Dilution is an unavoidable consequence of multi-material fusion processing, i.e. welding, cladding etc. In this paper we propose a novel method for controlled dilution experiments, analyzing microstructural trends of steel filler wire diluted with steel base metal. The highlight of this method is the control of processing conditions used to melt a pre-determined dilution of two high strength steels. The materials involved are S960QL base metal machining chips and a chopped under-matched wire consumable, which is used to increase the toughness of welded joints. These materials were combined in specific mass ratios in a prepared cavity and then melted by a pulsed laser beam. A high-speed RGB camera evaluated the relative spatial temperature of the melt surface. The molten mass then solidified into a uniform nugget, confirmed by energy dispersive x-ray spectrometry (EDS) to have a homogenous chemical composition (a ‘Snapshot’ nugget). Hardness values obtained for different dilution levels were compared to a narrow gap multi-layer laser weld (NGMLW), with a decreased dilution rate yielding a decreased hardness. The Snapshot microstructures created are similar to the different regions of the NGMLW, in the weld cap and in the body of the weld. Snapshot nuggets were also evaluated for non-metallic inclusion (NMI) size distributions relating to the dilution levels (NMIs are important indicators for acicular ferrite, which has been shown to increase impact toughness).

Laser materials processing, including thermal treatment and laser welding has been undergoing continuous growth in the manufacturing industry for decades. A laser beam offers high precision and energy transfer, capable of various processing. For many cases a Gaussian beam is applied, but lately development of more complex beam shapes has been developed, where e.g. multiple beams (beam splitting) can be used for increased tolerances during welding.

This thesis presents six papers (Papers A-F) on welding of high strength steels, laser pulse shaping, thermal treatments, and microstructural investigations. Different methods for obtaining a desirable weld were investigated through tailoring of the laser beam process. This affected the resulting temperature fields and thermal histories of the specimens. Experimental analysis was supported through various in-situ observation techniques and metallurgical studies.

Papers A-C present thermal processing and chemical manipulation to obtain the desired microstructure, by introducing and applying the here introduced Snapshot method. Paper A explores tailoring a laser pulse to mimic a hybrid welding process, Paper B elaborates the simulation to a multi-cycle process, and Paper C explores dilution. The manuscripts utilize a specialized experimental setup, optical analysis methods, and standard thermal measuring techniques. Metallographic analysis showed that thermal process optimization and/or dilution rate control during welding improved weld zone characteristics.

Improvements also include joint macrostructure characteristics, which are impacted by process stability, the theme of Papers D-F. Melt pool phenomena are studied in depth in Papers D and E. Paper D explores material ejections in a single beam welding scenario. Paper E investigates six beam shapes, from a single beam to a quad-beam arrangement. Paper F studies hybrid welding, a process that was simulated in Papers A-C but focused on the stability of the process instead of thermally guiding the microstructure. 

The studies complement each other in knowledge and methods. Welding of high strength steel is joining method-dependent, which imposes a unique thermal profile that affects the microstructures. The microstructure is also influenced by the chemical composition, an important point when multiple materials are used. The studies contribute an analysis of certain aspects of thermal and chemical effects of different laser-based processes to further optimize processing of specifically high strength steels, though the aspects can be generalized to other metals

Material loss during keyhole mode laser welding often leads to surface imperfections that can negatively impact component performance. High-speed imaging identified four types of material ejection attempts: classical spatter, re-captured spatter, protuberances, and scalloping. The momentum attributed to the melt body, which is influenced by the keyhole properties, dictates whether the ejection attempt is successful or if the material is re-captured. The relationship between the dynamics of the keyhole and melt pool was elaborated in an extended systematic description of melt ejection attempts, which were then classified. Ejection attempts were often observed to be proceeded by a co-current swelling of the melt, adjacent to the keyhole, followed by a melt depression. The melt swell would elongate into a melt column with a concentration of momentum, where excessive momentum causes the melt to neck and separate into spatter. Trajectory determines if the spatter becomes a permanent fixture or re-incorporates into the melt body, with the latter having the possibility to cause further melt body disturbances leading to more ejection attempts. If the melt column fails to neck and separate, or an additional force acts upon the column, a protuberance or a scallop could then form. Keyhole and melt pool fluctuations were sometimes observed to be accommodated, avoiding material ejection. In these cases, a stable weld could be obtained with large variations in the dimensions of the melt pool and the keyhole.

Laser beam welding of tailored blank butt joints of different sheet thickness generates asymmetric melt pool conditions. By employing two, three or four tailored laser beams, additional options for shaping the melt pool conditions can be offered. As observed by high speed imaging, in most multi-spot cases a large stable buttonhole was generated, by the trailing laser beams asymmetrically towards the thinner sheet. Correspondingly, the ablation pressure from the multiple boiling fronts has generated a fast melt jet, particularly along the thicker sheet. In many cases the boiling front kept open to the keyhole rear. The buttonhole differs from the Catenoid-like shape reported earlier. The walls are steeper and the horizontal shape can be asymmetric. The melt pool can switch between different stable modes. Inclined arrangement of three beams enabled even two separate, parallel boiling fronts and melt jets, combining behind the opening. Despite the large buttonhole, sound welds were achieved. Solely for four equal laser beams, arranged as a square, a melt pool without buttonhole was generated. Provided the driving forces from the ablation pressure along with the melt flow are sufficiently explored and understood, new opportunities to optimize the welding process are available.

The aim of this work was to investigate the microstructure and the mechanical properties of laser-welded joints combined of Dual Phase DP800 and DP1000 high strength thin steel sheets. Microstructural and hardness measurements as well as tensile and fatigue tests have been carried out. The welded joints (WJ) comprised of similar/dissimilar steels with similar/dissimilar thickness were consisted of different zones and exhibited similar microstructural characteristics. The trend of microhardness for all WJs was consistent, characterized by the highest value at hardening zone (HZ) and lowest at softening zone (SZ). The degree of softening was 20 and 8% for the DP1000 and DP800 WJ, respectively, and the size of SZ was wider in the WJ combinations of DP1000 than DP800. The tensile test fractures were located at the base material (BM) for all DP800 weldments, while the fractures occurred at the fusion zone (FZ) for the weldments with DP1000 and those with dissimilar sheet thicknesses. The DP800-DP1000 weldment presented similar yield strength (YS, 747 MPa) and ultimate tensile strength (UTS, 858 MPa) values but lower elongation (EI, 5.1%) in comparison with the DP800-DP800 weldment (YS 701 MPa, UTS 868 MPa, EI 7.9%), which showed similar strength properties as the BM of DP800. However, the EI of DP1000-DP1000 weldment was 1.9%, much lower in comparison with the BM of DP1000. The DP800-DP1000 weldment with dissimilar thicknesses showed the highest YS (955 MPa) and UTS (1075 MPa) values compared with the other weldments, but with the lowest EI (1.2%). The fatigue fractures occurred at the WJ for all types of weldments. The DP800-DP800 weldment had the highest fatigue limit (348 MPa) and DP800-DP1000 with dissimilar thicknesses had the lowest fatigue limit (<200 MPa). The fatigue crack initiated from the weld surface.

Separated keyholes generated during a multi-beam laser welding process were observed, analyzed, and quantified. Two high-speed cameras were utilized to capture process phenomena of bead on plate experiments, with a near vertical and a horizontal view. The films were analyzed for: the ability to form a unique keyhole for each beam, the time and location of observed fully penetrated keyhole, and other associated trends. The number of beams was correlated to fully penetrated, separate keyholes. Keyhole separations were observed to be stable while full penetration was less regular. The location of a fully penetrating keyhole correlated with the position of the stronger beam. The dynamic melt wall width separating the keyholes was measured with beam orientation and power ratios between beams having a significant impact. Beam orientation showed that the lagging inter-keyhole wall of a quad-beam process was impacted by the presence of a fully molten front in the in-line beam arrangement. For the cross-beam orientation, the lagging wall did not seem impacted by the melt front. Large power differences between leading beams for the quad-beam, in-line orientation formed a melt column in the location of the weaker beam and spattering from the column.

In welding, wire-feeding enables alteration of the resulting microstructure and, in turn, the mechanical behavior of the welded joint. For pipeline steel grades, very few commercial wires are matching at high strength and simultaneously ensure sufficient toughness. New wire chemistries need to be investigated. Promising consumable chemistries can be studied through metal cored wires. One promising concept is alloys that promote acicular ferrite instead of bainite. Interlocking instead of parallel laths can lead to higher toughness. In the gouge range of 15–19 mm, laser-arc hybrid welding has been studied for pipeline steel grades X80 and X100. For efficient mapping of various weld metal conditions, a simplifying “snapshot” method was developed. A pulse shaped laser beam melts wire pieces in a controlled manner, reproducing thermal cycles in welding. The weld metal tends to form bainite, but under certain conditions, complex microstructures with interlocking laths can be generated. Slow thermal cycles can lead to coalescence of the laths to coarser structures, while fast cycles favored finer structures and occasionally lath interlocking. The formation of acicular ferrite was difficult to achieve. Advanced wire chemistries lowered the hardness of the weld metal, as did preheating.

Filler wire metallurgy was modified through temporally shaped laser pulses, controlling cooling cycles in a recently developed method. Trends were identified through efficient mapping while maintaining representative thermal cycles of welding processes. A primary pulse melted preplaced filler wires while a secondary, linearly ramped-down pulse elevated the nugget to re-austenization temperatures. Ramped-down pulses resulted in linear cooling rates comparable with and exceeding furnace-based methods, between 50 and 300∘C/s. The linear decay of laser output power guided the temperature through a regime to obtain desired microstructures. For three very high-strength steel filler wire chemistries, quenching resulted in smaller plates with cross-hatched microstructures, accompanied by grain boundary ferrite. Finer bainite microstructures started forming for fast linear temperature decay, about 250∘C/s. Slower decay or a weaker third cycle formed coarser microstructures with coalescent sheaves and less cross-hatching.

Controlled dilution experiments were conducted in a novel manner that allowed for precise dilution of base material into the wire consumables, enabling for a prompt analysis of microstructural trends as dilution is altered. Different heat cycles and cooling rates of the wire material, without base metal additions, were shown to cause different microstructures, varying from parallel plates to random or interlocking[SR1]  orientation, with varying size of packets. The proposed method enables more controlled conditions with a known dilution value from mass percentages. Chopped filler wire is weighed and added to the base metal crucible, base metal chips are also weighed and added to the filler wire in specific mass percentages. A pulsed laser irradiates the metal, melting the mixture into a sample nugget. Lack of fusion is a benefit in this method as contamination from the base plate is negligible. The cooling rate is influenced by the pulse shape, and can be used to reheat the nugget, demonstrating the microstructural evolution in a complex thermal cycle. This method is demonstrated for S960QL steel with under-matched wire consumable, generally used for laser-arc hybrid processes to obtain high toughness, where a representative thermal cycle is needed. The thermal cycle is measured via a remote process, Dualscope, to evaluate the spacial temperature of the surface. The microstructures found using the snapshot method are similar to those found in the narrow gap multi-layer weld, different only in the size of the grains and packets.

Various metallurgical microstructures and their formation are studied in this thesis by using a laser beam to melt a variety of materials with different chemical compositions over a range of thermal cycles. The laser beam was used conventionally in a narrow gap multi-layer weld, used for welding large depths with filler wire additions, as well as a non-traditional simulated welding approach labelled here as the Snapshot method.

In laser beam welding, materials go through rapid heating and cooling cycles that are difficult to mimic by other techniques. In welding, any microstructural development depends on complex combinations of chemistry and thermal cycles but is also influenced by melt flow behavior. In turn, microstructural morphologies influence the mechanical behavior which can suffer due to inappropriate microstructural constituents. The Snapshot method, through control of thermal cycling and material composition, can achieve the same rates while guiding microstructural development to form tailored properties.

The tunable laser beam properties can be exploited to develop an experimental welding simulation (Snapshot method), which enables the complex interlinked chemical and thermal events which take place during welding to be studied in a controlled manner. Exploring the microstructural relationships to their thermal history provides a greater knowledge into tailoring microstructural compositions to obtain various required mechanical properties for laser welding, additive manufacturing and also non-laser welding techniques.

The feasibility of the Snapshot method is investigated in the three appended journal publications. High speed imaging and thermal recording have proved to be essential tools in this work, with analysis from optical microscopy and EDX/EDS to provide additional support. The Snapshot method is introduced as a concept in Papers I and II, demonstrating successfully guided thermal histories after obtaining molten material. Application of a second and third heating cycle, reheating the structure without melting, yielding altered microstructures. Reaching the austenitisation temperature range allowed for the simulation of complex multi-layer welding thermal histories. Geometrically non-uniform material additions are utilized in Paper III, which investigated the formation of microstructures through the chemical composition route. New chemical compositions were obtained by different degrees of dilution of the weld filler wire by the base material.