Softening Layer Produced by Shot Peening, Froc. of the 3rd International Conf. on Shot Peening, Oct. 611 (1987) (9) K. Iida & K. Tosha: Variation of Surface Residual Stress Induced by Shot Peening, J. of JSP 52, 3, 535 (1986)