A. Ureña

Influence of interface reactions on fracture mechanisms in TIG arc-welded aluminium matrix composites

Abstract This paper describes the influence that the interfacial reaction between the matrix and reinforcement has on the fracture behaviour of an aluminium alloy (2014) reinforced with SiC particles when it is arc welded. TIG arc-welding tests were carried out on 4-mm thick sheets of AA2014/SiC/Xp (where X is 6, 13 and 20 vol%, respectively), which were machined with the welding pool in the transverse position and tensile tested. In all specimens tested, the failure was located in the weld metal with a tensile strength lower than 50% of the parent material properties. Fractures of the tested welds were studied and compared with those of the parent composites. Both surface studies of the fracture (SEM) and transverse preparations of these (LM and SEM) were observed. From these studies, it was deduced that the proportion of interfacial failure increases in the weld material because of the formation of Al4C3, which reduces the strength of the matrix/reinforcement interface, limiting the number of cracked particles. Also, embrittlement of the matrix alloy was observed, caused by the increment of Si-rich phases and intermetallic compounds in it (Si and Al–Fe–Si).

High temperature soldering of SiC particulate aluminium matrix composites (series 2000) using Zn–Al filler alloys

Abstract The present paper evaluates the solderability of three discontinuously reinforced aluminium matrix composites. The tested composites were an aluminium alloy of the 2000 series (AA2014) reinforced with different percentages of silicon carbide particles (6, 13, and 20 vol.-% respectively). A similar study was carried out on the unreinforced aluminium alloy for comparative purposes. Three low temperature filler alloys of the Zn–Al system were used for soldering. Drop formation tests were performed to evaluate the wettability of the molten filler alloys and sample joints (single overlap specimens) were produced to determine solder penetration in the joint clearance. Microstructural studies of the joints were carried out to determine the effects of the solid reinforcement particles on molten pool behaviour and solidification mechanisms.

Nanoreinforced Epoxy Adhesives for Aerospace Industry

Adhesive joints of carbon fiber/epoxy laminates were studied using an epoxy resin as the adhesive. In order to enhance the mechanical and electrical properties of epoxy adhesives, they were modified by the introduction of carbon nanofibers (CNFs). Also, different surface treatments, such as grit blasting, peel ply, and plasma, were applied to the laminates. The CNFs addition slows down the curing reaction of the epoxy adhesive although the final conversion is still high. The contact angle of nanoreinforced adhesives on the surface of treated laminates is lower than that of the neat epoxy resin. However, this increase of wettability scarcely modified the lap shear strength. Plasma treatment causes an important increase of the surface energy of laminates, markedly increasing the joint strength. The fracture mechanisms of the adhesive joints tested in the present study are strongly dependent on the surface treatment applied to the laminates.

Study of the effect of substrate roughness on adhesive joints by SEM image analysis

The effect of adherend surface roughness on epoxy bonded aluminium joints has been studied. Several epoxy adhesives were tested to evaluate the influence of adhesive nature on the roughness effect. The aluminium pre-treatments applied were abrasion and impression processes, generating different texture levels. The abrasion with grinding papers of different grain sizes provides surfaces with high density of low summits. In contrast, the surfaces subjected to impression process present low density of very deep valleys. The roughness measurements were made by image analysis of micrographs obtained by Environmental Scanning Electron Microscopy (ESEM). In addition to the average roughness, other surface descriptors were determined to characterise completely the surfaces, allowing to evaluate the influence on the adhesive strength of different texture variables, such as the height, shape and density of the peaks and valleys which constitute the roughness profile. An optimum value of surface roughness was found for the joint strength measured by the lap shear tensile test. An increase of adherend roughness causes an increase of effective area but, at the same time, decreases the ability for adhesive penetration. It has also been shown that the joint strength depends on the main characteristics of the adherend surface, such as the density and depth of the protuberances. Finally, it has been found that the roughness effect seems be influenced by the adhesive nature. Different epoxy adhesives with similar mechanical properties present different joint strengths due to their different penetration abilities.

Diffusion bonding of Ti-6Al-4V alloy at low temperature: metallurgical aspects

The diffusion bonding of the Ti-Al-4V alloy at low temperature (850°C) has been studied. The principal objective of this investigation was the development of a diffusion bonding procedure suitable for Ti-6Al-4V alloy and capable of being used as part of a superplastic forming/diffusion bonding process. It was found that high-quality joints can be obtained by bonding at 850°C, with pressures of 4 MPa and times in the range 90–120 min. Mechanical properties of the joints were determined using cylindrical and plane test pieces. Tensile, shear and peeling tests were used to determine the strength of the joints. On bonding with the above conditions, the parent alloy strength was reached. Little reduction in these values was measured because the heat treatment was applied during bonding. A metallographic study by scanning electron microscopy and energy dispersive spectroscopy was performed to determinate the influence of the previous parameters on the microstructural changes that occur in the joint. Grain growth kinetics and ratio of bonding area were also studied. The results shows that a new method of diffusion bonding for Ti-6Al-4V alloy has been developed. This method can be carried out using lower bonding temperatures than in conventional processes.

Oxidation barriers on SiC particles for use in aluminium matrix composites manufactured by casting route: Mechanisms of interfacial protection

This paper is centred on a study of the interface reaction mechanisms which participate in the fabrication of an aluminium-SiC composite by a casting route, when reinforcements (particles, in this case) have been previously coated by oxidation with a SiO2 layer. The studies, which were carried out using transmission electron microscopy and differential scanning calorimetry, made it possible to propose a model of action of the SiO2 barrier in relation to the coating thickness and the reaction time. The first reaction that occurred in this SiC-SiO2-molten Al system was the formation of an Al-Si-O glassy phase which progressively consumed the SiO2 barrier, reducing the matrix-particle interface energy and favouring wetting of the SiC surfaces. When the oxidation coating was completely consumed, the SiC was preferentially dissolved by the glassy phase, inside which the formation of amorphous carbon was detected. These studies also show that carbon enrichment of the reaction layer activated the precipitation of metallic impurities (such as Fe or Cu) in the reaction. Longer reaction times (8 h) could also favour crystallization of the glassy phase to form mullite and the formation of microcrystalline alumina at the reaction interface.

Diffusion bonding of an aluminium-copper alloy reinforced with silicon carbide particles (AA2014/SiC/13p) using metallic interlayers

In this work, the application of solid state diffusion bonding to a SiC particulate reinforced aluminium-copper alloy (AA2014) has been studied. The use of metallic interlayers such as an aluminum-lithium alloy and pure silver, has been tested. Bonding interfaces were microstructural characterized using scanning electron (SEM) and transmission electron microscopies (TEM). Joint strengths were evaluated by shear mechanical tests, completed with fractographic studies to determine the failure mechanisms of each kind of joint.

Rheological Behaviour of Nanoreinforced Epoxy Adhesives of Low Electrical Resistivity for Joining Carbon Fiber/Epoxy Laminates

Epoxy resins reinforced with carbon nanofibers (CNF) and nanotubes (CNT) were prepared and evaluated as adhesives of carbon fiber/epoxy laminates. Different percentages of nanofiller (0.1–3 wt%) have been tested. The viscosity of the non-cured nanoreinforced epoxy mixtures increased with the nanofiller content. On the other hand, the thermal treatment at high temperatures of the mixtures of amino-functionalized CNTs and epoxy monomer also caused an increase of their viscosity — this is likely due to the chemical reaction between the oxirane groups of the epoxy and the amine groups of the nanofiller. The joint strength of the carbon fiber/epoxy laminates bonded with nanoreinforced epoxy adhesives was analyzed by means of the single lap shear test. The shear strength of these joints was similar to that of the one made with unfilled epoxy resin. However, observation by Scanning Electron Microscopy of the fracture surfaces of the adhesive joints confirmed that the incorporation of carbon nanofillers caused the cohesive fractures inside the laminates (light-fiber tear failure). The electrical conductivity was drastically increased by the addition of nanofillers, especially CNTs.

Solid-state transformations during diffusion bonding of copper to iron

Solid-state bonding between dissimilar metals, produced at elevated temperatures with the application of a bonding pressure, causes structural changes in the microstructure of the zones nearest to the bond interface. These metallurgical transformations, produced by interdiffusion in the vicinity of the bond, decide the final properties of the joint. In the present paper, such diffusional transformations have been investigated for diffusion-bonded joints of Armco iron and copper with different oxygen contents (ETPC and OFLPC). The formation of iron oxide (wustite) has been observed in the ETPC-Armco iron joints. This oxide did not appear in OFLPC-Armco iron diffusion-bonded joints. This suggests that iron oxide forms by reaction of iron with oxygen dissolved in the ETPC base metal. The formation of copper particles in the iron base matrix, near the bond interface, has been observed. This may be due to two different processes: the solid-state precipitation of copper into iron and the eutectoid reaction (γ →ε +α) at bonding temperatures above 900° C.

Active coatings for SiC particles to reduce the degradation by liquid aluminium during processing of aluminium matrix composites: study of interfacial reactions

The application of a surface coating on SiC particles is studied as an alternative means of solving problems of reactivity between SiC reinforcements and molten aluminium and problems of low wetting which limit the application of casting routes for fabrication of Al–SiCp composites. The selected active barrier was a ceramic composed of SiO2, which was generated by controlled oxidation of the SiC particles. The coating behaves as an active barrier, preventing a direct reaction between molten aluminium and SiC to form Al4C3 as the main degradation product. At the same time, the SiO2 provokes other interfacial reactions, which are responsible for an improvement in wetting behaviour.