Laser welding of boron steels for light-weight vehicle applications

Laser beam welding has gained a significant interest during the last two decades. The suitability of the process for high volume production has the possibility to give a strong advantage compared to several other welding methods. However, it is important to have the process in full control since various quality issues may otherwise occur. During laser welding of boron steels quality issues such as imperfections, changes in local and global geometry as well as strength reduction can occur. The aspects that need to be considered are strongly depending on alloy content, process parameters etc. These problems that can occur could be fatal for the construction and the lowest level of occurrence is wanted, independent of industry. The focus of this study has been to investigate the properties of laser welded boron steel. The study includes laser welding of boron alloyed steels with strengths of 1500 MPa and a recently introduced 1900 MPa grade. Focus has been to investigate weldability and the occurrence of cracks, porosity and strength reducing microstructure that can occur during laser welding, as well as distortion studies for tolerances in geometry. The results show that both conventional and 1900 MPa boron alloyed steel are suitable for laser welding. Due to the martensitic structure of welds the material tends to behave brittle. Cracking and porosity do not seem to be an issue limiting the use of these steels. For tolerances in geometry for larger structures tests has been done simulating laser welding of A-pillars and B-pillars. Measurements have been done with Vernier caliper as well as a more advanced optical method capturing the movements during the welding sequence. Results from the tests done on Ushaped beams indicates that depending on the geometry of the structure and heat input distortions can be controlled to give distortions from 1 to 8 mm, at a welding length of 700 mm. This means that important geometry points can be distorted several millimeters if the laser welding process not is controlled

Laser welding of ultra-high strength steel and a cast magnesium alloy for light-weight design

There is a strong industrial need for developing robust and flexible manufacturing methods for future light-weight design. Better performing, environmental friendly vehicles will gain competitive strength from using light weight structures. In this study, focus has been on laser welding induced distortions for ultra-high strength steel (UHSS) where trials were performed on single hat and double hat beams simulating A-pillar and B-pillar structures. Furthermore, also laser welding induced porosity in cast magnesium alloy AM50 for interior parts were studied. For UHSS, conventional laser welding was done in a fixture designed for research. For cast magnesium, single-spot and twin-spot welding were done. Measurements of final distortions and metallographic investigations have been performed. The results show that the total weld metal volume or the total energy input were good measures for predicting the distortions within one steel grade. For comparing different steel grades, the width of the hard zone should be used. The relation between the width of the hard zone, corresponding to the martensitic area of the weld, and the distortions is almost linear. Additionally, compared with continuous welds, stitching reduced the distortions. For cast magnesium, two-pass (repeated parameters) welding with single-spot gave the lowest porosity of approximately 3%. However, two-pass welding is not considered production friendly. Twin-spot welding was done, where the first beam provided time for nucleation and some growth of pores while reheating by the second beam should provide time for pores to grow and escape. This gave a porosity of around 5%. Distortions and porosity are the main quality problems that occur while laser welding UHSS and cast magnesium, respectively. Low energy input seems to generally minimize quality issues. Laser welding shows high potential regarding weld quality and other general aspects such as productivity in light-weight design for both high strength steel and cast magnesium

Vad påverkar bankers räntemarginaler? : En studie av räntemarginaler i Norden och vad som påverkar bankers räntemarginaler i Norden under 2000-talet

Denna uppsats undersöker med hjälp av Ho & Saunders (1981) ränte- marginalsmodell de nordiska länderna Sveriges, Norges och Danmarks banker och deras räntemarginaler. Undersökningen sträcker sig mellan 2001 och 2011 och totalt 167 banker är representerade i undersök- ningen. Resultaten av undersökningen visar att faktorerna bankernas dolda räntebetalningar och bankernas kapitaltäckningskvot påverkar bankernas bruttoräntemarginal positivt. Undersökningen visade även att den svenska marknadsstrukturen leder till högre räntemarginaler än i Norge och Danmark allt annat lika, vilket tyder på att den svenska bankmarknaden är sämre konkurrensutsatt än Norges- och Danmarks bankmarknader

Controlled metal transfer from a wire by a laser-induced boiling front

The addition of wire is an option during laser welding, laser cladding or laser additive manufacturing. By high speedimaging of leading wire addition during fibre laser keyhole welding it was observed that for the 40 experiments underconsideration the wire tip always established a concave boiling front. The front appears similar to a keyhole front and issort of a continuation of the keyhole, owing to the leading wire employment. For most of the parameters the melt istransferred downwards from the wire tip into the melt pool surrounding the keyhole front. In other words, hardly anyuncontrolled spatter to the sides was observed. A trailing wire would normally tend to a completely different behaviour.Typical as well as limiting phenomena of the wire melt transfer mechanism are presented and discussed. Controlledvertical melt transfer of the wire through the ablation pressure from a laser-induced boiling front, either in contact withthe workpiece surface or positioned higher above, can be a desirable mechanism of metal deposition for the differenttechniques, namely welding, surface treatment or LAM. By suitable choice of the laser power density above the boilingthreshold, the here observed mechanism can be applied in a controllable manner. An interesting technique option islateral beam oscillation for example by a galvanometer optics which shears off the melt in a manner similar to remotefusion cutting. The process limits become different to the static technique. The wire melt transfer technique has thepotential to be developed further towards a highly controllable remote drop transfer, e.g. in terms of direction.Upprättat; 2016; 20151117 (alka)</p