Microstructure and Properties of Wire Arc Additive Manufacturing of Inconel 625

In the present investigation, wire arc additive manufacturing of Inconel 625 was carried out with the cold metal transfer variant of the metal inert gas process. The heat input varied between 0.46 and 0.63 kJ/mm, which is a rather low heat input with low deposition rate. The built walls were subjected to Charpy V and crack tip opening displacement (CTOD) fracture toughness testing, in addition to microstructure examination with light microscope and scanning and transmission electron microscope. The results obtained show that hardness increases from the base metal level of 210, via the heat-affected zone (in the building plate) with HV of 220, to the weld metal, with a hardness of around 240–250. All individual Charpy V values fall within the range from 160 to 200 J, while the CTOD fracture toughness is within the range from 0.49 to 1.05 mm. The microstructure examination revealed the microsegregation of certain elements to the interdendritic regions, causing three different particle types to form. Particles with a spherical morphology were identified as spinel (MgAl2O4). Some of the spinel particles were surrounded by disc-shaped precipitates, which were identified as (NbTi)(CN), having the same orientation as the spinel.publishedVersio

Laser Beam and Laser-Arc Hybrid Welding of Aluminium Alloys

Aluminium alloys are widely used in many industries due to their high strength-to-weight ratios and resistance to corrosion. Due to their specific thermophysical properties and intricate physical metallurgy, these alloys are challenging to weld. Work-hardened alloys may experience strength loss in heat-affected zones (HAZ). The strength of precipitation-hardened alloys is severely damaged in both HAZ and weld metal due to coarsening or full dissolution. The high thermal conductivity and reflectivity of aluminium causes lower laser beam absorptivity with lower processing efficiency. Weld imperfections such as porosity, humping, and underfills are frequently formed due to the low melting point and density promoting high liquidity with low surface tension. Porosity is the most persistent imperfection and is detrimental for mechanical properties. In this work, extensive review was made on laser beam and laser-arc hybrid welding of aluminium alloys. Solidification cracking, evaporation of alloying elements, porosity and keyhole stability, and other challenges are studied in detail. The current development of laser welding of aluminium alloys is not so mature and new discoveries will be made in the future including the use of newly developed laser systems, welding consumables, welding methods, and approaches.publishedVersio

External hydrogen embrittlement assessment of pipeline base metal and heat affected zone through slow strain rate tensile testing

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A Review on Laser-Assisted Joining of Aluminium Alloys to Other Metals

Modern industry requires different advanced metallic alloys with specific properties since conventional steels cannot cover all requirements. Aluminium alloys are becoming more popular, due to their low weight, high corrosion resistance, and relatively high strength. They possess respectable electrical conductivity, and their application extends to the energy sector. There is a high demand in joining aluminium alloys with other metals, such as steels, copper, and titanium. The joining of two or more metals is challenging, due to formation of the intermetallic compound (IMC) layer with excessive brittleness. High differences in the thermophysical properties cause distortions, cracking, improper dilution, and numerous weld imperfections, having an adverse effect on strength. Laser beam as a high concentration energy source is an alternative welding method for highly conductive metals, with significant improvement in productivity, compared to conventional joining processes. It may provide lower heat input and reduce the thickness of the IMC layer. The laser beam can be combined with arc-forming hybrid processes for wider control over thermal cycle. Apart from the IMC layer thickness, there are many other factors that have a strong effect on the weld integrity; their optimisation and innovation is a key to successfully delivering high-quality joints.publishedVersio

A study on the influence of impurity content on fatigue endurance in a 6082 Al-alloy

The paper investigates the influence of different impurity/scrap elements on the high cycle fatigue (HCF) properties of 6082 Al-alloy. Accordingly, HCF tests are carried out on the different variants- GA1, GA2 and GA3 (designed as a function of varying impurity content)- of this alloy, which resulted in a lower fatigue limit in GA2/GA3 compared to GA1. To explain such behavior, detailed fractographic and microstructural investigations are conducted which bring out that although the crack initiation and propagation modes are mostly transgranular irrespective of alloy variant and stress levels, intergranular crack initiation is promoted in GA2/GA3 at 150 MPa in contrast to occurrence of run-out in GA1 at the same stress level. This phenomenon is found to be facilitated by intermetallic particles anchoring the grain boundaries. These observations point out the possibility of strong sensitivity of stress and alloy variant (varying impurity content) to fatigue life. The difference in fatigue properties as a function of alloy variant could be attributed to the variation in initial microstructure/particle size distribution as well as the slip character. In light of these, a fracture mechanism map is generated which underlines the different mechanisms responsible for fatigue crack initiation among different alloy variants.publishedVersio

Root formation and metallurgical challenges in laser beam and laser-arc hybrid welding of thick structural steel

Single-pass laser beam welding (LBW) of steel components with wall thickness of > 10 mm is of high interest due to enhanced productivity. Deep penetration LBW provides excessive hardness and certain quality issues such as root humping in flat position, which is associated with disability of surface tension to sustain melt dropout. High hardness is associated with fast cooling rates and shortage of filler wire transportation to the root of the fusion zone. Use of laser-arc hybrid welding (LAHW) can promote acicular ferrite by adding filler metal and additional heat input from the arc. However, LAHW may promote humping and adjustment of many parameters is required hindering its application. In this work, a 16 kW disk laser was used in butt welding of 12 mm and 15 mm thick plates with different bevelling geometries. Root humping occurred within a wide range of process parameters providing narrow process window. Twelve millimeter thick plates were successfully welded with a single-pass technique providing good quality of root by using zero air gap regardless bevelling geometry. Welding of 15 mm plates was more challenging, and the process was sensitive even with a slight parameter change. Improved results were achieved with application of small air gap. Acceptable hardness in both weld metal and heat affected zone (< 290 HV) was achieved for both plate thicknesses providing good toughness of minimum 27 J at −50°C.publishedVersio

Pastoral Herding Strategies and Governmental Management Objectives: Predation Compensation as a Risk Buffering Strategy in the Saami Reindeer Husbandry

Previously it has been found that an important risk buffering strategy in the Saami reindeer husbandry in Norway is the accumulation of large herds of reindeer as this increases long-term household viability. Nevertheless, few studies have investigated how official policies, such as economic compensation for livestock losses, can influence pastoral strategies. This study investigated the effect of received predation compensation on individual husbandry units’ future herd size. The main finding in this study is that predation compensation had a positive effect on husbandry units’ future herd size. The effect of predation compensation, however, was nonlinear in some years, indicating that predation compensation had a positive effect on future herd size only up to a certain threshold whereby adding additional predation compensation had little effect on future herd size. More importantly, the effect of predation compensation was positive after controlling for reindeer density, indicating that for a given reindeer density husbandry units receiving more predation compensation performed better (measured as the size of future herds) compared to husbandry units receiving less compensation

Fatigue investigation of complex weldments by the means of the local strain energy density approach

publishedVersio

OMAE2003-37370 SENT SPECIMENS AN ALTERNATIVE TO SENB SPECIMENS FOR FRACTURE MECHANICS TESTING OF PIPELINES

ABSTRACT During installation operations offshore pipes are often strained beyond yielding. Due to the high loading condition and the high costs of these operations it is important with accurate defect assessment analysis to avoid delays caused by unnecessary repairs or failure because of flaws that should have been detected and repaired. There is therefore a need for an accurate assessment procedure that can be a tool for defect assessment analysis for this application. It is commonly accepted that the fracture toughness is dependent on the geometry constraint at the crack tip. The traditional single edge notch bend (SENB) specimens have a high geometry constraint, and give lower bound fracture toughness for all geometries. For reeling operations these fracture toughness values are often too low to be used in defect assessment of reeling operations. The same is the assumption of plastic collapse when the net section stress is equal to the average between the yield strength and tensile strength. In this paper, the single edge notch tension specimen (SENT) is presented as an alternative fracture mechanics specimen. This specimen has a geometry constraint that is much closer to flaws in pipes than SENB specimens, which will give more realistic fracture properties of the pipe. In the procedure for defect assessments we present, both the fracture toughness and plastic collapse properties are taken from testing of SENT specimens. FE simulations and full scale testing verify the procedure

Room temperature creep mechanism of a Pb-Sn-Sb lead alloy

Lead alloys are the most common materials adopted for the production of subsea power cable sheathing. The sheathing is a layer of stable and watertight metal, which serves to prevent the electrical failure of the cable. During the predicted operational life of the cables of several decades, these experience strains due to the installation process, the oceanic currents and the thermal expansion of the cable. The low melting temperature of such alloys, around 600 K, imply that creep deformation will occur when subjected to loading even at room temperature. The goal of the present study is to investigate the tensile behavior of the Pb-Sb-Sn alloy of interest in order to predict the correlation between strain rate and stress level. A mechanical characterization was performed through tensile testing at different strain rates of specimens cut from power cable sheathing. Due to the extreme ductility of the material, the use of digital image correlation was necessary to compute an acceptable approximation of the in-plane strain field on the surface of the specimens. The results were implemented in finite element method environment using Abaqus and Isight to calibrate a creep model able to reproduce at best the behavior of the material. Such model was also positively tested in the case of a relaxation test. In addition, a tensile test of several steps at different loads was executed with the aim of extrapolating and interpreting the steady state creep exponents at different creep regimes and the indications that these can provide on the deformation mechanisms of the alloy.publishedVersio