A. Morri

Linear Friction Welding of a 2024 Al Alloy: Microstructural, Tensile and Fatigue Properties

The possibility of using linear friction welding (LFW) to produce high quality joints on an aerospace grade aluminium alloy (AA2024) was evaluated. In this solid state joining process the bonding of two flat edged components is achieved through frictional heating, induced by their relative reciprocating motion, under an axial compressive force. The Al joints were subjected to microstructural and mechanical characterization, including hardness and tensile tests. S-N probability curves were also computed after preliminary axial fatigue tests. No post-weld heat treatment was performed. The microstructural analyses showed substantially defect-free joints, with a relevant plastic flow in the thermo-mechanically altered zone. Maximum hardness decrease in the joint zone was approximately only 5% in respect to the base material. The joint efficiency was about 90% with respect to the ultimate tensile strength, with a slight reduction in the elongation to failure. Good fatigue performances were also detected.

A Study on Similar and Dissimilar Linear Friction Welds of 2024 Al Alloy and 2124Al/SiCP Composite

The aim of the present work is to evaluate the possibility of using the Linear Friction Welding (LFW) technique to produce similar and dissimilar joints between a 2024 Al alloy and a 2124Al/25vol.%SiCP composite. In this solid state joining process the bonding of two flat edged components is achieved through frictional heating induced by their relative reciprocating motion, under an axial compressive force. Microstructural characterization of the welds was carried out by optical and scanning electron microscopy, to investigate the effect of LFW both on the aluminium alloy matrix and the reinforcement particles. The mechanical behaviour of the welded specimens was studied by means of hardness and tensile tests. The mechanisms of failure were investigated by SEM analyses of the fracture surfaces. LFW joints in MMCs resulted substantially defect free, with a uniform particle distribution, while a partial lack of bonding at the corners was observed in the others welds. The hardness decreased by approximately 10% in the welded zone, with some data fluctuations due to the complex microstructural modifications introduced by the LFW process. The joint efficiency, evaluated in respect to the UTS, was 90% for the Al alloy joints and 80% for the MMC joints. A decrease in the elongation to failure was measured in all the LFW specimens, probably related to the orientation of the plastic flow in the TMAZ, where the fracture generally occurred.

Forming of Metal Matrix Composites

Metal matrix composites (MMCs), through a proper selection of matrix and reinforcement, offer unusual combinations of physical and mechanical properties such as high specific strength and stiffness, high thermal and electrical conductivity, high wear resistance, good corrosion resistance, and good impact and fatigue properties, together with superior thermal stability when compared to the unreinforced matrix alloys. Near net shape forming and improvements in microstructure, strength, and ductility can be achieved in discontinuously reinforced MMCs through the application of primary or secondary plastic deformation processes such as extrusion, rolling, forging, superplastic forming, or friction processing. Thermomechanical processing can be directly a part of the production process or follow a primary stage of fabrication or consolidation. Processing can be mainly divided in hot and cold working, the latter often followed by annealing. The set of processing conditions (e.g., temperature, total deformation, deformation rate) is of crucial importance with respect to the resultant microstructural and consequently mechanical properties, and must be optimized with respect to the system to be processed (matrix alloy, reinforcing hard phase, reinforcement size, and volume content).

Numerical and Experimental Study of Residual Stresses in a Linear Friction Welded Al-SiCp Composite

In the present study, the process modelling of AMCs linear friction weldment was carried out. Four major stages of the process (Part 1: Warm-Up; Part 2: Osci-Forging; Part 3: Forging; Part 4: Cool-Down) were identified and simulated consecutively to generate the temperature field and residual strains distribution within the model. An eigenstrain model calibrated by the neutron diffraction results was also employed to capture the permanent deformation distribution. Good agreement between the process modelling and the experimental measurements was found.

Gas-Liquid In Situ Production of Ceramic Reinforced Aluminum Matrix Nanocomposites

The present study aims at evaluating the gas bubbling method, based on the use of dry air as a gaseous phase, for the production of Al based metal matrix nanocomposites through a proper gas-liquid reaction. In particular, Al2O3 reinforcement particles were in-situ synthesized in molten commercially pure Al through a gas bubbling oxidation technique. Dry air was injected in the melt in order to induce a controlled oxidation of the molten matrix. SEM-EDS analysis on the produced samples revealed the presence of alumina particles, ranging from the nanoto the micrometric size, demonstrating the feasibility of the process. A hardness increase on the produced samples confirmed the strengthening effect of the in-situ produced ceramic particles.

High temperature tensile behaviour of the A354 aluminum alloy

The high temperature tensile behaviour of the A354 casting aluminum alloy was investigated also evaluating the influence of secondary dendrite arm spacing (SDAS). Cast specimens were produced through a gradient solidification equipment, obtaining two different classes of SDAS, namely 20-25 µm (fine microstructure) and 40-50 µm (coarse microstructure). After hot isostatic pressing and T6 heat treatment, the samples underwent mechanical characterization both at room and high temperature (200 °C). Results of tensile tests and hardness measurements were related to the microstructural features and fractographic characterization, in order to investigate the effect of microstructure and high temperature exposure on the mechanical behaviour of the alloy.

Analysis of the Influence of Fretting on the Fatigue Life of Interference Fitted Joints

The present paper deals with the fatigue behaviour of shaft hub interference fitted joints. The investigation is carried out by means of a down-scaled ad-hoc specimen, which has been developed by the authors in order to perform accelerated fatigue tests. The specimen is similar to those suggested by standard ISO 1143, but it consists of two parts, joined together by shrink fitting. In a previous work, the authors compared the performances of a plain specimen manufactured according to ISO 1143 and those of the shrink fitted specimen. Both the plain and the shrink fitted specimens were made of C40 EN 10083 low carbon steel. The outcome of that research was that shrink fitting determines a degradation of the fatigue response of the assembly, with respect to those of the plain specimen. Moreover, such decrease cannot be predicted by means of FEA alone, since it is partly due to fretting phenomena. In fact, fretting takes place on the mating surface between the shaft and the hub, and especially in the vicinity of the end of the contact. The present paper deals with the observation of failed and survived specimens by means of optical and SEM microscopes, in order to determine the actual tribological characteristics of the contact surface. For instance, the amplitude of the sliding zone observed experimentally is compared with that given by FEA for different choices of the contact formulation. Since fretting is often associated with the presence of secondary fatigue cracks, which do not propagate, the authors set up an experimental method for locating the secondary cracks prior to sectioning the specimen for microscope observation.Copyright

Superplastic Deformation of Twin Roll Cast AZ31 Magnesium Alloy

The aim of the present work was to evaluate the potential for superplastic deformation of the AZ31 magnesium alloy produced by Twin Roll Casting (TRC), a continuous casting technology able to convert molten metals directly into a coiled strip. In order to develop a superplastic microstructure, the TRC sheets were heated at 400 °C for 2 h, then rolled by multiple passes with re-heating between them, with a total thickness reduction of about 60%. The superplastic behaviour of the alloy was studied by tensile tests, carried out at in the temperature range from 400 °C to 500 °C and with initial strain rates of 1•10-3 s-1 and 5•10-4 s-1. The microstructural and fractographic characterization of the alloy was carried out by means of optical (OM) and scanning electron microscopy (SEM). The tensile tests evidenced a superplastic behaviour of the processed AZ31 Mg alloy, with a maximum elongation to failure of about 500% at 460 °C, with a strain rate of 5•10-4 s-1. The microstructure of the alloy after superplastic deformation showed fine and equiaxed grains, with a large fraction of high angle boundaries. Analyses of the fracture surfaces evidenced flow localization around the grains, suggesting that grain boundary sliding (GBS) was the main deformation mechanism. Failure occurred by cavitation, mainly at the higher testing temperature, due to the prevailing effect of grain growth.

Friction Welding of Particle Reinforced Aluminium Based Composites

The widespread use of metal matrix composites (MMC) is often limited due to the difficulties related to their joining by means of traditional fusion welding processes. The aim of this work was to evaluate the effect on microstructure and mechanical properties (hardness and tensile strength) of two different friction welding techniques used for joining two Al-based metal matrix composites. In particular, Friction Stir Welding was applied to a 6061 (Al-Mg-Si) alloy matrix, reinforced with 20vol.% of Al2O3 particles (W6A20A), while Linear Friction Welding was applied to a 2124 (Al-Cu-Mg) alloy matrix reinforced with 25vol.% of SiC particles (AMC225xe). Both the welding processes permitted to obtain substantially defect-free joints, whose microstructures was found to be dependent on both the initial microstructure of the composites and the welding processes. Hardness decrease was in the order of 40% for the FSW joint and of 10% for the LFW joint, mainly due to overaging of the matrix induced by the frictional heating, while the joint efficiency in respect to the ultimate tensile strength was 72% and 82%, respectively. Elongation to failure increased in the FSW joint due to coarsening of precipitates, whereas it decreased in the LFW joints due to the fibrosity in the thermomechanically altered zone. Fracture surface analysis showed good matrix/reinforcement interface for both composites.

Friction Stir Welding of Aluminium Based Composites Reinforced with Al2O3 Particles

This paper presents the results of microstructural and mechanical characterization of Friction Stir Welding joints of two aluminum-based particles reinforced composites. The composites were FSW in the extruded and T6 heat treated condition. No post-weld heat treatment was carried out on the FSW joints. Hardness, tensile, low-cycle fatigue and impact tests were carried out. Microstructural and fractographic characterization were performed both on the base and FSW material, in order to investigate the effect of the solid-state welding process on the reinforcement particles and aluminum matrix. The FSW produced high quality joints with good microstructural characteristics: the welded zone displayed a refinement of the Al matrix grain size and reinforcement particles, and a better particle distribution. The FSW specimens showed high efficiency, both in the tensile, impact and fatigue tests.