Increasing the use of Ultra High Strength Steels (UHSS) in vehicle bodystructures is important for reducing weight and cutting CO2 emissions. This thesis investigates challenges in resistance welding that can be a barrier to implementing UHSS as a replacement for low strength steels in vehicle structures. Empirical research has been performed to offer new approaches for improved joint strength and to increase knowledge on cracking mechanisms in resistance projection welding and resistance spot welding of UHSS. By optimising the current build-up phase and peak current during the first milliseconds of weld time, it was shown that the strength could be improved by up to two-fold for projection welded joints. An approach to improve the ductility and strength of resistance spotwelds in UHSS using reduced cooling time was unsuccessful. The reduced cooling rate after weld metal solidification did not fully create the desired softened microstructure. The study on the surface cracking mechanism in resistance spot welded dual-phase UHSS showed that cracking is linked to the galvanization method. It is proposed that formation of aluminium oxide layers on the electrode tips increases the surface temperature and thereby increases the probability for liquid metal embrittlement and surface cracking. / <p>QC 20150526</p>

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In this master thesis, performed at Ovako Group R&amp;D (Hofors) a new machining test method for ranking heats of calcium-treated steels, M-Steels, has been proposed.  A total of six different heats of 42CrMo4-M steel in the quenched-and-tempered (Q+T) condition were tested. The proposed method uses dry turning with coated cemented carbide tools at a cutting speed of 225 m/min, 2 mm depth of cut, and a feed rate of 0.35 rev/min. The machining test is tuned so that M-Steels within medium-sized bar diameters and a small hardness interval can be differentiated by monitoring the flank wear progression of the cutting tool. This method is easily taught to a CNC-operator, it mimics real workshop conditions and can be repeated with relative robustness and ease of execution. It is possible to get a ranking where both the M-Steel treatment and hardness are included.  The major differentiator of tool wear performance was found to be a combination of wear of the outermost Al2O3 coating and the build-up and renewal of protective slag deposits on the tool rake. The rake face wear was found similar between the M-Steels samples, so it is thought that the effect of the slag deposits, as thermal barriers, affect the plastic deformation and wear of the flank face. Given the slag deposits sub-micron thickness, this indirect method of recording the flank wear is highly recommended to showcase the positive effects of the M-Steel in machining The inclusion content was evaluated according to ASTM E45 and with a scanning electron microscope (SEM) automated inclusion assessment. The link between the machinability performance of the M-Steels and their inclusion characteristics could not be clarified in this work.  Teknisk-naturvetenskapliga fakulteten

Laser-Directed Energy Deposition (L-DED), an Additive Manufacturing (AM) processused for the fabrication of parts in a layer-wise approach has displayed an immense potential over the last decade. The aerospace industry stands as the primary beneficiary due to the L-DED process capability to build near-net-shape components with minimal tooling and thereby producing minimum wastage because of reduced machining. The widespread use of Alloy 718 in the aero-engine application has prompted huge research interest in the development of L-DED processing of this superalloy. AM processes are hindered by low build rates and high cycle times which directly affects the process costs. To overcome these issues, the present work focusses on obtaining high deposition rates through a high material feed. Studying the influence of process parameters during the L-DED process is of prime importance as they determine the performance of in-service structures. In the present work, process parameters such as laser power, scanning speed, feed rate and stand-offdistances are varied and their influence on geometry and microstructure of Alloy 718 single-track deposits are analyzed. The geometry of deposits is measured in terms of height, width and depth; and the powder capture efficiency is determined by measuring areas of deposition and dilution. The microstructure of the deposits shows a column ardendritic structure in the middle and bottom region of the deposits and equiaxed grains in the top region. Nb-rich segregation involving laves and NbC phases, typical of Alloy718 is found in the interdendritic regions and grain boundaries. The segregation increases along the height of the deposit with the bottom region having the least and the top region showing the highest concentration of Nb-rich phases due to the variation in cooling rates. A high laser power (1600 W – 2000 W) and a high scanning speed (1100 mm/min) are found to be the preferable processing conditions for minimizing segregation. Another approach to minimize segregation is by performing post-build heat treatments. The solution treatment (954 °C/1 hr) and double aging (718 °C/8 hr + 621 °C/ 8 hr) standardized for the wrought form of Alloy 718 is applied to as-built deposits which showed a reduction in segregation due to the dissolution of Nb-rich phases. Upon solution treatment, this reduction is accompanied by precipitation of the delta phase, found predominantly in top and bottom regions and sparsely in the middle region of the deposit.

Additive Manufacturing (AM), commonly known as 3D Printing, is a disruptive modern manufacturing process, in which parts are manufactured in a layer-wise fashion. Among the metal AM processes, Powder Bed Fusion (PBF) technology has opened up a design space that was not formerly accessible with conventional manufacturing processes. It is, now, possible to manufacture complex geometries, such as topology-optimized structures, lattice structures and intricate internal channels, with relative ease. PBF is comprised of Electron Beam Melting (EBM) and Selective Laser Melting (SLM) processes. Though AM processes offer several advantages, the suitability of these processes to replace conventional manufacturing processes must be studied in detail; for instance, the capability to produce components of consistent quality. Therefore, understanding the relationship between the AM process together with the post treatment used and the resulting microstructure and its influence on the mechanical properties is crucial, to enable manufacturing of high-performance components. In this regard, for AM built Alloy 718, only a limited amount of work has been performed compared to conventional processes such as casting and forging. The aim of this work, therefore, is to understand how the fatigue properties of EBM and SLM built Alloy 718, subjected to different thermal post-treatments, is affected by the microstructure. In addition, the effect of as-built surface roughness is also studied. Defects can have a detrimental effect on fatigue life. Numerous factors such as the defect type, size, shape, location, distribution and nature determine the effect of defects on properties. Hot Isostatic Pressing (HIP) improves fatigue life as it leads to closure of most defects. Presence of oxides in the defects, however, hinders complete closure by HIP. Machining the as-built surface improves fatiguelife; however, for EBM manufactured material, the extent of improvement is dependent on the amount of material removed. The as-built surface roughness, which has numerous crack initiation sites, leads to lower scatter in fatigue life. In both SLM and EBM manufactured material, fatigue crack propagation is transgranular. Crack propagation is affected by grain size and texture of the material.

Schriftenreihe, die Tagungsbände und Beiträge des Instituts für Füge- und Montagetechnik beinhaltet. / Publication series containing conference proceedings and contributions from the Institute of Joining and Assembly Technology.

The current patterns of consumption are threatening the availability of natural resources. These natural resources, such as raw materials and biomass, are critical for different economic activities. ProductService System (PSS) has received attention as a solution to drive resource efficiency by extending the life cycle of products. Given the many barriers to shifting from product to PSS-based offering, the literature has acknowledged the importance of partnerships to support Small and Medium-SizedManufacturers (SMMs) during the life cycle of PSS. However, the role played by the partner in supporting the SMM, as well as how are they supporting to overcome existing barriers, remains unclear. To address this gap, the present thesis to aims comprehensively identify the multifaceted role of external partners throughout the life cycle of PSS developed by SMM. The research follows a cross-sectional design to investigate three industry sectors located in Finland, Norway, and Sweden: forestry, farming, and mining machinery manufacturers. Through semi-structured qualitative interviews with CEOs and directors, data was collected and analyzed by thematic analysis. The results show the existence of eight key partners along the life cycle of PSS: Dealers, Suppliers, End Customers, Service Providers, Industry Hubs, Parent Company, New Partners (such as KIBS firms),and Other, such as the ones associated with delivery. These partners were found to support SMMs by co-creation (joint activities involving the exchange of resources between the partner and SMM), education (knowledge sharing from the partner to the SMM), market-related roles (activities associated with the end user, such as sales, marketing, and service provision), and supply chain (roles liked to supply of components and logistics). Partners can drive the shift from current product-oriented PSStowards use- and result-oriented ones, which have a higher potential of creating environmental benefits. The findings of this thesis contribute to the existing literature in many ways. First, it clarifies how partners are supporting SMMs along the life cycle of PSS. Second, it shows that some partners, such as dealers, are more important through the life cycle than other partners. Third, it shows that SMMs opt for different partners to perform the same role. Therefore, it opens the opportunity for future research questions to investigate the causes behind such choices. In terms of practical implications, the thesis support SMMs to optimize and diversify their network of partners by considering partnership alternatives. From the partner’s point of view, it shows the expectations of the SMMs for their partners. Therefore, it can be used to improve the existing solutions aiming to better address the SMMs’ needs.

Sustainability and sustainable production methods are becoming more important in the manufacturing industry all around the world. Within this trend, United Nations’ formulated 2030 Sustainable Development Goals are used as a target and guideline for developing a roadmap in manufacturing companies’ schedules. For this purpose, there are some existing methods used in practice such as Triple Bottom Line (TBL) management, which is based on the environmental, economic, and social aspects of sustainability. The purpose of this study is aiming to provide insights into the role of the TBL management approach in supporting companies to achieve the UN 2030 sustainability goals. As well as contributing to the ongoing discussions on the importance of sustainable practices in achieving the significance of the study. The findings of this study, gathered through a combination of qualitative primary and secondary data collection, show that there are some similarities and differences between the sustainability practices depending on the SDGs. Approaches of the companies vary with factors such as legislations, culture, and other formal and informal institutions of the country. It is found that most of the Dutch companies act more proactive regarding the SDGs whereas Turkish organisations are more reactive to change. It can be seen that Turkish organisations are very much still primarily focused on the economical perspective of TBL. Dutch organisations tend to prioritise the social and the environmental aspects more than their Turkish counterparts. It can also be identified that Turkish organisations tend to be incredibly focused on legislations, whereas Dutch organisations are more often moving towards their own visions and how they want to position themselves in the market. Even though there is a significant difference between the Dutch and Turkish organisations, it can be identified that the economical aspect plays a major role in both cases. Yet the way they work towards this differs strongly.

The demand for Advanced High Strength Steel (AHSS) in the automotive industry is increasing day by day. It is mainly motivated by the fact that AHSS can be used as thin sheets while having high strengths. It enables weight reduction of the automobiles which consequently increases the fuel efficiency and has proven to be less harmful to the environment. It is also expected that AHSS will have even higher strength in the near future. Cold work tools steels with 5 wt.% Cr are commonly used to process AHSS. Therefore, the tool steel must meet the challenges in the future, i.e. have even higher hardness, compressive strength and toughness. One way of increasing the mechanical properties of the tool steel is by improving the heat treatment parameters. However, it is not possible without a deeper understanding of the heat treatment process. Therefore, this work presents investigations related to phase transformations occurring in a 5 wt.% Cr cold work tool steel during heat treatment. Furthermore, the influence of austenitisation and tempering temperatures on the microstructure and mechanical properties were investigated. The studies revealed that a higher austenitisation temperature can be used to achieve a higher hardness, good compressive strength and adequate toughnessof the steel. However, too high austenitisation temperature may result inexcessive coarsening of prior austenite grains which reduced the impact toughness. It was also found that retained austenite can transform during tempering by two different mechanisms. Firstly, when tempering at 525°C, carbides precipitate in retained austenite lowering its stability and permitting a transformation to marten site on cooling. Secondly, when tempering at 600°Cfor extended holding time retained austenite isothermally transforms to ferrite and carbides. This occurs by precipitation of carbides in retained austenite followed by a final transformation to ferrite and carbides.These results were used to understand the standard tempering procedure of the 5 wt.% Cr cold work tool steel. Furthermore, alternative heat treatment procedures are discussed based on the important findings presented in this thesis.

This bachelor thesis was made in collaboration with Scania. The objective was to find the cause for defects found in some rear axle welds. It was known axle material was coated with anti-corrosive oil. Oils were examined through ICP-AES, and then compared to the composition found on the surface of the steel samples. Elements found in the oils vastly differed from one another. One of the oils contains large amounts of aluminium while the other contains high levels of calcium. When samples surfaces were analysed using EDS, phases consisting of aluminium and calcium were observed. These results indicate that the wrong anti-corrosive had been used for the axle material which gave substandard welds. The oil used contained elements with a low vaporisation temperature, such as calcium. This causes instabilities in the keyhole, leading to collapse. Collapse of the keyhole facilitates the formation of defects.