I REPRINTED BY I Reprinted from PROGRESS November 1963 METAL IMPROVEMENT CO. "The Magazine of Materials and Process Engineering'' Published by the American Society for , for 37,000 member-readers h .

Peening Increases Strength of Welded Aluminum

The fatigue strength of longitudinal butt welds in aluminum plates can be increased significantly by peening the weld and the heat affected zone. Because of surface compressive stresses introduced by peening, the improvement in fatigue life is greater than that produced by thermal relief. (G23n, Q7a; Al-b, 7-51)

REsIDufi mm wmucm in magnitude of the residual stresses (tensile weldments by local thermal expansion, plastic residual stresses in the region of the weld are deformation and subsequent shrinkage on balanced by compressive stresses in regions cooling. In the region near the weld, the re- away from ihe weld), the halves of the speci- sulting tensile residual stresses have a magni- mens were machined to size before . tude approaching the strength. Welded samples were shot peened on both Welding stresses significantly reduce the fatigue strength of aluminum welds, particu- larly longitudinal butt welds. But because Fig. 1- (shown) or harn- thermal treaments to relieve these stresses rner peening significantly increases are impractical for many large welded assem- the fatigue life of welded aluminum. blies, peening can and should be considered as an alternate method of improving the fatigue strength. ' Butt Welded ~axk~ler To determine how much peening increases the fatigue strength, we prepared longitudinal butt welded joints from 0.40 in. thick 5456- H321 plate. The abutting edges were beveled to provide a single V-groove, and the groove welded automatically in two passes employ- ing 5556 electrode; then the weld was back chipped and f3ed with a third pass. Since removal of any material from the edges of the test section after welding would reduce the *Research Engineer, Alma Research Laborato- ra'es, New Kensington, Pa. Several samples were also hammer peened using rounded, single point chisels in impact hammers. A lighter peening was obtained Shot peened with % and Y4 in. diameter tools in a pneu- As welded matic hammer, and a heaeer hammer peen- Thermally stress relieved ing was obtained with a 3/16 in. diameter tool in an electric hammer. Residual Stresses Measured Residual (112 bv Typical levels were deter- mined mechanically by saw cut and layer removal methods. Figure/2 shows how shot peening or thermal stress r,elief affects residual stresses at a location adjacent to the weld. Surface residual stresses at this location were about 25,000 psi (compression) for the shot peened specimen, zero for the specimen given a thermal stress relief, and 5000 psi (tension) for the as welded specimen. In the weld itself, the maximum residual stresses of the latter specimens were about 17,000 psi tension. Failures Not in Peened Area Axial-stress fatigue tests - conducted at minimum stress to maximum stress ratios of -0.5 and 0.0 in Templin structural fatigue machines (50,000 Ib capacity) - produced fail- IXPS 02fside the pee~edarea i:: everj. instance. The ioads were transmitted to the specimen through keys pressed into the specimen by I+--- Specimen ~hickness-I tightening bolts through the butt ends. Be- Fig. 2- Shot peening produces cause of this, it was necessary to machine the relatively high compressive resid- weld beads flush in the butt ends. This ual stresses at the surface; a dressing usually removed most of the shot thermal treatment reduces resid- ual welding stresses to a low peened surface from these areas. The origin magnitude. Data were obtained of failure was either at the edge of the bolt on a residual stress sample holes, or more commonly, at areas of fretting (shown) of welded 5456-H321 plate. 1 under the loading key (see Fig. 3). Failures i in as welded and stress relieved fatigue speci- sides by moving them at a constant speed mens occurred in the weld within the test under a nozzle (12 in. away) emitting S230 section. steel shot propelled by 90 psi air pressure. Even when failures occurred prematurely This treatment resulted in a maximum in- in butt ends, both shot and hammer peening tensity of Alrnen 0.013A at the center of the improved the fatigue strength more than did weld. As shown in Fig. 1, the shot peened a thermal stress relief. This is shown in Fig. area did not extend to the edges of the welded 4. For each stress ratio, the longest lived plates. In a few instances, the shot peened shot peened specimen had a fatigue strength samples were straightened by additional peen- about 100% higher than the curve for as ing to a tolerance of % in. on center bow. welded material. Hammer peened weld Generally, warpage was within this limit if metal subjected to a 0.0 stress ratio loading the side which was concave after welding was also was increased significantly in fatigue peened &st. strength. @ Fig. 3-Fatigue failures ot all shot peened test specimens were outside the peened area, and most were caused by fretting under the loading key.

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50 -- awl 8 40 0 d Fig. 4-The effect of SZ shot peening, hammer ~30 r peening or thermal E stress relief on the ;5 fatigue life of longi- 5 20 M tudinal butt welds in 0- & 5456H321 aluminum is LLCO shown here. Strefs-ra- 10 = minimum stress maximum stress. Strength values repre- 0 sent maximum stress 10' 10' lo6 10' lo8 10' 105 106 10' lon applied intestingcycle. Cycles EXECUTIVE OFFICE 10 Forest Avenue METAL IMPROVEMENT COMPANY, INC. Paramus, New Jersey 07652 SUBSIDIARY OF CURTISS-WRIGHT CORPORATION TeI: (201) 843-7800 Telex: 6291 1203

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