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.

Thin-walled steel sections made from high strength thin cold-reduced G550 steel to Australian Standard AS 1397-1993 under compression are investigated experimentally and theoretically in this thesis. This thesis describes three series of compression tests performed on box-section stub columns, box-section long columns and lipped channel section columns cold-formed from high strength steel plates in 0.42 mm or 0.60 mm thickness with nominal yield stress of 550 MPa. The tests presented in this thesis formed part of an Australian Research Council research project entitled: Compression Stability of High Strength Steel Sections with Low Strain-Hardening. For the fix-ended stub column tests, a total of 94 lipped-square and hexagonal section stub columns were tested to study the influence of low strain hardening of G550 steel on the compressive section capacities of the column members. For the pin-ended long column tests, a total of 28 box-section columns were tested to study the stability of members with sections which undergo local instability at loads significantly less than the ultimate loads. For the fix-ended lipped channel section columns, a total of 21 stub and long columns were tested to study the failure resulting from local and distortional buckling with interaction between the modes. A numerical simulation on the three series of tests using the commercial finite element computer program ABAQUS is also presented as part of this thesis. The post-buckling behaviour of thin-walled compression members is investigated. The effect of changing variables, such as geometric imperfections and end boundary conditions is also investigated. The ABAQUS analysis gives accurate simulations of the tests and is in good agreement to the experimental results. Theoretical studies using finite strip methods are presented in this thesis to investigate the buckling behaviour of cold-formed members in compression. The theoretical studies provide valuable information on the local and distortional buckling stresses for use in the interaction buckling studies. The finite strip models used are the semi-analytical and spline models. As expected for the stub columns tests, the greatest effect of low strain hardening was for the stockier sections where material properties play an important role. For the more slender sections where elastic local buckling and post-local buckling are more important, the effect of low strain hardening does not appear to be as significant. The pin-ended and fix-ended long column tests show that interaction, which is between local and overall buckling in the box sections, and between local and distortional buckling in the open channel sections, has a significant effect on their member capacities. The results of the successful column tests and ABAQUS simulation have been compared with the design procedures in the Australian & New Zealand Standard for Cold-Formed Steel Structures AS&NZS 4600 and the North American Specification for Cold-Formed Steel Structural Members prepared by the American Iron and Steel Institute. The stub column tests show that the current design rules give too conservative predictions on the compressive section capacities of the column members; whereas the long column tests show that the current column design rules are unconservative if used in their current form for G550 steel. Three design proposals are presented in this thesis to account for the effects of high strength thin steels on the section and member capacities.

No description available.

Thin-walled steel sections made from high strength thin cold-reduced G550 steel to Australian Standard AS 1397-1993 under compression are investigated experimentally and theoretically in this thesis. This thesis describes three series of compression tests performed on box-section stub columns, box-section long columns and lipped channel section columns cold-formed from high strength steel plates in 0.42 mm or 0.60 mm thickness with nominal yield stress of 550 MPa. The tests presented in this thesis formed part of an Australian Research Council research project entitled: Compression Stability of High Strength Steel Sections with Low Strain-Hardening. For the fix-ended stub column tests, a total of 94 lipped-square and hexagonal section stub columns were tested to study the influence of low strain hardening of G550 steel on the compressive section capacities of the column members. For the pin-ended long column tests, a total of 28 box-section columns were tested to study the stability of members with sections which undergo local instability at loads significantly less than the ultimate loads. For the fix-ended lipped channel section columns, a total of 21 stub and long columns were tested to study the failure resulting from local and distortional buckling with interaction between the modes. A numerical simulation on the three series of tests using the commercial finite element computer program ABAQUS is also presented as part of this thesis. The post-buckling behaviour of thin-walled compression members is investigated. The effect of changing variables, such as geometric imperfections and end boundary conditions is also investigated. The ABAQUS analysis gives accurate simulations of the tests and is in good agreement to the experimental results. Theoretical studies using finite strip methods are presented in this thesis to investigate the buckling behaviour of cold-formed members in compression. The theoretical studies provide valuable information on the local and distortional buckling stresses for use in the interaction buckling studies. The finite strip models used are the semi-analytical and spline models. As expected for the stub columns tests, the greatest effect of low strain hardening was for the stockier sections where material properties play an important role. For the more slender sections where elastic local buckling and post-local buckling are more important, the effect of low strain hardening does not appear to be as significant. The pin-ended and fix-ended long column tests show that interaction, which is between local and overall buckling in the box sections, and between local and distortional buckling in the open channel sections, has a significant effect on their member capacities. The results of the successful column tests and ABAQUS simulation have been compared with the design procedures in the Australian & New Zealand Standard for Cold-Formed Steel Structures AS&NZS 4600 and the North American Specification for Cold-Formed Steel Structural Members prepared by the American Iron and Steel Institute. The stub column tests show that the current design rules give too conservative predictions on the compressive section capacities of the column members; whereas the long column tests show that the current column design rules are unconservative if used in their current form for G550 steel. Three design proposals are presented in this thesis to account for the effects of high strength thin steels on the section and member capacities.

Residual compressive stresses can be used to increase the lifetime of parts under cyclic stress as they negate the applied tensile stresses that cause crack initiation and propagation in the material. The goal of this project was to investigate methods to induce stresses, their advantages and disadvantages as well as depth and magnitude of induced stresses, and also to find methods of analyzing the induced residual stresses. This was done on behalf of Epiroc Drilling Tools AB in order for them to induce stresses on the insides of their long, narrow and hollow rods, where stress induction is difficult. Shot peening was used as a reference as that is the method currently in use by the company. The results show that the two most promising methods are cavitation peening and laser shock peening; two relatively new methods with large magnitudes and depth of induced stress as well as a great capability of inducing stresses on the hard-to-reach insides of the rods. Ultrasonic needle peening, ultrasonic shot peening as well as induction hardening, cryogenic treatment and friction stir processing were also investigated. Methods of analyzing the stresses include X-ray diffraction and slitting, hole drilling and ultrasonic methods.

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.