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. 2020 May 12;13(10):2216.
doi: 10.3390/ma13102216.

Effect of Various Peening Methods on the Fatigue Properties of Titanium Alloy Ti6Al4V Manufactured by Direct Metal Laser Sintering and Electron Beam Melting

Hitoshi Soyama  1 , Fumio Takeo  2
Affiliations
Free PMC article

Effect of Various Peening Methods on the Fatigue Properties of Titanium Alloy Ti6Al4V Manufactured by Direct Metal Laser Sintering and Electron Beam Melting

Hitoshi Soyama et al. Materials (Basel). .
Free PMC article

Abstract

Titanium alloy Ti6Al4V manufactured by additive manufacturing (AM) is an attractive material, but the fatigue strength of AM Ti6Al4V is remarkably weak. Thus, post-processing is very important. Shot peening can improve the fatigue strength of metallic materials, and novel peening methods, such as cavitation peening and laser peening, have been developed. In the present paper, to demonstrate an improvement of the fatigue strength of AM Ti6Al4V, Ti6Al4V manufactured by direct metal laser sintering (DMLS) and electron beam melting (EBM) was treated by cavitation peening, laser peening, and shot peening, then tested by a plane bending fatigue test. To clarify the mechanism of the improvement of the fatigue strength of AM Ti6Al4V, the surface roughness, residual stress, and surface hardness were measured, and the surfaces with and without peening were also observed using a scanning electron microscope. It was revealed that the fatigue strength at N = 107 of Ti6Al4V manufactured by DMLS was slightly better than that of Ti6Al4V manufactured by EBM, and the fatigue strength of both the DMLS and EBM specimens was improved by about two times through cavitation peening, compared with the as-built ones. An experimental formula to estimate fatigue strength from the mechanical properties of a surface was proposed.

Keywords: Ti6Al4V; additive manufacturing; cavitation peening; direct metal laser sintering; electron beam melting; fatigue; laser peening; post-processing; shot peening.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

Figure A1
Figure A1
X-ray diffraction pattern for the Ti6Al4V manufactured by DMLS and EBM through cavitation peening (CP), laser peening (LP), shot peening (SP) grinding (G), and cavitation peening after grinding (G + CP), compared with the as-built specimen.
Figure A2
Figure A2
Schematic diagram of X-ray diffraction, considering the surface roughness and depth of the contribution to the diffracted X-ray.
Figure A3
Figure A3
Aspects of the surface of the as-built specimen, before the fatigue test.
Figure 1
Figure 1
Geometry of the fatigue specimen for the displacement-controlled plane bending fatigue test. The thickness was 2.6 ± 0.2 mm for the direct metal laser sintering (DMLS) specimen and 2.0 ± 0.2 mm for the electron beam melting (EBM) specimen.
Figure 2
Figure 2
Schematic diagram of the peening section of the cavitation peening system.
Figure 3
Figure 3
Schematic diagram of the peening section of the laser peening system.
Figure 4
Figure 4
Schematic diagram of the peening head of the recirculating shot peening accelerated by a water jet.
Figure 5
Figure 5
Aspects of the specimen manufactured by DMLS, observed by a laser confocal microscope. (a) As-built; (b) cavitation peening; (c) laser peening; (d) shot peening; (e) grinding; (f) grinding and cavitation peening.
Figure 6
Figure 6
Aspects of the specimen manufactured by EBM, observed by a laser confocal microscope. (a) As-built; (b) cavitation peening; (c) laser peening; (d) shot peening.
Figure 7
Figure 7
Surface roughness of the as-built Ti6Al4V manufactured by DMLS and EBM and the Ti6Al4V manufactured by DMLS and EBM through cavitation peening (CP), laser peening (LP), shot peeing (SP), grinding (G), and cavitation peening after grinding (G + CP). (a) Arithmetical mean roughness Ra. (b) Maximum height of the roughness profile Rz.
Figure 8
Figure 8
Surface hardness of the as-built Ti6Al4V manufactured by DMLS and EBM and the Ti6Al4V manufactured by DMLS and EBM through cavitation peening (CP), laser peening (LP), shot peeing (SP), grinding (G), and cavitation peening after grinding (G + CP).
Figure 9
Figure 9
Surface residual stress of the as-built Ti6Al4V manufactured by DMLS and EBM and the Ti6Al4V manufactured by DMLS and EBM through cavitation peening (CP), laser peening (LP), shot peeing (SP), grinding (G), and cavitation peening after grinding (G + CP).
Figure 10
Figure 10
Improvement of the fatigue properties of Ti6Al4V manufactured by DMLS and EBM through cavitation peening (CP), laser peening (LP), shot peening (SP) grinding (G), and cavitation peening after grinding (G + CP), compared with the as-built specimen.
Figure 11
Figure 11
Aspects of the surface near the fracture of the specimen manufactured by DMLS, observed by scanning electron microscope (SEM). (a) As-built (σa = 301 MPa, N = 162,400); (b) cavitation peening (σa = 370 MPa, N = 291,600); (c) laser peening (σa = 400 MPa, N = 352,200); (d) shot peening (σa = 361 MPa, N = 764,800); (e) grinding (σa = 350 MPa, N = 141,000); (f) grinding and cavitation peening (σa = 450 MPa, N = 310,000).
Figure 12
Figure 12
Aspect of the surface near the fracture of the specimen manufactured by EBM, observed by SEM. (a) As-built (σa = 224 MPa, N = 510,300); (b) cavitation peening (σa = 328 MPa, N = 319,300); (c) laser peening (σa = 320 MPa, N = 420,900); (d) shot peening (σa = 350 MPa, N = 505,700).
Figure 13
Figure 13
Aspects of the fractured surface of the specimen manufactured by DMLS, observed by SEM. (a) As-built (σa = 301 MPa, N = 162,400); (b) cavitation peening (σa = 370 MPa, N = 291,600); (c) laser peening (σa = 400 MPa, N = 352,200); (d) shot peening (σa = 361 MPa, N = 764,800); (e) grinding (σa = 350 MPa, N = 141,000); (f) grinding and cavitation peening (σa = 450 MPa, N = 310,000).
Figure 13
Figure 13
Aspects of the fractured surface of the specimen manufactured by DMLS, observed by SEM. (a) As-built (σa = 301 MPa, N = 162,400); (b) cavitation peening (σa = 370 MPa, N = 291,600); (c) laser peening (σa = 400 MPa, N = 352,200); (d) shot peening (σa = 361 MPa, N = 764,800); (e) grinding (σa = 350 MPa, N = 141,000); (f) grinding and cavitation peening (σa = 450 MPa, N = 310,000).
Figure 14
Figure 14
Aspects of the fractured surface of the specimen manufactured by EBM, observed by SEM. (a) As-built (σa = 224 MPa, N = 510,300); (b) cavitation peening (σa = 328 MPa, N = 319,300); (c) laser peening (σa = 320 MPa, N = 420,900); (d) shot peening (σa = 350 MPa, N = 505,700).
Figure 14
Figure 14
Aspects of the fractured surface of the specimen manufactured by EBM, observed by SEM. (a) As-built (σa = 224 MPa, N = 510,300); (b) cavitation peening (σa = 328 MPa, N = 319,300); (c) laser peening (σa = 320 MPa, N = 420,900); (d) shot peening (σa = 350 MPa, N = 505,700).
Figure 15
Figure 15
Schematic diagram of the thickness of the specimen for the calculation of the bending stress.
Figure 16
Figure 16
Relationship between the experimental fatigue strength and fatigue strength estimated from the surface roughness, surface hardness, and surface residual stress.
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