Fatigue behavior of SiC particulate reinforced spray-formed 7XXX series Al-alloys

Abstract

This paper presents the fatigue results obtained in high strength spray-formed 7XXX series Al-alloys reinforced with the addition of silicon carbide particulate. This is one in a series of papers reporting on experimental studies completed to analyze the effects of processing and composition on numerous properties of these 7XXX series Al-alloys. In this study, testing was performed in tension on specimens with a constant amplitude axial fatigue cycle of R = −1.0. Test results were also compared with results available in the literature. The metal matrix composite materials displayed a large increase in fatigue strength in comparison to commercial ingot metallurgy 7075-T6 and T7 alloys. The enhanced fatigue performance of the spray formed alloys is attributed to low crack growth rates, resulting from inhomogeneous slip.

Introduction

The 7XXX series aluminum alloys are widely used in aerospace, automotive, and other high performance structural applications requiring a significantly high strength to weight ratio. As a result of this characteristic and the associated industry demand, much research has been focused on the improvement of their mechanical properties and the elimination of disadvantages associated with the coarse grains and macrosegregation of ingot metallurgy (IM) alloys. Material processing techniques that incorporate high cooling rates, such as powder metallurgy (PM), have been used to manufacture 7XXX series aluminum alloys with improved characteristics such as fine, segregation-free microstructures. However, PM aluminum alloys are associated with the formation of fracture inducing oxides on the powder surface, adversely affecting ductility [1]. The spray metal forming process has successfully minimized this problem by incorporating powder atomization and consolidation into a single step within an inert atmosphere [2]. As a result, spray formed aluminum alloys have substantially reduced oxide content, and thereby possess increased ductility [3]. For this as well as other benefits, the spray forming process has received considerable attention as an alternative method for the synthesis of a variety of structural materials [4], and has been utilized in this and associated studies.

In addition to the relevance of processing technique, the properties of the 7XXX series alloys are significantly affected by the details of chemical composition and the inclusion of nonmetallic fibers, whiskers, or particulates. Aluminum alloys reinforced with ceramic particles are in the metal matrix composite (MMC) class of materials, which continues to make major industrial impacts [5]. These materials can be altered to yield superior properties by controlling the amount and type of incorporated reinforcements within the metal matrix. Ceramic particle reinforcement results in isotropic properties, unlike fiber reinforced composites which exhibit severely anisotropic mechanical properties. Aluminum and its alloys are widely used in the fabrication of MMCs which have been gaining importance in recent years [6], [7], [8]. The addition of refractory silicon carbide particulate (SiCp) to aluminum matrices, for instance, has been associated with improvements in strength, elasticity, wears resistance, and corrosion resistance [9]. Although a reduction in ductility and fracture toughness often accompanies this improvement in elastic modulus and other properties, the significant increase in strength continues to be the motivation behind the addition of high volume fraction carbides to these alloys.

Additionally, the ability to process SiCp-reinforced aluminum alloy composites by conventional metallurgical (casting, extrusion, PM) and machining processing techniques has increased their usage and sparked their more recent consideration for applications where fatigue properties are critical [10]. Unfortunately, processing by casting, extrusion, and PM often results in inherent defects such as porosity, shrinkage, oxide inclusions, and segregation of the carbide particles that can reduce properties such as toughness and low temperature ductility [5]. A distinct advantage of the spray forming process for the production of ceramic particle-reinforced aluminum composites, in contrast, is the relatively uniform distribution of the reinforcing particles within the matrix [2], [11]. Unlike conventional casting or PM methods, stirring and mixing of the reinforcement into the matrix is not needed with spray forming. Sharma et al. previously determined that the use of spray forming to include ceramic silicon carbide particulates to form reinforced aluminum-based MMCs resulted in an increase in elastic modulus of Al–Zn–Mg–Cu alloys by as much as 50% in comparison to the constituent matrix alloy [12]. This work has motivated the current study, which investigates the fatigue behavior of spray formed SiCp reinforced 7XXX series aluminum alloys that possess the process related ductility benefits and the particulate induced increase in elastic modulus.

Fatigue resistance is a crucial design criterion for high performance structural applications and, as a result, several fatigue properties of SiCp-reinforced aluminum alloy composites have been investigated extensively [13]. Research beginning almost twenty years ago showed that under certain conditions the presence of reinforcement can increase fatigue life. Bonnen et al. [14] verified that the reinforcement of 15 vol.% SiCp in a naturally aged powder metallurgy (PM) aluminum alloy improved the resistance to stress-controlled fatigue. Similar improvement was observed for a PM 2124 aluminum alloy reinforced with 20 vol.% silicon carbide (SiC) whiskers under load-controlled fatigue tests [15]. Different MMC alloys with two different forms of reinforcements, i.e., particulate and whisker, were both found to be superior in high-cycle fatigue as compared to the unreinforced matrix alloys.

Chawla et al. [16] examined the effect of SiC volume fraction and particle size on the fatigue behavior of 2080 aluminum alloy fabricated using powder metallurgy and determined that fatigue resistance improved by increasing the volume fraction between 10 and 30% and decreasing the particle size to 5 μm. This behavior was credited to the increase in obstacles for dislocation motion and the decrease in strain localization as interparticle spacing decreased with particle size. Hall et al. [17] obtained comparable results when investigating particle size and volume fraction of 2124 aluminum alloy reinforced with SiC particles. Strength and fatigue life increased as reinforcement particle size decreased and volume fraction increased.

Uematsu et al. [10] focused on the fatigue behavior of SiCp reinforced 2024 aluminum alloy composites, fabricated by powder metallurgy, at elevated temperature. Fully reversed axial fatigue tests were performed at 150 and 250 °C using smooth specimens of SiCp-reinforced aluminum alloy composites with different particle sizes at a constant 10 wt.% of SiC particles. They reported a decrease in fatigue strength, regardless of particle size, with increasing temperature. Crack initiation was found to depend on temperature and particle size and small crack growth rates were an order faster at 250 °C than at ambient temperature and 150 °C in all materials studied. Softening and associated loss in matrix strength at elevated temperatures were the primary causes for the observed temperature and particle size dependence of fatigue behavior.

Kaynak and Boylu [18] completed a comparative study of the fatigue behavior of squeeze casted SiCp-reinforced aluminum composite specimens versus the matrix aluminum alloy containing 12 wt.% Si. They considered three different weight percentages of SiC particulates (5, 10, and 15%) in the size range of 40–60 μm and determined that fatigue resistance improved with increasing content of SiC particulates. SiC particulates were found to improve fatigue resistance mainly by acting as barriers to cracks and/or deflecting the growth plane of cracks resulting in decreased crack propagation rates.

Contrary to the improved resistance in stress-controlled fatigue, low-cycle fatigue resistance of reinforced composites under strain-controlled tests has been shown to be inferior to that of unreinforced matrix alloy [19], [20], [21]. The low-cycle fatigue behavior of a SiCp reinforced Al–Si cast alloy with two different volume fractions has been investigated under strain-controlled conditions with and without tensile mean strains by Koh et al. [22]. They found that the composites and the unreinforced matrix alloy showed cyclic hardening behavior. The composite containing higher volume fraction of SiC particles exhibited a more pronounced strain-hardening rate leading to shorter fatigue life at a given strain amplitude. The inferior strain-life of the composite can be attributed to the low ductility and the corresponding poor resistance to cyclic plasticity caused by the brittle reinforcement. Still, it has been reported that the SiCp reinforced PM aluminum alloys possess superior low-cycle fatigue resistance to the corresponding composite processed by casting. This is considered to be due to interfacial segregation or intermetallic formation at SiC particle interfaces in the cast composites [15].

Although a substantial amount of research has been completed to investigate the fatigue behavior of SiC reinforced Al–Zn–Mg–Cu alloys, the majority of this work has considered alloys produced by powder metallurgy techniques. There is far less published work focused on similar spray formed alloys [23], [24], [25]. The present study was undertaken to investigate the fatigue behavior of high strength SiCp reinforced Al–Zn–Mg–Cu alloys manufactured by spray forming. The objective is the development of insight into the effects of zinc, copper, manganese, and chromium content, as well as heat treatment, on the tensile and fatigue behavior of the MMCs. A more complete understanding of the effects of SiCp on the fatigue life of the reinforced metal matrix composites is required in order to ensure the effective use of these spray-formed alloys.

This paper presents and describes the results of comparative fatigue tests of SiCp reinforced 7XXX series alloys of different chemical composition and heat treatments. The study is aimed at the characterization of the fatigue and fracture mechanics behavior of rapidly solidified ultra-high strength alloys, with special emphasis on critical variables affecting their damage tolerance properties.