Nelson Guedes de Alcântara

Parameters optimization for friction spot welding of AZ31 magnesium alloy by Taguchi method

Friction spot welding (FSpW) is a solid state welding process suitable for producing spot-like joints, especially in lightweight materials, which are particularly interesting due to the weight saving potential. The plunging of an especially designed non-consumable and rotating tool creates a connection between overlapped sheets through frictional heat and plastic deformation. Minimum material loss is observed, and therefore a fully consolidated joint with flat surface (no keyhole) is obtained. In the current study, the effect of FSpW parameters, such as rotational speed, plunge depth and dwell time, on lap shear strength of AZ31 magnesium alloy joints was investigated. The optimization of input process parameters was carried out through Taguchi approach of DOE. Analysis of variance was applied to determine the individual importance of each parameter. Main effect plots were used to indicate the best levels for maximizing lap shear strength. The results show that tool plunge depth has the higher effect on the weld strength, followed by rotational speed and dwell time.

Fatigue Behavior and Physical Characterization of Surface-modified Ti-6Al-4V ELI Alloy by Micro-Arc Oxidation

Anodizing has proven an effective method for preparing bioactive titanium and has been the subject of many studies regarding the performance and biological characterization of the layers obtained. However, the fatigue behavior of titanium alloys, after undergoing this process is still poorly studied. This study aims to investigate the influence of MAO (Micro-Arc Oxidation) process on the fatigue properties of titanium alloy Ti-6Al-4V. Therefore axial fatigue tests were performed to obtain SxN curves, of specimens in polished and anodized (MAO processed, phosphate salt solution, potential of 290V) conditions. Roughness measurements, SEM, Raman spectra and X-ray photoelectron spectroscopy (XPS) analyses were used to characterize the features of the modified surface. SEM was also used to analyze the fatigue fractures of the tested specimens. The MAO process, with the parameters used in this investigation, had no influence on the fatigue behavior of the Ti-6Al-4V alloy, when compared to specimens without surface modification.

Thermodynamic simulation as an assistant tool in the development of iron based hardfacing materials

A thermodynamic model was used to investigate the solidification of a hardfacing material in Fe-Cr-C-Nb system designated CNO, and developed for cladding components subjected to severe abrasive wear by welding. Microstructural characterization of the alloy showed that the theoretical simulations carried out to predict the amount and volume fraction of the phases using the module Scheil of MT-DATA Software gave very realistic results in the case of the rapid solidification of the hardfacing alloy CNO. The results obtained in this study indicate that the utilization of this tool may provide a good understanding of alloys solidification. As a consequence, it is possible to enable the refinement of alloy composition during the early stages of hardfacing materials development, leading to a decreasing of overall cost and time of manufacturing of hardfacing electrodes.

Prediction of Friction Stir Welding defect-free joints of AISI 304 austenitic stainless steel through axial force profile understanding

Friction Stir Welding (FSW) joints of AISI 304 austenitic stainless steel (304 SS) using position controlled mode were investigated in order to understand and relate axial force behavior to welding quality. Joints were produced using two tools and four combinations of specific parameters. The results showed coherence between the axial force profiles and the low-magnification overviews of the welded joints. For defect-free joints, only a natural oscillation on axial force occurred after tool plunge. In contrast, abnormal or abrupt oscillations were directly associated with common welding defects, such as voids and nugget collapse.

Caracterização microestrutural e propriedades mecânicas da liga de alumínio 2198-T851 em configuração tailored blank soldada por FSW - Friction Stir Welding

In both aeronautic and automobile related industries, development and application of light-weighted metallic alloys have been pursued with the intention of reducing product weight and consequent fuel consumption. Due to their dimension and structural requirements, components and sections from airplanes and automobiles are produced through fusion-based welding processes. Still, solid-state based joining processes are preferable because they avoid defects intrinsic to fusion, and have therefore been an object of study in the last couple of decades. The objective of this work was the characterization of a friction stir welded (FSW) butt-joint of aluminum alloy 2198-T851 in tailored blanks configuration (sheets with different thicknesses). The methodology in which characterization was based involved the study of mechanical properties, through tensile tests and cross-section microhardness profiles determination, and the comprehension of the relation between microstructural changes caused by the welding process and resulting properties was sought. It was concluded that, due to metallurgical phenomena induced by the welding process, microstructural changes such as grain refinement, work-hardening and dissolution/formation of second phases as precipitates granted the attainment of a weld whose mechanical properties in tension (except elongation) equaled or exceeded those of base material, showing the feasibility of using the FSW parameters considered in this work.

Caracterização microestrutural e comportamento mecânico das ligas de alumínio AA2139 T3 e T8 soldadas por fricção rotativa com mistura

This work aimed to study the microstructural characterization and mechanical behavior of AA2139 T3 and T8 aluminum alloys joined by FSW - Friction Stir Welding. The microstructural analysis has been done using optical microscope and scanning electron microscopy, and the mechanical analysis were evaluate through microhardness testing and conventional tensile test using the ARAMIS system . Each alloy were welded using the same welding parameters and the same tool in order to evaluate the joint quality for both heat treatment used for this work, T3 and T8. The result showed sounds joints for both heat treatments, although with better mechanical properties for the alloy AA2139 T3.

Effect of in service weld repair on the performance of CrMo steel steam pipelines

This article presents a literature overview and discusses a practical case involving the effect of in service weld repair on the performance - behavior under creep conditions and remanent life - of low alloy CrMo steel steam pipelines with longstanding use. According to the specialized literature, several authors argue that the in service weld repair of low alloy CrMo steel steam pipes can be done successfully, while others disagree, reporting a reduction of up to 30% or more in the remanent life. According to the case analyzed here, which involves the weld repair of a 1¼Cr½Mo ferrite-pearlite steel steam pipe operating for 20 years at 480 °C, in service weld repair leads to the intragranular precipitation of Fe2MoC-type carbides and decreases the solute concentration of the ferrite in the base metal adjacent to the welded joint, thereby considerably reducing the pipelines remanent life, since the estimated rupture time for the service-aged material is 3.6 times that estimated for the weld-repaired joint.

Effect of friction spot welding parameters on the joint formation and mechanical properties of Al to Cu

Friction spot welding is an appealing technique for joining dissimilar materials, such as aluminum and copper that have significant differences in physical and mechanical properties. To optimize the welding process, a full-factorial design was employed. It is found that in addition to the plunge depth, the interaction between the rotational speed and the plunge depth significantly influences the lap-shear strength of the Al/Cu dissimilar joints. Further investigations on macro- and microstructures show that increasing the plunge depth could deform the Cu sheet into a concave shape to form a mechanical interlocking, and thus increase the joint lap-shear strength; increasing the tool rotational speed, however, may compromise this effect because of the formed tunnel defects on the interface due to high thermal exposure.

Microstructure and corrosion resistance of radial friction welded supermartensitic stainless steels

Abstract In this work, supermartensitic stainless steel pipes were radial friction (RF) welded and their corrosion behaviours were studied based on potentiodynamic polarisation and double-loop electrochemical potentiokinetic reactivation tests. Measurements were performed on samples taken from the base metal (BM), weld interface and consumable ring (CR) of the RF weldment. The corrosion properties are discussed in terms of their resulting metallurgical microstructure. The precipitation of Cr carbides that takes place during the tempering treatment induces a substantial Cr depletion value (Ir/Ia = 54.22%). On the other hand, CR and weld interface regions, which had their microstructure transformed and their Cr carbide precipitates redissolved by the RF welding thermomechanical cycle, present a low level of Cr depletion (Ir/Ia < 1%). The AC microstructure, which is composed of a mixture of virgin martensite and stable retained austenite, presents an increase in pitting corrosion resistance compared to the tempered structure of the BM region. It was also observed that the δ-ferrite decreases the pitting resistance of the weld interface region.

Microestrutura e resistência à corrosão de aços inoxidáveis supermartensíticos soldados por fricção radial

Neste trabalho, tubos de aco inoxidavel supermartensitico (AISM) foram soldados por friccao radial (SFR) e suas propriedades de corrosao foram estudadas atraves de ensaios de polarizacao anodica em solucao de 3,5 % de NaCl e da tecnica eletroquimica de reativacao potenciocinetica de ciclo duplo (DL-EPR). As medidas foram realizadas em amostras extraidas de diferentes regioes da solda SFR, i.e., metal de base (MB), interface de soldagem e anel consumivel (AC). As propriedades de corrosao foram discutidas em termos da microestrutura metalurgica resultante em cada regiao. A precipitacao de carbonetos de Cr que ocorre durante o tratamento de revenimento causa um empobrecimento em Cr substancial na estrutura revenida do MB (Ir/Ia = 54,22%). Por outro lado, as regioes do AC e da interface de soldagem, que tiveram suas microestruturas transformadas e seus precipitados de carboneto de Cr redissolvidos pelo ciclo termomecânico de soldagem SFR, apresentam um baixo grau de empobrecimento em Cr (Ir/Ia < 1%). A microestrutura do AC, composta de uma mistura de martensita temperada e austenita retida, apresenta maior resistencia a corrosao localizada do que a estrutura revenida do MB. Tambem foi observado que a ferrita-δ diminui a resistencia a corrosao por pites na regiao da interface de soldagem.