The relationship between extrusion die line roughness and high cycle fatigue life of an AA6082 alloy
Introduction
High cycle fatigue performance (>105 cycles) of most metallic alloys is governed largely by the process of fatigue crack initiation (FCI), which (in turn) depends on the localization of plastic deformation under cyclic loading conditions below the nominal yield strength of the alloy [2]. Localization of cyclic plastic deformation and FCI in engineering alloys subject to high cycle fatigue is most often associated with features near the surface of the test specimen, such as roughness, flaws, inclusions and other inhomogeneities within the microstructure. The influence of die line surface roughness on the fatigue properties of AA6082 alloy extrusions is the primary subject addressed in this paper.
The increased demand for fuel efficiency of automobiles has led the automotive industry to invest in the evaluation of light weight material substitutions for critical components. The abundance, cost, strength and weight of aluminum, specifically hollow extruded aluminum profiles, make them suitable for manufacturing replacements for heavier ferrous components. Hollow extruded aluminum profiles have already been incorporated into impact critical components, such as frames and bumpers; and their use could be expanded into cyclically loaded applications if the fatigue behavior of these profiles is well understood. Possible future applications of hollow extruded AA6082 aluminum profiles in automobiles include engine cradles and suspension components or assemblies. The use of hollow extruded aluminum in such fatigue critical components will most likely involve the as-extruded surfaces; therefore it is important to explicitly study the effects of surface roughness, originating from the extrusion process in the form of die lines, on fatigue life.
The formation of die lines on the surface of extruded profiles may degrade fatigue properties. Die lines can create stress concentrations at the surface of the extrusion, leading to premature fatigue crack initiation at their roots. The origins of die line formation and the optimization of extrusion variables to reduce die lines has been studied extensively [3], [4], [5], [6], [7], [8]. These studies have mainly focused on the effects of die lines on anodizing and aesthetic finishes of extruded profiles for architectural applications. Even though surface roughness is generally understood to strongly influence fatigue life of metallic materials [9], [10], [11], the influence of die line roughness on the fatigue properties of extruded aluminum profiles has received little attention. The removal of surface recrystallized layers by machining in fatigue critical aerospace applications could be a reason for the lack of attention to die line effects on fatigue. Also, many extruded components are fusion welded, and effects of fusion welds may supersede surface roughness in fatigue failure.
Seam welds are solid-state welds that form as a result of material flow around die mandrel supports, and these welds extend along the entire length of an extruded profile. As-extruded specimens taken transverse to the extrusion direction in a hollow profile will have both seam welds and die lines oriented transverse to the loading direction. Polishing allows the effects of seam welds on fatigue life to be examined independently of the die line stress concentrations.
The results presented here address the effect of die line roughness on the high cycle fatigue behavior of hollow extruded aluminum profiles. Die line effects were determined by comparing specimens where the die line surface roughness was removed by grinding and polishing with previously reported fatigue properties of specimens from the same extrusion having as-extruded surfaces [1]. Tensile fatigue specimens taken transverse to the extrusion direction, i.e. transverse to the direction of die lines, were the main focus of the study, but a few as-extruded and polished specimens were taken longitudinal to the extrusion direction for comparison. To further understand the effects of die line stress concentrations, some polished specimens taken longitudinal to the extrusion direction were artificially notched to various depths and cycled at a moderate stress level until failure. Elastic stress concentrations originating at die lines were calculated using finite element analysis. The stress concentration values were used to predict the fatigue run-out stresses at 10 million cycles for as-extruded transverse specimens.
Section snippets
Experiment
Fifteen sections of press-spray-quenched extruded AA6082, each approximately 150 cm in length, were obtained from Hydro Aluminum, Holland, MI. These 15 sections represent the total output from one fully extruded billet. The chemistry of the billet is shown in Table 1 and the extruded profile cross-section is shown in Fig. 1. The thicknesses of walls A, B, and C were 2.5, 2.8, and 2.7 mm, respectively; and the orientations and locations of fatigue specimens with respect to seam welds are indicated.
Fatigue of as-extruded and polished specimens
The fatigue test results for polished L-NW, T-NW and T-W specimens are presented as the data points in Fig. 5a–c. Previously reported results for comparable as-extruded specimens are also provided in Fig. 5a–c as curves, which are power law fits to the data [1]. For the as-extruded samples, fatigue crack initiation was almost always associated with the as-extruded surface, and only rarely did crack initiation occur at the corners or on the milled edge of the specimen gage section. For the
Summary and conclusions
Die lines play a significant (even dominant) role in fatigue failure of as-extruded AA6082 when a cyclic tensile stress component lies normal to the die line orientation. The influence of die lines on fatigue run-out stress amplitudes can be adequately predicted using elastic stress concentrations calculated from surface profilometer measurements and FEA.
Comparison of fatigue results for as-extruded, polished, and polished and notched specimens suggests that fatigue lives significantly below
Acknowledgments
The authors would like to express their gratitude to Hydro Aluminium, ASA for the support of this research. Also, contributions from Mike LaCourt in the micro-milling of notched specimens were greatly appreciated.
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