Effect of thermomechanical processing defects on fatigue and fracture behaviour of forged magnesium

Abstract

The microstructural origins of premature fatigue failures were investigated on a variety of forged components manufactured from AZ80 and ZK60 magnesium, both at the test specimen level and the full-scale component level. Both stress and strain-controlled approaches were used to characterize the macroscopically defect-free forged material behaviour as well as with varying levels of defect intensities. The effect of thermomechanical processing defects due to forging of a industrially relevant full-scale component were characterized and quantified using a variety of techniques. The fracture initiation and early crack growth behaviour was deterministically traced back to a combination of various effects having both geometric and microstructural origins, including poor fusion during forging, entrainment of contaminants sub-surface, as well as other inhomogeneities in the thermomechanical processing history. At the test specimen level, the fracture behaviour under both stress and strain controlled uniaxial loading was characterized for forged AZ80 Mg and a structure-property relationship was developed. The fracture surface morphology was quantitatively assessed revealing key features which characterize the presence and severity of intrinsic forging defects. A significant degradation in fatigue performance was observed as a result of forging defects accelerating fracture initiation and early crack growth, up to 6 times reduction in life (relative to the defect free material) under constant amplitude fully reversed fatigue loading. At the full-scale component level, the fatigue and fracture behaviour under combined structural loading was also characterized for a number of ZK60 forged components with varying levels of intrinsic thermomechanical processing defects. A novel in-situ non-contact approach (utilizing Digital-Image Correlation) was used as a screening test to establish the presence of these intrinsic defects and reliably predict their effect on the final fracture behaviour in an accelerated manner compared to conventional methods.

Nondestructive Methodology for Identification of Local Discontinuities in Aluminide Layer-Coated MAR 247 during Its Fatigue Performance

“…Increasing demands within the aviation industry for a new generation of turbine blade materials led to the development of new diagnostic techniques. Advanced techniques, such as the blade-surface images analysis [ 21 ], optoelectronic and thermographic methods [ 22 ], eddy current and ultrasonic methods [ 23 ], vibrothermography [ 24 ] or even Digital Image Correlation [ 25 ], allow for the identification of cracks, intrinsic defects, subsurface defects, pores and potential areas of crack initiation. The variety of destructive and nondestructive methods for nickel-based superalloy inspection allows us to identify potential reasons for the material decohesion.…”

A Method for Comparing the Fatigue Performance of Forged AZ80 Magnesium

“…In the majority of the available literature, the selection of these parameters has not been optimized in each study; in general, the temperature selection was such that it was high enough to provide sufficient material flow whilst not inducing any incipient melting defects, and the rate was constrained by pragmatic limitations of the forging equipment and avoiding edge cracking of the billet. Several recent works by Gryguć et al [3,4,[15][16][17][18][19][21][22][23][24] have investigated varying the processing parameters of Mg forging with the specific objective of improving the strength, fatigue performance, and improving the homogeneity of properties throughout the forging.…”

A Novel Method for High Temperature Fatigue Testing of Nickel Superalloy Turbine Blades with Additional NDT Diagnostics

“…Increasing demands of the aviation industry led to the development of new diagnostic techniques, that allow to identify surface cracks, intrinsic defects, subsurface defects, pores, and potential areas of crack initiation. These techniques include: blade-surface images analysis [8], optoelectronic and thermographic methods [9], eddy current and ultrasonic methods [10], vibrothermography [11], or even digital image correlation [12].…”

“…The mechanical properties and service life of turbine blades were mostly evaluated using standard specimens [12][13][14][15][16] and numerical stress-strain analysis or model predictions [2,[17][18][19]. However, the results obtained from standard specimen testing are different from these obtained from the full-scale turbine blade testing due to its structural complexity [20].…”