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Concentrically braced frames (CBFs) have been widely used in seismic areas as efficient structural systems to provide both lateral stiffness and strength. They dissipate earthquake energy through the inelastic deformation of the braces in both tension and compression. While these frames are efficient in providing lateral stiffness and strength, their inelastic mechanism is not ductile when compared to other systems such as moment resisting frames (MRFs). This student proposes a new approach to enhance the ductile behavior of CBFs by locally heat treating gusset plate connections or braces. In this method, the steel is heated locally to austenitizing temperature and then cooled with the appropriate rate to achieve the desired material properties. In gusset plate connections, to permit the rotation imposed from brace buckling, the conventional approach is to use linear fold lines, which can result in overly large plates. A more compact design uses elliptical fold lines, but both designs can lead to damage to welds with surrounding components. To enhance the performance of the gusset plate connection, a yield path is defined with a locally weakened zone within a high strength steel gusset plate. The weakened zone, created through heat treatment concentrated the inelastic deformation, resulting in an efficiently sized connection in which the failure mechanism is tightly controlled. A design methodology for the heat treated gusset plate is proposed, and finite element analysis is used to analyze the behavior of the heat treated gusset plates. In conventional braces, repeated buckling leads to deterioration and low-cycle fatigue which limits the ductility capacity of the CBF, compared to MRFs. As a novel approach, heat treatment is used to increase the local yield strength in the brace. Through this method, the buckling is permitted to occur, but an enhancement in the buckling behavior is intended. Various heat treated configurations are investigated, and finite element analysis is used to compare the behavior of heat treated braces. / Thesis / Master of Applied Science (MASc)

It is well known that within materials science and engineering, the advancement of steels is subject to the conflicting objectives of achieving high strength, energy absorption, and ductility within a single material. Multilayer metal composites (MLMCs), combining multiple advanced high strength steels (AHSSs), are promising candidates for designing materials that can achieve these mechanical property combinations which are unattainable by monolithic steels. However, the mechanical behaviour and corresponding properties of MLMCs are challenging to predict, due to the number of variables within the design space of the composite. Variables such as alloy design, number, thickness, configuration of layers, and interfacial bonding strength, all impact the potential mechanical properties. Accordingly, this work addressed the fracture behaviour of a multilayer AHSS composite, consisting of carbide-free bainitic (CFB) steel and high Mn twinning-induced plasticity (TWIP) steel, in both sequential deformation and co-deformation of layers to determine the potential advantages of a multilayer structure. In tensile deformation, a balanced combination of high strength (ultimate tensile strength (UTS) of 1290 MPa) and high ductility (total elongation (TE) of 23%) was achieved with a sandwich structure configuration consisting of two outer layers of the TWIP steel and an inner core layer of the CFB steel. The composite consisted of equal volume fractions of each constituent steel. The TE achieved by this structure exceeds that which previous studies would predict, which suggest that the elongation of a composite is controlled by the elongation limits of the monolithic hard layer (which in the case of the CFB steel is 13%). In the sandwich configuration, the soft outer layers contributed to increased ductility of the composite by inhibiting crack formation in the hard layer and exerting a compressive stress on the inner CFB core. The increased compression caused the CFB to yield at a lower stress (than it would in monolithic conditions), allowing it to plastically deform further, and the composite to have a greater total elongation. This was attributed to the strong interfacial bond, which enabled the layers to co-deform without any delamination. A bilayer composite consisting of the same volume fractions (as the sandwich configuration), demonstrated the same UTS, but a total elongation of 13%. The reduced ductility is a result of smaller compressive forces on the CFB, as well as, crack formation in the CFB at the 13% elongation (the TE of monolithic CFB), which led to immediate fracture of the sample. In tensile deformation with a pre-existing crack (double-edge notched tension (DENT)), the bilayer composite exhibited a high essential work of fracture (EWF)/cracking resistance. In the sandwich configuration, the outer TWIP layers exerted a compressive stress on the inner CFB core, which was possible due to the strong interfacial bond. This compressive stress and the thin layer configuration caused the CFB core to fracture in a ductile manner. The impact energy absorption of the sample was investigated by Charpy impact testing, and the procedure of crack propagation analyzed by three-point bending. High energy absorption was achieved with a notch positioned in the TWIP layer, in which the composite exceeded the energy absorption of either monolithic steel. The sample absorbed the energy through plastic deformation of the two layers, as the interface prevented crack formation in the CFB layer. When the notch was positioned in the CFB layer, the impact energy absorption was nearly equal to that of the monolithic TWIP steel. In this configuration, the composite absorbed the energy through dissipation of the propagating crack along the interface, causing delamination and subsequent bending of the TWIP layer. In assessing the experimental results in this work, it was determined that in both deformation conditions (sequential and co-deformation), the composite is sensitive to the layer configuration. To produce an optimal and balanced combination of mechanical properties (strength, energy absorption, and ductility), it is critical to inhibit or at minimum, delay crack initiation within the CFB (hard steel) layer. Overall, this research shows that the experimental multilayer composite is promising for developing an AHSS structure that can demonstrate properties unattainable by monolithic steels. / Thesis / Master of Applied Science (MASc) / Advanced high strength steels are generally limited by competing mechanical properties of strength and impact energy absorption. Combining hard and soft phase microstructures within one material (i.e. dual-phase steel) thermodynamically restricts the material by the composition and the possible heat treatment conditions. It also leads to large strain gradients resulting in void formation and failure. Instead, multilayer composites can be designed with each layer independently exhibiting a monolithic microstructure that optimizes each desired mechanical property. The bonding strength between the layers can also be adjusted, altering the distribution of stresses when the material is deformed. This research aimed to analyze a multilayer metal composite that combined a soft-phase austenitic steel exhibiting high energy absorption with a hard-phase carbide-free bainitic steel exhibiting high strength. The material was evaluated in two conditions: i) under co-deformation where the layered structure was deformed parallel to the interface and ii) under sequential deformation, where stress was applied to one layer at a time. The results indicated that in both conditions, the composite was sensitive to the configuration of the layers. It demonstrated the potential to exhibit a combination of high strength and high energy absorption capabilities in sequential deformation. In co-deformation, certain configurations of the composite were able to exhibit increased ductility and fracture resistance (improved from the monolithic hard steel). In both cases, the critical design factor was that crack initiation and propagation must be restricted in the hard material to achieve balanced mechanical properties of strength and energy absorption.

In an effort to increase the fuel efficiency of cars more widespread use of higher strength steels is seen for their high strength-to-weight ratio. Thesesteels are more limited in their formability and tendency to harden than conventional steels, complicating manufacturing. This thesis summarizes the available research on resistance projection nut welding to higher strength steels and investigates the accuracy of the simulation program SORPAS when simulating projection nut welds to AlSi-coated Boron steel. It was found that the greatest difficulties in welding coated ultra high strength steels were the metallurgical effects of both the high alloying content of the steel and the coatings interacting with the weld when melting. Although SORPAS was an intuitive program to use for resistance welding and had a wide library of materials available, it was not found to be able to predict the resistance characteristics or results of projection nut welds to coated Boron steel without significant changes to default material parameters. The biggest issue was the delaying effect the coating layer had on the peak resistance, something not observed experimentally. Better results are suggested to be possible after experimentally ensuring the properties of the materials used and importing those values into SORPAS. / I ett försök att öka bilars bränsleeffektivitet används i allt större utsträckning stål med högre hållfasthet på grund av deras goda förhållande mellan styrka och vikt. Dessa stål är mer begränsade i sin formbarhet och tendens att härda än konventionella stål, vilket försvårar tillverkning. Denna uppsats sammanfattar den tillgängliga forskningen om motståndssvetsning med projektionsmutter i höghållfasta stål och undersöker noggrannheten hos simuleringsprogrammet SORPAS vid simulering av projektionsmuttersvetsar i AlSi-belagt borstål. Det konstaterades att de största svårigheterna vid svetsning av belagda ultrahöghållfasta stål var de metallurgiska effekterna av både stålets höga legeringsinnehåll och beläggningarnas interaktion med svetsen vid smältning. Trots att SORPAS var ett intuitivt program att använda för motståndssvetsning och hade ett brett bibliotek av tillgängliga material kunde det inte förutsäga motståndsegenskaperna eller resultaten av projektionsmuttersvetsar mot belagt borstål utan betydande förändringar av standardmaterialparametrarna. Det största problemet var den fördröjande effekt som beläggningsskiktet hade på maximala resistansen, något som inte observerades experimentellt. Bättre överenstämmelse föreslås vara möjlig efter att experimentellt säkerställa egenskaperna hos de material som används och importera dessa värden till SORPAS.

Recently, Zn-XAl-YMg coatings have emerged as lighter-weight substitutes for traditional Zn-based coatings for the corrosion protection of steels; however, little is currently known concerning the interactions between the oxides present on advanced high strength steel (AHSS) surfaces and the Zn-Al-Mg bath. In the current contri- bution, the selective oxidation and reactive wetting of a series of C-Mn AHSS were determined with the objective of providing a quantitative description of this pro- cess. The process atmosphere pO2 was varied using dew points of −50◦C, −30◦C and −5◦C. The surface oxide chemistry and morphology were analysed by means of SEM and XPS techniques. Reactive wetting of the selectively oxidized surfaces using a Zn-1.6 wt.% Al-1.6 wt.% Mg bath was monitored as a function of annealing time at 60 s, 100 s and 140 s at 800◦C. The resulting bare spot defects in the Zn-1.6 wt.% Al-1.6 wt.% Mg coating were assessed by means of SAM-AES and FIB, while coating adhesion was analysed by 180◦ bend tests. Annealing the steel substrates resulted in the formation of surface MnO, which varied based on pO2 and Mn alloy content, and that this MnO greatly reduced the wettability of the steel by the Zn-1.6 wt.% Al- 1.6 wt.% Mg bath, resulting in bare spot defects. It was determined that the reactive wetting of the steel substrate was dependant on the oxide morphology and oxidation mode, which was a function of both alloying content of Mn in the steel and annealing pO2 process atmosphere (dew point). Finally, it was concluded that the bare spot area percentage on the coated panels was statistically invariant for annealing times of between 60 s and 140 s at 800◦C. / Thesis / Master of Applied Science (MASc) / Metallic coatings are applied to steels that are not naturally corrosion resistant. The aim of this research was to determine how well a coating containing zinc, aluminum and magnesium adhered to high strength automotive steel. It was deter- mined that manganese oxides formed on the steel during heating prior to applying the metallic coating. The manganese oxides prevented good adhesion between the steel and the coating, resulting in bare spot defects in the coating. The bare spot defects are undesirable as they leave the steel exposed and therefore susceptible to corrosion and are unsightly when painted.

Nowadays there is a strong demand for lighter vehicles in order to increase the pay load. Through this the specific fuel consumption is decreased, the amount of greenhouse gases is lowered and the transport economy improved. One possibility to optimize the weight is to make the components from high strength steels and join them by welding. Welding is the main joining method for fabrication of a large proportion of all engineering structures. Many components experience fatigue loading during all or part of their life time and welded connections are often the prime location of fatigue failure.Fatigue fracture in welded structures often initiates at the weld toe as aconsequence of large residual stresses and changes in geometry acting as stress concentrators. The objective of this research is to increase the understanding of the factors that control fatigue life in welded components made from very high strength steels with a yield strength of more than 800 MPa. In particular the influences of the local weld toe geometry (weld toe radius and angle) and residual stress on fatigue life have been studied. Residual stresses have been varied by welding with conventional as well as Low Transformation Temperature (LTT) filler materials. The three non-destructive techniques Weld Impression Analysis (WIA), Laser Scanning Profiling (LSP) and Structured Light Projection (SLP) have been applied to evaluate the weld toe geometry.Results suggest that all three methods could be used successfully to measure the weld toe radius and angle, but the obtained data are dependent on the evaluation procedure. WIA seems to be a suitable and economical choice when the aim is just finding the radius. However, SLP is a good method to fast obtain a threedimensional image of the weld profile, which also makes it more suitable for quality control in production. It was also found that the use of LTTconsumables increased fatigue life and that residual stress has a relatively larger influence than the weld toe geometry on fatigue strength of welded parts.

Multi-material structures are increasingly used in vehicle bodies to reduce weight of cars. The use of these lightweight structures is driven by requirements to improve fuel economy and reduce CO2 emissions. The automotive industry has replaced conventional steel components by lighter metals such as aluminium alloy. This is done together with cutting weight of structures using more advanced strength steels. However, sound joining is still difficult to achieve due to differences in chemical and thermal properties.   This research aims to develop a new innovative welding technique for joining aluminium alloy to ultra-high strength steels. The technique is based on friction stir welding process while the non-consumable tool is made of an ordinary tool steel. Welding was done by penetrating the rotating tool from the aluminium side without penetrating into the steel surface. One grade of Al-Mg aluminium alloy was welded to ultra-high strength steels under lap joint configuration. Different types of steel surface coatings including uncoated, hot-dipped galvanised and electrogalvanised coating have been studied in order to investigate the influence of zinc on the joint properties. The correlation among welding parameters, microstructures, intermetallic formation and mechanical properties are demonstrated in this thesis.  Results have shown that friction stir welding can deliver fully strong joints between aluminium alloy and ultra-high strength steels. Two intermetallic phases, Al5Fe2 and Al13Fe4, were formed at the interface of Al to Fe regardless of surface coating conditions. The presence of zinc can improve joint strength especially at low heat input welding due to an increased atomic bonding at Al-Fe interface. The formation of intermetallic phases as well as their characteristics has been demonstrated in this thesis. The proposed welding mechanisms are given based on metallography investigations and related literature. / <p>QC 20170519</p>

A aplicação de chapas de aço como revestimento em matrizes de forjamento a quente tem sido estudada como uma alternativa inovadora, dentro de um projeto de cooperação internacional titulado “Evaluation of Sheet Metal Covers to Improve Tool Life in Forging”. Pesquisa realizada em parceria com a Alemanha pelo programa BRAGECRIM. Essas chapas serviriam como um metal de sacrifício em matrizes de forjamento a quente, que pretende substituir tratamentos superficiais de alto custo. As limitações geométricas e propriedades mecânicas das chapas garantiriam o sucesso ou não da gravura conforme o design exigido pela ferramenta, como também o número de ciclos de forjamento mantendo-se a integridade. Desta forma, o conhecimento das propriedades mecânicas em chapas e ductilidade torna-se fundamental para garantir os limites de aplicabilidade. Portanto, este trabalho tem como objetivo avaliar as propriedades mecânicas e os aspectos metalúrgicos em diferentes temperaturas de duas chapas de alta resistência DP600 e Hardox450®, os quais possuem diferentes microestruturas e ductilidade. Esta escolha se deve as diferentes aplicações que ambos os materiais possuem na indústria automobilística, devido as suas propriedades físicas, com a hipótese que influenciará na aplicação como máscaras que irão proteger as matrizes. Para isso foram realizados ensaios de dureza após ciclos de aquecimento e tração em diferentes temperaturas, com taxa de deformação controlada, simulando as condições térmicas que o material irá suportar durante o processo de forjamento. Para avaliar a ductilidade das chapas foi realizado o ensaio de estiramento biaxial. Uma análise pelo método de elementos finitos foi utilizada no ensaio de estiramento de punção esférico Erichsen, o qual foi possível pela análise de laboratório validar os experimentos e então a realização da simulação de estampagem de uma geometria bi radial em formato de copo. Os resultados de tração e dureza mostram que o DP600 possui considerável resistência mecânica em altas temperaturas com boa ductilidade, porém não maior que o Hardox450®, que perde em termos de ductilidade devido a fenômenos de fragilização em altas temperaturas. A simulação numérica permitiu avaliar como seria o produto estampado em uma geometria 3D, sobre os aspectos geométricos da chapa e os efeitos de anisotropia do DP600, como também suas tensões. / The application of sheet metal cover in hot forging dies has been studied as an innovative alternative, within an international project titled "Evaluation of sheet metal covers to improve tool life in forging". Research carried out in partnership with Germany under the BRAGECRIM program. The sheet metal would apply as a sacrificial membrane in the hot forging die, which intended to replace expensive surface treatments. The geometrical limitations and mechanical properties of the plates would guarantee the success or failure of the engraving according to the design required by the tool, as well as the number of forging cycles maintaining the integrity. In this way, the knowledge of the mechanical properties of the sheet metals and ductility becomes fundamental to guarantee the limits of applicability. Therefore, this work has as objective to evaluate the mechanical properties and the metallurgical aspects in different temperatures of two advanced High Strength Steel Sheets DP600 and Hardox450®, which have different microstructures and ductility. This choice is due to the different applications that both materials have in the automotive industry due to their properties, with the hypothesis that will influence the application as masks that will protect dies. Therefore, a hardness test has performed after heating cycles and tensile tests at different temperatures, with a controlled strain rate, simulating the thermal conditions that the material will withstand during the forging process. Biaxial stretching test to evaluate the ductility of the plates were performed. An inverse analysis by the finite element method was used in the Erichsen biaxial stretching test, in which it has been possible to validate the experiments and then to perform the stamping simulation of a bi-radial geometry in a cup format. The results show that the DP600 has considerable mechanical resistance at high temperatures with good ductility, but not higher than the Hardox450®, which is inferior in terms of ductility due to embrittlement phenomena at high temperatures. The numerical simulation allowed an evaluation of how the product can be stamped in a 3D geometry, the geometric aspects of the plate, the anisotropy effects of the DP600, as well as stress distributions.

Friction Bit Joining (FBJ) is a new technology that can be used to join dissimilar materials together. This ability makes it a good candidate for creating light weight structures for the automotive industry by combining lightweight materials such as aluminum to stronger materials like advanced high-strength steels. The automotive industry and many other industries have great interest in reducing structure weight to increase fuel efficiency. The purpose of this research is to make FBJ of GADP 1180 to AA 7085-T76 a production ready process by (1) better understanding the effects of process parameters, bit design and tool design on joint strength and reliability especially as they relate to different joint configurations; (2) determining if consecutive FBJ joints on a part will be additive in strength; (3) improving surface finish for better coating adhesion so that joints can be made to withstand extended corrosion testing; and (4) determining the failure modes and fatigue life of joint components at high and low load amplitudes. No universal parameter set for optimizing peak load for T-peel, cross tension, and lap-shear tension configurations were found. Due to the extreme load conditions of T-peel and the smaller margin of safety it is better to optimize for T-peel. However, strength and reliability were still improved across the board. Cutting features and tapered shanks were found to not always be necessary. Removing cutting features from the bit design increased peak weld cycle loads, but a stiffer machine can overcome this. Consecutive FBJ joints on a part are mostly additive in nature. When the weakest joint fails, its load is distributed to the remaining joints and will limit the peak load of the whole part. If all joints are "good" then the peak load will be approximately additive. Most of the stress is localized on the side of the bit opposite of the pulling direction. Failure modes in lap-shear tend to change from weld nugget pullouts in single weld specimens to aluminum material failures in multi-weld specimens. This is because of the added stiffness that additional material and welds provide to resist coupons bending and creating a peeling action. Surface finish was improved by development of a floating carbide cutting system which cut aluminum flash as it was generated around the head of the bit. A new internal drive design provided the ability to drive bits flush with the aluminum top layer if desired with minimal reductions in strength. Flush bits provided benefits in safety, cosmetics, and coating adhesion.