The Strength and Metallography of a Bimetallic Friction Stir Bonded Joint between AA6061 and High Hardness Steel

12.7-mm thick plates of 6061-T6511 aluminum alloy and high hardness steel (528 HV) were successfully joined by a friction stir bonding process using a tungsten-rhenium stir tool. Process parameter variation experiments, which included tool design geometry, plunge and traverse rates, tool offset, spindle tilt, and rotation speed, were conducted to develop a parameter set which yielded a defect free joint. Laboratory tensile tests exhibited yield stresses which exceed the strengths of comparable AA6061-to-AA6061 fusion and friction stir weld joints. Scanning electron microscopy and energy dispersive X-ray spectroscopy analysis also show atomic diffusion at the material interface region

Joining copper to stainless steel by friction stir diffusion process

Dissertação apresentada na Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa para obtenção do Grau de Mestre em Engenharia MecânicaOne of the major trends in welding and joining technology is to join dissimilar materials taking advantage of individual materials properties. Among these, copper to stainless steel joining has significant industrial applications and importance where e.g. the highest electrical and thermal conductivities are required to engineering materials associated to good corrosion resistance. However, joining these materials is difficult due to their very different chemical composition and thermo-physical properties. Additionally, they easily form intermetallic phases that deteriorate the mechanical strength of the joint. Thus investigating the feasibility of applying alternative processing technologies is relevant. Solid state processes have been investigated for this application, namely friction stir welding. Recently, a variant has been exploited where local diffusion is the fundamental joining mechanism triggered by friction stir. The advantage of friction stir diffusion process is the minimal detrimental effect on both materials, preventing some critical identified problems. Since this process is not well developed, this thesis aimed to study friction stir diffusion process (FSDP) to join copper to stainless steel. Lap joints were produced varying processing parameters, namely, rotation and travel speeds and axial forging force. The effect of processing parameters on the width of effective joining was studied, as well the joints characterization for mechanical resistance properties and microstructural features at the interface. The thermo-mechanical conditions and time during the FSDP resulted in an interface with diffusion between both materials below 3 μm. The shear strength of the lap joints depends on the material thickness involved, but joining efficiencies up to 73.8 % were achieved

Copper to stainless steel welding in lap joint

Master’s final work to obtain the master’s degree in Mechanical EngineeringO desenvolvimento ao longo dos últimos 30 anos de soldadura em estado-sólido tem oferecido inúmeros benefícios em comparação com as técnicas mais tradicionais, especialmente para materiais cuja composição e propriedades são muito diferentes. A soldadura de materiais dissimilares permite tirar partido das melhores propriedades de cada material, enquanto a soldadura em estado-sólido permite que esta seja exequível, algo dificilmente obtido ou com piores resultados com o uso de outras técnicas para certos materiais. A ligação entre aço inoxidável e cobre tira partido da elevada resistência do aço inoxidável e da elevada condutividade térmica do cobre. Uma junta dissimilar em que o seu estudo em soldadura em estado-sólido ainda não está muito aprofundado. A revisão da literatura apresenta a relevância da soldadura de cobre com aço inoxidável na indústria e explica o processo de friction stir spot welding, como também a sua variante probe-less friction stir spot welding. Também são apresentados estudos que recorreram a friction stir welding e friction stir spot welding entre ligas de cobre e aço, que permite uma visão geral das ferramentas e parâmetros utilizados, como também quais as técnicas de caracterização usadas e os resultados obtidos para este par numa soldadura em estado sólido. O objetivo do presente trabalho foi estudar as propriedades morfológicas, microestruturais e mecânicas das juntas entre cobre-DHP e aço inoxidável AISI 304, obtidas através de probe-less friction stir spot welding. Os parâmetros de soldadura, incluindo a velocidade de rotação (870–1500 rpm), tempo de estabilização (20 e 60 s) e posição do material, foram variados para encontrar uma janela de parâmetros que permitisse a união desses dois materiais. Foram utilizadas duas ferramentas sem pino, diferindo no diâmetro da base (10 e 12 mm). Durante a execução das soldaduras procedeu-se à medição da temperatura. Após a sua produção, as soldaduras foram submetidas a inspeção visual, análise macro e microestrutural e ensaios de microdureza e tração-corte. O posicionamento do material foi um fator determinante para se alcançarem soldaduras. Nenhum dos ensaios realizados com cobre na chapa superior produziu soldaduras consistentes, muito devido à diferença da condutividade térmica dos materiais. Dos 16 ensaios realizados foram obtidas com sucesso 6 soldaduras. A redução de diâmetro da ferramenta permitiu que as soldaduras fossem também obtidas para valores mais baixos de velocidade e tempo de estabilização. A temperatura registada durante o processo aumentou com o aumento dos parâmetros, estando de acordo com as observações feitas através da inspeção visual sobre a intensificação das alterações morfológicas. A análise da macroestrutura permitiu verificar que não existe mistura entre os dois materiais, isto é, a interface permaneceu muito idêntica quando do início do processo de soldadura. Ainda nesta análise, verificou-se que o tempo de estabilização teve mais impacto no comprimento de ligação do que a velocidade de rotação. A análise microestrutural mostrou que o cobre passou por um processo de recozimento, onde se verifica o crescimento de grão. Ou seja, no cobre apenas se identificou uma zona termicamente afetada que é significativamente afetada pelo tempo de estabilização, quer em dimensão quer no aumento do grão. As micrografias obtidas por microscopia eletrónica de varrimento revelaram descontinuidades micro-dimensionais e o aparecimento do fenómeno de micro-mechanical interlocking. Através da análise por espectrometria de dispersão de energias de raios-X, verificou-se que a ligação foi alcançada por mútua difusão entre a composição elementar (Cu e Fe) dos dois materiais. O aço inoxidável sofreu um aumento de dureza, no entanto com uma tendência de diminuição com o aumento dos parâmetros. Além disso, verificou-se uma maior sensibilidade na variação da dureza para o aço inoxidável do que para o cobre com a variação dos parâmetros, já que este está sujeito aos efeitos da deformação e temperatura. No caso do cobre, a dureza diminuiu em relação ao valor do material base, com uma redução ainda mais acentuada à medida que os parâmetros aumentavam, algo que está de acordo com as observações feitas da microestrutura. 57.16 HV0.2 foi o valor médio mais baixo registado para o cobre e 283.48 HV0.2 a dureza média mais alta para aço inoxidável, correspondente às condições com introdução mais elevada e mais baixa de calor, respetivamente. Foi também observada repetibilidade entre a maioria dos ensaios de tração-corte, com o modo de falha mais comum a ser nugget pull-out. A força registada pelas soldaduras variou de 4.1 a 5.8 kN, sendo a dureza e o comprimento de ligação as duas características que mais influenciaram este valor.The development of solid-state welding over the last 30 years has offered numerous benefits compared to the more traditional techniques, especially for materials whose composition and properties are vastly different. The welded components composed of stainless steel and copper take advantage of the high strength and corrosion resistance of stainless steel and the high thermal and electrical conductivity of copper. The aim of the present work was to study the morphological, microstructural, and mechanical properties of copper-DHP to stainless steel AISI 304 friction stir spot welds. The welding parameters, including rotational speed (870–1500 rpm), welding time (20 and 60 s), and material position, were varied to find a window of parameters allowing sound joining of these materials. Two probe-less tools were used, differing by the shoulder diameter (10 and 12 mm). After their production, the weld trials were submitted to visual inspection, macro and microstructural analysis, and microhardness and tensile-shear testing. All the trials performed with copper as the top plate were not consistent, as no bonding was achieved at the weld interface. The temperature measured, during friction stir spot welding, increased by increasing the welding time, tool diameter, and rotational speed, which agreed with the morphological changes on the weld surface. In addition, the macrostructural analysis of the joints' transverse sections showed no material mixing or macro-defects. The copper’s microstructural analysis only exhibited a heated affected zone, which was significantly affected by the welding time. Micro-discontinuities and micro-mechanical interlocking features were seen on the weld interface, with mutual diffusion occurring between the two materials. The stainless steel showed a microhardness increase compared to the base material but decreased with increments in the welding temperature. For copper, in agreement with the grain size evolution, the microhardness decreased, and the reduction was more intense with the increase in the welding temperature.N/

Recent Developments in Non-conventional Welding of Materials

Welding is a technological field that has some of the greatest impact on many industries, such as automotive, aerospace, energy production, electronics, the health sector, etc. Welding technologies are currently used to connect the most diverse materials, from metallic alloys to polymers, composites, or even biological tissues. Despite the relevance and wide application of traditional welding technologies, these processes do not meet the demanding requirements of some industries. This has driven strong research efforts in the field of non-conventional welding processes. This Special Issue presents a sample of the most recent developments in the non-conventional welding of materials, which will drive the design of future industrial solutions with increased efficiency and sustainability

Advance in Friction Stir Processed Materials

This reprint presents the current state of knowledge and the latest advances in the development of microstructure and material properties using modern FSP (Friction Stir Processing) and related technologies such as FSW (Friction Stir Welding). The chapters of this reprint contain valuable results of research on changes in the microstructure and properties of materials caused by the use of the above technologies. Detailed analysis of these results allowed for the formulation of constructive conclusions of scientific and technological importance. The issues described in here present a significant cognitive and application potential and indicate the problems and implementation challenges faced by users of FSP and related technologies

Developments in Magnetic Pulse Welding

Magnetic Pulse Welding is a solid state joining technology based on impact, which allows to produce overlap joints both in planar and tubular geometries. The technology has seen an increased interest in recent years, especially as a result of the industrial need to joint dissimilar materials (metallic and non-metallic) which easily form brittle intermetallic phases when welded by fusion-based processes. However, no significant improvements on existing equipments have been reported, which are normally sized for endurance, compromising the machine efficiency. In fact these are normally equipped with large storage capacitors banks, which are sometimes insufficient for dissimilar material combinations that require more energy to weld In this study existing equipments were analysed to understand the key components aiming at its optimization. A prototype machine was developed and assembled envisaging higher discharge energies efficiency. The equipment was tested and validated in tubular transitions due to the facility to produce the coils in laboratory facilities but also due to the industrial applications identified. This joining process is known to need a conductive flyer material to allow inducing current for the magnetic interaction which projects the flyer against the target to produce a weld. Thus, tube to tube and tube to rod welds were produced in AA6063 in similar and dissimilar metallic joints to Ti6A4V. AA7075 to carbon fibre reinforced polymer tubes transitions were also successfully produced especially when Cu or Ni ductile interlayers were used. The developed prototype equipment was compared to a commercial machine to identify the optimization achieved and to compare characteristics of the welds produced. For this, the joints were characterized both structural and mechanically. The prototype machine proved to have a higher efficiency needing less than 15% of the energy required on the commercial machine to produce similar aluminium transitions (reducing from 16 kJ to 2 kJ). The machine also proved to be efficient in producing dissimilar joints, such as aluminium to titanium transitions and metal to non-metal transitions

Atomistic modelling of Fe-Al and α-AlFeSi intermetallic compound interfaces

The joining of aluminum and steel has been considered an efficient solution for building light-weight technology, particularly in the automotive, aerospace and shipbuilding industries. It is an immense challenge to join these materials together due to the significant differences in the physical and chemical properties of aluminum and steel. The development of intermetallic compound (IMC) layers has a huge impact on the strength of the aluminum-steel joint. The development of IMCs at the aluminum and steel joint is greatly influenced by the welding methodology and temperature reached during the welding process. It is thermodynamically possible to develop certain IMCs depending on the composition and phase diagram of aluminum and steel alloys. For this reason, understanding the mechanical nature of the IMCs is pivotal to improve the welding methodologies. In this work, atomistic simulations were performed on Fe2Al5, Fe4Al13 and α-AlFeSi bulk and interface structures. We started with the construction of atomistic bulk structures of Fe2Al5 and Fe4Al13 and calculated the mechanical properties using density functional theory (DFT) calculations. A comparative study was performed to identify the mechanical behavior of these compounds. Moreover, comparisons were also made with other experimental, semi-empirical and ab-initio methods to test the reliability of the calculations. Due to the complex nature and large atomic structures of Fe-Al IMCs, using ab-initio methods could be very computationally expensive. To make computational calculations fast and accurate, a semi-empirical potential based method has also been used in this work. The main objective of this study was to test the reliability of modified embedded atoms method (MEAM) potentials and suitability for finding good initial structures for Fe-Al interfaces. It was concluded that MEAM and semi-empirical methods are not reliable for inferring mechanical features of Fe-Al IMCs. However, MEAM was found to be reasonable for finding good initial guesses for the Fe-Al interface structures. Lastly, a systematic study was performed to identify the virtual tensile and shear strengths of Fe-Al and α-AlFeSi interfaces using DFT. Interface structures were optimized using the fast inertial relaxation engine (FIRE), which was very successful in optimizing these complex interfaces with a large number of atoms. After the optimization of the interface structures, virtual tensile and shear strength calculations were performed. An extended version of the so-called Universal Binding Energy relation (UBER) was used to fit the energy-displacement curve for virtual tensile strength and a Fourier series for the virtual shear strength predictions. The results indicated the potential negative effect of the Fe-Al IMCs on the strengths of the aluminum-steel joint

Disk laser welding of metal alloys for aerospace

2011 - 2012Laser welding is the logical processing solution to accomplish different needs. Improvements at the design stage are actually aimed to remove any mechanical fastening, thus moving towards a technology which would not increase the joint thickness; moreover, a number of benefits in comparison with conventional welding methods are provided when considering laser beams, since deep penetration is achieved and the energy is effectively used where needed, thus melting the interface to be joined rather than excessively heating up the base metal, which would suffer from thermal distortion and degradation of metallurgical properties otherwise. Further advantages are achieved in laser welding with thin disk sources, since high output power, high efficiency and good beam quality are simultaneously delivered, unlike traditional laser systems; costs are significantly reduced in comparison with lamp-pumped laser systems. As a consequence, specific interest is shown in aerospace where strict specifications apply. Nevertheless, a number of issues must be addressed, depending on the material to be welded, as many variables and sub processes concerning fusion and vaporization are involved in laser welding and a delicate balance between heating and cooling is in place within a spatially localized volume. Therefore, extensive studies are required to manage both the stability and the reproducibility of the overall process, before introducing any change in industrial environments. Methods, experimental results and discussions concerning laser welding of common metal alloys for aerospace are provided in this Ph.D. thesis. A general view of applications and basic advantages of laser welding is first given, with mention to diagnostics and safety. Hence, the principles of laser emission are examined, with respect to the architecture of the sources, beam geometry, quality and efficiency, in order to better portray the benefits of a thin disk laser concept. Processing dynamics of laser welding are explained afterward, referring to conduction and key-hole mode, instability, gas supply and leading governing parameters such as laser power, welding speed, defocusing and beam angle to be considered in the experimental work. Procedures are provided for proper bead characterization, from preliminary examinations including non destructive tests such as fluorescent penetrant inspections and radiographic tests, to sample preparation and eventual mechanical assessment in terms of tensile strength and Vickers micro hardness in the fused zone. A straightforward description of the design of experiment approach and the response surface methodology is given, so to introduce the testing method to be taken, as well as the steps for data elaboration via statistical tools. Hence, four case studies about metal aerospace alloys are presented and discussed in their common seam configuration: autogenous butt and overlapping welding of aluminum alloy 2024; autogenous butt welding of titanium alloy Ti-6Al-4V; dissimilar butt welding of Haynes 188 and Inconel 718; dissimilar overlapping welding of Hastelloy X and René 80. All of the welding tests were conducted at the Department of Industrial Engineering at the University of Salerno; a Trumpf Tru-Disk 2002 Yb:YAG disk-laser source with a BEO D70 focusing optics, moved by an ABB IRB 2004/16 robot was employed. When needed, additional tests for the purpose of specific bead characterization were conducted by Avio and Europea Microfusioni Aerospaziali. As general procedure for each topic, the operating ranges to be examined are found via preliminary trials in combination with the existing literature on the subject. Then, special consideration is given to the processing set-up, the resulting bead profile, possible imperfections, defects and overall features; consistent constraint criteria for optimization of the responses are chosen on a case-by-case basis depending on materials and seam geometry and referring to international standards as well as customer specifications for quality compliance. Optimal combinations of the input welding parameters for actual industrial applications are eventually suggested, based on statistical tools of analysis. Convincing reasons are provided to give grounds to improvements in real applications. Moreover, based on the results, a proper device for bead shielding, to be conveniently adjusted depending on both geometry and materials to be welded has been designed, produced and patented (SA2012A000016). As concerning aluminum welding, a comprehensive description is given for laserrelated issues: reflectivity and thermal conductivity influence on the material response is illustrated; the porosity evolution is discussed with respect to thermal input and defocusing; a theory for softening in the fused zone is provided through energy dispersive spectrometry and estimations of magnesium content in the crosssection. Optimization is performed for butt configuration of 1.25 mm thick sheets; the discussion about the interactions among the governing factors is deepen with reference to overlapping welding. With respect to titanium welding, optimization is performed for 3 mm thick butt welding; the resulting micro structure in the weld is discussed since it is thought to be closely related to the mechanical properties. In particular, special care is taken of the grain size as a function of the governing factors. Dissimilar welding of super alloys is considered for gas turbine components; for this specific purpose, laser welding is expected to offer a valid alternative to arc and electron beam welding, whose weaknesses are pointed out. Given their actual application in the engine, Haynes 188 and Inconel 718 are examined in butt welding configuration, whilst an overlapping geometry is preferred for Hastelloy X and René 80. Considerable tolerances are matched, thus promoting the suggested range of the operating variables. [edited by author]XI n.s

Material bond, formation, and growth, in Al/Mg compound castings

This thesis was submitted for the award of Doctor of Philosophy and was awarded by Brunel University LondonAl/Mg compound castings possess a number of benefits compared to conventional single material castings. High specific strength, low weight, economic viability and the possibility for a “tailored-to-need” design have been cited. In compound castings, the material bond is of great importance to the mechanical stability and soundness of the compound casting and thus ultimately their economic and practical viability. Whilst aluminium and magnesium have been successfully joined by a number of processes, little is known about the formation of the material bond in the compound casting process and how to influence it. Consequently, in this study a methodology was devised to assess the factors influencing the formation of this material bond. The results of the experimentation were characterised using SEM, EDS, XRD, DSC measurements and mechanical testing. As similar to the solid state joining of the two metals, the primary constituents of the material bond were found to be the intermetallic phases β(Al3Mg2) and γ(Al12Mg17). Aluminium’s intrinsic oxide layer did not inhibit the formation of a bond at the interface between the two metals. In fact, the oxide layer was found to be reduced by the magnesium melt upon contact, forming MgO. The resultant MgO is then dispersed in the surrounding melt. Dissolution of the solid aluminium into the magnesium melt and the subsequent precipitation of the material bond from the liquid phase was identified to be the main mechanism behind the formation of the material bond. A direct correlation between the amount of aluminium and thickness of the material bond exists. Based on the results from experimentation, a step-by-step model was developed to explain and understand the underlying mechanism and processes that contribute to the formation and growth of the material bond in Al/Mg compound castings. The different steps were identified as: 1. Initial contact of melt and solid metal 2. Dissolution of the solid metal and solidification of the melt 3. Growth of the material bond due to solid state diffusion The addition of silicon and zinc resulted in the formation of phases according to the corresponding ternary phase diagrams. The addition of silicon resulted in the formation of an Mg2Si phase. The formation of this phase was found to be an exception, resulting from a diffusion reaction process rather than precipitation from the liquid phase. Unlike silicon, zinc was dissolved by the magnesium and crystallised during solidification on the crystal lattice of the intermetallic phases β and γ. The zinc- rich phases τ1 and φ were only formed in the presence of an abundance of zinc and displayed a very low solidification range of 367-433°C. The low melting temperature and rapid and imbalanced diffusion of zinc caused the formation of evenly shaped voids (similar to the Kirkendall effect). This was only observed with zinc concentrations greater than 15wt%. Based on the observations made during experimentation and assessment of the compound castings, a model was developed to explain the formation of these aforementioned voids. The temperature distributions during casting, solidification and cooling at the solid/liquid interface were measured and simulated with the CAE software package Magmasoft 5.31. As the formation of the interface is a complex interaction of temperature, dissolution, diffusion and solidification it cannot be precisely simulated. Despite a number of discrepancies between simulation and reality, a simulated and measured distribution was determined, that was in relatively good accordance, especially at higher temperatures. The melt composition near the solid/liquid interface was changed by the dissolution of aluminium into the magnesium melt. As a result, the melt near the solid/liquid interface solidified last, thus causing shrinkage, which negatively affected the soundness of the bond. Furthermore, soundness was impaired by fractures and cracks, thought to originate during cooling or handling of the castings. Mechanical strength of the material bond was approximated with Vickers micro-hardness measurements and push-out testing. Regardless of the used alloys and parameters, the bond displayed high brittleness with the push-out tests revealing that the material bond always fails in the aluminiumrich region. The magnesium- rich phases γ and φ (in the presence of zinc) showed signs of increased ductility (compared to the aluminium- rich phases β and τ1). Overall push-out resistance of the bond varied between 5-25MPa. Although no correlation between the thickness of the material bond and pushout resistance was evident, push-out resistance was found to be greatly affected by the soundness of the castings. Micro-hardness of the intermetallic phases in the material bond was up to 300Hv higher than that of the used aluminium and magnesium alloys. The magnesium- rich phases β and φ were found to exhibit a slightly lower hardness than the aluminium- rich phases β and τ1, whilst retaining some ductility. Several problems/drawbacks of compound castings have been identified in this study. Especially the low mechanical strength of less than 25MPa, which is considerably below than that of other joining techniques, is to be seen as problematic. Moreover, the material bond was found to be highly susceptible to galvanic corrosion, with signs of corrosion already being present on the samples after preparation. Lastly, fractures and pores were commonly found within in the bond of the compound castings, reducing the soundness of it. These problems/drawbacks will need to be overcome if Al/Mg compound castings are ever to become a viable alternative to already established and proven joining techniques