In floors without dilation joints drying shrinkage cracking can occur. Shrinkage reducing agents (SRAs) can reduce the amount of drying shrinkage and improve the durability of the concrete structure. The mechanism of shrinkage reduction and the influences on the hydration process and development of mechanical properties of the SRA Sika Control 40 were researched. Shrinkage reducing agents lower the surface tension of water which reduces the capillary tensile stresses and ultimately reducing the drying shrinkage. Experiments were performed on cement paste samples to investigate influences on the hydration process of cement (surface tension, rate of hydration, porosity, shrinkage, flexural and compressive strength). In addition experiments were performed on concrete samples to investigate the influences of applying shrinkage reducing agents on the mechanical properties of concrete (compressive strength, shrinkage, elastic modulus). Overall, the experimental results showed that applying Sika Control 40 is an effective method for reducing drying shrinkage when concentrations of 1.56 w% and 2.61 w% are used. The shrinkage reducing agent has positive effects on the surface tension, weight loss and drying shrinkage for concentrations of 1.56 w% and 2.61 w% of cement. It has negative effects on rate and degree of hydration, porosity, pore size distribution, compressive and flexural strength for concentrations of 1.56 w% and 2.61 w%. The elastic modulus lacks adequate data to observe a trend, however the results are within the range of expected values for concrete for mixtures 156 and 261. , An SRA concentration of 0.52 w% of cement resulted in unpredictable and contradicting results. ... Read more

In recent years, Wire and Arc Additive Manufacturing (WAAM), is gaining attractions both from academia and industry, because it has the potential to substitute or complement the traditional manufacturing methods. Traditional materials and methods are reaching their limitation when demanding and special applications are considered.

The current growth in WAAM applications will boost the WAAM consumable material development in the coming future. WAAM applications mostly refer to small batch production or prototypes, which often requires special wire compositions or can benefit from tailoring the consumable composition for the desired components. On one hand, solid wire production is only economically viable when large volumes are involved. On the other hand, metal-cored wires are particularly suitable to produce tailored or small-batch consumable compositions, which is very attractive for WAAM. However, only limited research has been carried out in the use of metal-cored wires in additive manufacturing. In this research, the focus is to explore the use of metal-cored wire in WAAM applications. Two chemical compositions, one ferrous, medium carbon low steel alloy (AM-XC-45), and one non-ferrous, Stellite 6 (cobalt-based superalloy), metal-cored wires were investigated based on industrial interests at RAMLAB.

The microstructure of the WAAM AM-XC-45 thin wall was characterized using optical microscopy and scanning electron microscopy. Pearlite, ferrite, bainite, and martensite are present in the deposited wall. Columnar grains are found near the fusion line. The repeated thermal cycles cause the grains to become finer from the top to bottom layers. The mechanical properties including microhardness and tensile strength were tested and compared with the traditional processing methods like casting, milling, and forging. It showed a comparable or superior microhardness and tensile strength to the traditional process whereas the relative lower elongation of the deposited AM-XC-45 thin wall indicates that further post heat treatment is needed to improve the ductility of the part.

Stellite 6 is a cobalt-based superalloy, which has good wear and corrosion resistance and retains these properties at high temperatures. In this study, Stellite 6 metal-cored wire (WEARTECH® WT-6 GMAW-C, AWS A5.21 ERCCoCr-A) is selected and optimized to obtain results comparable to the commonly employed and more costly laser deposition, in terms of microstructure and mechanical properties. The optimal deposition parameters were developed using the S355 steel substrate, and then adopted on depositing on the AISI 420 stainless steel substrate. Characterization showed that Stellite 6 layer mainly contains Co-Cr-Fe solid solution with FCC crystal structure as the matrix and the Cr7C3 and Cr3C2 were identified in the microstructure through SEM, EDS, and XRD. These carbides can contribute to the strength. In addition, the identified Co4W2C carbides can contribute to the wear properties of the Stellite 6 WAAM deposits. The dilution and hardness of the WAAM deposit can reach the same level as laser deposition.

Additionally, a 3D temperature distribution model is developed by adopting the moving mesh technique to numerically study the WAAM process. The model helps to gain a better understanding of the physical phenomena (heat and mass transfer) during the WAAM deposition. The AM-XC-45 was used to validate the developed model. The simulated and experimental results were compared. The dilution and HAZ of WAAM deposited single bead was used as validation criteria. The A1 line (1100K) simulation is in a good agreement with the experimental result. The fusion line (1800K) shows some deviation. Compared with laser deposition, the main differences between the two processes lie on the heat source, the boundary conditions, and the material responses to the process. ... Read more

The pipeline production rate and the pipeline’s superior mechanical properties are the crucial parameters for the offshore pipe laying company. Pulling on the bead, i.e. the action of moving the pipe on the firing line to the subsequent weld stall as the vessel moves forwards allows a fast and reliable pipe welding process. During this movement, the ends of the pipes are intermittently not supported and the bending moment in the weld is created as a consequence. Such operation imposes tensile loads in the welds, that are especially high for the 3 mm thick beadweld and the subsequent 3 mm thick hot pass (6mm thick in total). An important question arises whether imposing loads above the beadweld material yield capacity affects the overall fatigue performance of the girth weld. It is in the best interest of the offshore industry to assure that the pipeline welds properties during their 20-30 years operation lifetime are not adversely affected during its installation. This study investigates the effect of pulling on the bead on the weld’s fatigue behaviour.
The overstressing simulation of the test coupons, Finite Element Analysis (FEA) of the loaded beadweld and the small-scale specimens comparative fatigue study were conducted to investigate the effect of straining history on the full girth weld fatigue performance. Additionally, the holistic approach was implemented to investigate the consolidated effects of other factors: local weld root misalignment (Hi-Lo), weld porosity and microstructure quality. Four test coupons were welded together and overstressed to simulate the pulling on the bead action. To assess the stress-strain state in the beadweld and hot pass during the overstressing simulation the FEA analysis was done. The FEA results allowed to quantitatively determine the exact locations of the elastically and plastically strained beadweld material. Full welds were checked against the porosity and subsequently, 46 fatigue specimens were fabricated and tested in the 4-point-bending fatigue loading setup. Additionally, the fatigue crack observation techniques were implemented in order to separate fatigue crack initiation and growth phases out of the total fatigue life. The optical observations (with the optical-digital microscope) and the Potential Drop techniques were used. Tensile and hardness tests were done to compare the strength properties of specimens with different straining and porosity histories. Finally, the microstructure and fracture surfaces of the fatigue broken specimens were studied.
The research done in this study gives a good overview of the fatigue behaviour of the girth welds and provides insights into the straining history effects. Plastic straining was found not to affect the fatigue performance more than currently acceptable elastic straining does. Moreover, plasticity was found to reduce the negative effect of porosity. Additionally, the prevalent influence of Hi-Lo on the crack initiation phase was confirmed, making it the driving negative factor for the fatigue resistance. ... Read more

Wire and arc additive manufacturing (WAAM), a relatively new metal 3D-printing technology, allows for fabrication of shapes that were not possible to produce a few years ago. Using 6-axis industrial robots with a gas metal arc welding (GMAW)-head, Dutch start-up MX3D prints metal in every direction mid-air. By welding small parts of material at a time along a laser guided - CAD-generated path, self-supporting cross-sections of very complex shapes, such as internally reinforced non-prismatic hollow sections can be manufactured fully automatically. Unlike powder-bed based manufacturing, this technology is not bounded by size limits of a final product, which creates opportunities for fabrication of large-scale structural components and finally entire structures. It is MX3D’s aim, in collaboration with key industry partners, to 3D-print world’s first fully functional bridge spanning 12 metres, to be installed in the city centre of Amsterdam in the course of 2019. The bearing elements of the bridge consist of tubular shaped elements, together building up to a complex-shaped free-form steel structure. These tubular members mainly behave as axially compressed bearing elements, nonetheless it is yet not known how the stability of WAAM-members can be assessed. Several standards for testing (and characterisation) of additively manufactured materials are published by different national and international committees, e.g. ISO/ASTM 529 and CEN-CT 438, but a comprehensive set of international standards has not been achieved yet. It is widely recognised that there is a strong link between manufacturing process parameters and material properties, which requires special attention in this research area. So far there is a lack of sufficient test results to cover a specific manufacturing process. In this research the material properties of MX3D’s dot-by-dot and continuously printed members are studied. This pioneering research provides insight in material and geometrical properties relevant for the stability of (ER)308LSi stainless steel tubular columns. A tubular column is used as existing basic structural element to assess the stability of wire and arc additively manufactured steel members. Advanced 3D-laserscanning technology is applied to characterise and analyse geometrical imperfections. A clear understanding of bending stiffness and buckling behaviour is established by performing four-point flexural bending tests and flexural buckling tests on tubular columns, diameter 33.8mm, thickness 3.7 mm. Tensile properties of milled WAAM specimens are analysed both in - and perpendicular to - their print direction for dot-by-dot and continuously printing. It is verified whether or not the structural properties are in compliance with the existing steel standard (EC3). Additional Vickers hardness measurements and metallographical analyses are performed to examine the microstructure of column samples. Finally, a buckling design graph is proposed based on own experiments, aiming towards a safe model suitable for stability calculations of additively manufactured tubular columns.
It is concluded from experiments that the material properties of 3D-printed steel differ from that of conventionally produced steel. Following the thermal gradient of the weld pool, large columnar grain structures are detected giving rise to anisotropy. Stiffness, strength and ductility prove to be dependent on the print direction.
Whereas the tensile strength top expected values, the obtained stiffness is significantly lower than that of commonly applied steel grades. Inaccuracies of the printing process result in local wall thickness variations and a relatively high out-of-straightness, both negatively affecting the buckling capacity of printed tubular columns.
Yet it should be acknowledged that due to rapid advancements in improving the printing process, the geometrical imperfections are soon expected to reduce drastically. It is recommended to apply active cooling during manufacturing to enhance mechanical properties even further. By combining topology optimisation and WAAM, an optimal material layout can be found ánd manufactured, consequently an excellent material efficiency can be achieved. A promising prospect for future construction projects. ... Read more

Due to EU regulations on fuel consumption, reducing the weight of vehicles has become one of the most important goals of car manufacturers in Europe. Among them, Toyota Motor Europe is one of the worldwide leaders in the research for a sustainable future. Materials like fiber-reinforced plastics and aluminum play a significant role in the research for lightweight design, thanks to their very good strength-to-weight ratio. However, joining these materials efficiently together is still a challenge. When thermoplastic composites are used, direct joining with the metal substrate can be obtained using welding technologies which melts the thermoplastic at the interface.

Ultrasonic welding is well-known for being a fast, reliable and effective technology for metal/metal or plastic/plastic joining. In this study, a collaboration between Toyota and TU Delft, ultrasonic plastic welding was investigated as candidate joining technology for aluminum/thermoplastic joints in automotive applications. The goal was to understand the main mechanisms involved in the adhesion and how they affect the performance of the joint. Initially, the technique proved to be successful, but moderate strengths were obtained. Therefore, several surface pre-treatments of the aluminum were analyzed to improve performance in terms of strength and durability of the joint; mechanical, chemical and physical treatments were carried out. With laser structuring, strengths comparable to adhesive bonded joints were obtained, but in a much shorter process time. Other treatments such as conversion coating, sandblasting and plasma led to considerable improvements as well.

The encouraging results achieved represent an important step in the development of ultrasonic plastic welding for multi-material joining in the automotive industry. Additional research could help Toyota and other car manufacturers realizing a better design to further decrease weight and CO2 emissions of vehicles. ... Read more