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

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

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

Arctic Materials - Materials Technology for Safe and Cost- Effective Exploration and Operation Under Arctic Conditions, 2012

Oil and gas exploration and operation under arctic conditions poses several specific challenges. These are related to low temperatures, large temperature variations, and large deformations. The sensitive environment also puts strict requirements with regards to safety. Materials technology plays a vital role in ensuring that these challenges can be met, combing requirements for high safety and cost-effective solutions. The main goal for the Arctic Materials Project is to establish criteria and solutions for materials application under arctic conditions. Important specific R&D tasks will be: 1) Improve the understanding of and establish models to assess low temperature fracture risk in carbon steels down to -60°C. 2) Establish criteria for materials to be used in structures that may be subject to large deformations. 3) Establish solutions for efficient joining technologies. 4) Develop and qualify solutions for polymer thermal and corrosion coatings for arctic conditions. 5) Develop solutions for light weight structures utilising high strength steel, composite materials, and hybrid solutions. The results from the project will be basis for input to standardisation bodies, and form requirements to be put on materials performance. The potential for increased value creation based on R&D on the Norwegian Continental Shelf is estimated to 2000 billion NOK, of which much is related to possible activities in northern areas. The Arctic Materials Project will be an important contribution in making the realisation of this potential possible. Norwegian industry partners participating in the project will also strengthen their position in relation to being preferred partners in projects covering the whole arctic region

Metallurgical Aspects in the Welding of Clad Pipelines—A Global Outlook

In the present work, the metallurgical changes in the welding of clad pipelines are studied. Clad pipes consist of a complex multi-material system, with (i) the clad being stainless steel or a nickel-based superalloy, (ii) the pipe being API X60 or X65 high-strength carbon steel, and (iii) the welding wire being a nickel-based superalloy or stainless steel in the root and hot pass, with a nickel or iron buffer layer, followed by filling with carbon steel wire. Alternatively, the corrosion resistant alloy may be used only. During production of the clad pipe, at the diffusion bonding temperature, substantial material changes may occur. These are carbon diffusion from the carbon steel to the clad, followed by the formation of hard martensite at the interface on cooling. The solidification behavior and microstructure evolution in the weld metal and in the heat-affected zone are further discussed for the different material combinations. Solidification behavior was also numerically estimated to show solidification parameters and resulting solidification modes.publishedVersio

A coupled diffusion and cohesive zone modelling approach for numerically assessing hydrogen embrittlement of steel structures

Simulation of hydrogen embrittlement requires a coupled approach; on one side, the models describing hydrogen transport must account for local mechanical fields, while on the other side, the effect of hydrogen on the accelerated material damage must be implemented into the model describing crack initiation and growth. The present study presents a review of coupled diffusion and cohesive zone modelling as a method for numerically assessing hydrogen embrittlement of a steel structure. While the model is able to reproduce single experimental results by appropriate fitting of the cohesive parameters, there appears to be limitations in transferring these results to other hydrogen systems. Agreement may be improved by appropriately identifying the required input parameters for the particular system under study.acceptedVersio

Dry hyperbaric welding of HSLA steel up to 35 bar ambient pressure with CMT arc mode

Hyperbaric welding plays a significant role in subsea pipeline installations and repairs for transport of oil and gas from the offshore field to the market. The effect of ambient pressure, from 1 to 35 bar, on penetration depth and microstructure evolution in dry hyperbaric welding of X70 pipeline steel has been investigated. It was found that penetration depth is increasing with increased ambient pressure due to enhanced melt flow by using the cold metal transfer (CMT) arc mode. Increase ambient pressure lowered process stability causing more spattering strongly affecting current/voltage characteristics of the arc. Numerical simulation showed very fast cooling rate regardless ambient pressure effect causing hard microstructure. Application of lower alloyed wire provided lower hardenability and higher fraction of the allotriomorphic ferrite with high acicular ferrite volume fraction. Chemical analysis revealed positive effect of low oxygen/nickel with high silicon containing wire for acicular ferrite nucleation in weld metal at any process parameters due to higher activity of inclusions.acceptedVersio

A coupled diffusion and cohesive zone modelling approach for numerically assessing hydrogen embrittlement of steel structures

Simulation of hydrogen embrittlement requires a coupled approach; on one side, the models describing hydrogen transport must account for local mechanical fields, while on the other side, the effect of hydrogen on the accelerated material damage must be implemented into the model describing crack initiation and growth. The present study presents a review of coupled diffusion and cohesive zone modelling as a method for numerically assessing hydrogen embrittlement of a steel structure. While the model is able to reproduce single experimental results by appropriate fitting of the cohesive parameters, there appears to be limitations in transferring these results to other hydrogen systems. Agreement may be improved by appropriately identifying the required input parameters for the particular system under study

Metallurgical Aspects in the Welding of Clad Pipelines—A Global Outlook

In the present work, the metallurgical changes in the welding of clad pipelines are studied. Clad pipes consist of a complex multi-material system, with (i) the clad being stainless steel or a nickel-based superalloy, (ii) the pipe being API X60 or X65 high-strength carbon steel, and (iii) the welding wire being a nickel-based superalloy or stainless steel in the root and hot pass, with a nickel or iron buffer layer, followed by filling with carbon steel wire. Alternatively, the corrosion resistant alloy may be used only. During production of the clad pipe, at the diffusion bonding temperature, substantial material changes may occur. These are carbon diffusion from the carbon steel to the clad, followed by the formation of hard martensite at the interface on cooling. The solidification behavior and microstructure evolution in the weld metal and in the heat-affected zone are further discussed for the different material combinations. Solidification behavior was also numerically estimated to show solidification parameters and resulting solidification modes