PSI - Issue 23

Barbara Nasiłowska et al. / Procedia Structural Integrity 23 (2019) 583 –588 Nasiłowska B. / Structural Integrity Procedia 00 (2019) 000 – 000

587

5

5. Influence of shot-peening on mechanical strength

Static tensile tests of 5754 aluminium flat specimens with a front weld proved that strength properties were improved under an influence of shot-peening. During the welding process, an undercut of the ridge is performed, which results in introduction of the grinding scratches. They constitute geometric notches thereby they are a place for stress concentration. Shot-peening of the surface layer introduce a new state of compressive stress. Although the surface roughness undergoes changes, the grinding scratches are refined, which results in an increase in mechanical strength of 5754 aluminium alloy.

Table 2. Mechanical properties of 5754 aluminium alloy

Aluminium alloy with a shot-peened weld

Aluminium alloy with a non shot-peened weld

Tensile strength R m [MPa]

113

109

Yield strength R p0,2 Elastic modulus G

60

49,66

4,48  10 4

3,47  10 4

A cleavage fracture of the specimens welded with TIG method from 5754 aluminium alloy, originated during the static tensile test (Fig.5), occurred in the recrystallization zone. Static fractures before the surface shot-peening were characterized with a layered structure of the material resulted from rolling. While the necking was being extended, the shear of material layers occurred until the cleavage fracture was originated. Local sheers of the material also prove the material ability to considerable plastic strain in the analyzed welded joints. In the fracture of the tensile static tests, a numerous secondary fractures were observed.

a

b

Fig. 5. Cleavage fractures after the static tensile test of 5754 aluminium alloy with a front weld made with TIG method, for (a) non shot-peened specimen; (b) shot-peened specimen

Analysis of the microfractography originated after tensile tests of 5754 aluminium alloy with a front weld strengthened in the surface shot-peening process showed delamination of the surface layer at the dept h of approximately 150 µm (Fig. 6). There also occurred numerous allocations and inter-metallic phases (Fig.7), the dimensions and arrangement of which may considerably influence the mechanical strength (Orłowicz et al. (2006)) . Alloys of 5754 aluminium containing magnesium tend to originate phases on the basis of Al 8 Mg 5 , Mg 2 Si, Al 2 CuMg, Al 5 SiMg 8 Cu 2 compounds. Addition of magnesium in the aluminium alloys allows strengthening of the solid solution α(Al) with tiny allocations of Mg 2 Si phase during thermal processing, namely, the welding process. An excessive content of magnesium may, however, adversely influence

Fig. 6. A secondary crack in the upper layer

mechanical properties of the alloy, due to its high susceptibility to originating oxides (Ravi et al. (1998)). In the presence of iron, such inter- metallic phase as α(Al 15 Fe 3 Si 2 ), β(Al 5 FeSi) and π(Al 8 Mg 3 FeSi 6 ) originate in the alloy.