A Numerical Study into the Effect of Machining on the Interaction between Surface Roughness and Surface Breaking Defects on the Durability of WAAM Ti-6Al-4V Parts

Abstract

The airworthiness certification of military aircraft requires a durability analysis be performed using linear elastic fracture mechanics (LEFM). Furthermore, such analyses need to use a valid small crack growth equation. This paper focuses on the effect of rough surfaces and the effect of machining the surface on the durability of AM parts using LEFM and a valid small crack growth equation for the material. To this end, this paper analyses the effect of surface roughness on wire and arc additively manufactured (WAAM) Ti-6Al-4V titanium parts and the effect of machining on the durability of a part. The analysis reveals that the life of the component is a relatively strong function of the degree of surface roughness, and that the durability of a specimen is a strong function of the local radius of the curvature of the trough. It also appears that surfaces with tall narrow roughness will not overly benefit from partial machining of the surface.

Analysis of the Effect of Machining of the Surfaces of WAAM 18Ni 250 Maraging Steel Specimens on Their Durability

“…Practitioners in the field of additively manufactured structures have long been aware [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] that the interaction of surface-roughness with surface-breaking material discontinuities and surface-breaking porosity/lack of fusion can significantly degrade the durability and damage tolerance (DADT) of an additively manufactured part. Indeed, ref.…”

“…Indeed, this observation is consistent with the seminal finding reported by Schijve [20], and subsequently confirmed in [21][22][23][24], that for conventionally built parts internal cracks grow much slower than the surface-breaking cracks. Whereas [17][18][19] suggested that machining the rough surfaces may improve the DADT of an AM part, the studies presented in [2,15] suggested that partial machining may be of little benefit. However, the analysis presented in [2] examined an idealized (rough) surface where the surface roughness had a wave-like (sinusoidal) pattern.…”

“…Whereas [17][18][19] suggested that machining the rough surfaces may improve the DADT of an AM part, the studies presented in [2,15] suggested that partial machining may be of little benefit. However, the analysis presented in [2] examined an idealized (rough) surface where the surface roughness had a wave-like (sinusoidal) pattern. Consequently, noting that: i.…”

“…The United States Air Force (USAF) MIL-STD-1530D [25] states that the certification of a load-bearing part must be based on analysis, and that the role of testing is to validate the analysis. Whereas [17][18][19] suggested that machining the rough surfaces may improve the DADT of an AM part, the studies presented in [2,15] suggested that partial machining may be of little benefit. However, the analysis presented in [2] examined an idealized (rough) surface where the surface roughness had a wave-like (sinusoidal) pattern.…”

“…Here it should be stressed that in contrast to the author's prior study [2], the analysis reported in the present paper is based on actual (measured) surface roughness and uses the same crack growth equation that has been shown [1] to predict crack growth in specimens with as-built surface roughness.…”