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Laser deposition welding with cored wire

Aim of the development

Laser buildup welding is already used today in the offshore sector for corrosion protection of hydraulic cylinders or in oil and gas production for wear protection of drill heads. Filler materials in powder form are predominantly used, since they can be fed coaxially via a carrier gas or via radially arranged powder nozzles. The disadvantage of wire-actuated filler materials is that they are usually fed laterally and thus lead to a direction-dependent application. New processing heads allow coaxial wire feed and thus direction-independent processing. The wire-shaped filler materials can be divided into solid and cored wires. Metal powder filler wires consist of either a folded metal strip or a seamless metal tube that is first filled with a metal powder by vibration and finally drawn to the desired final diameters. Compared to the use of solid wires made of duplex steel in laser buildup welding, the use of cored wires is rarely mentioned in the literature. However, cored wires offer the advantage that they can be adapted quickly and inexpensively via the composition of the filling. During processing, the cladding and the filling melt and alloy with each other. The alloy composition can therefore be easily varied for metal powder cored wires by using a matched filling. The combination of duplex steel metal powder filler wires and laser cladding offers a way to apply low-cost corrosion-resistant coatings with high strength. Due to the high energy densities and the low heat input, the laser cladding process allows the coating of thin-walled and sensitive semi-finished products. Using the metal powder filler wires, an individual alloy can be adapted cost-effectively and economically for a corresponding area of application. When coating duplex steels, whether with powder or wire, the resulting microstructure becomes inhomogeneous with increasing number of layers. This means that the cooling intervals become considerably longer, which increases the austenite content and the risk of intermetallic phase formation and dendrite growth. Such a microstructure has a negative effect on the mechanical and corrosive properties of the coating, which means that this area in the component later represents a weak point. Therefore, the use of hot wire and an adapted intensity distribution is intended to control the energy input more precisely layer by layer. In the optimum case, an intrinsic heat treatment already takes place during the application process. In addition to a comprehensive materials engineering analysis, the cooling rates are determined in parallel. Here, the experience already gained in a previous project is used.

Advantages and solutions

The following focal points were defined for the successful completion of the research project: - Qualification of the performance capability in the processing of solid and cored wires by means of hot wire and adapted intensity distribution - Increase of the process reliability and deepening of the process understanding by means of non-contact temperature measurement and material-technical analysis -. Elaboration of a process characteristic on basic geometry elements (single tracks and two-dimensional buildup) by buildup welding tests as a function of the material and the manufacturing form of the filler metal - Determination of technological process windows for buildup welding with solid and cored wire for coaxial material feed - Testing of the transferability of the derived process characteristic to more complex, In order to avoid the laboratory character of the investigations and to do justice to the industrial purpose, the transferability of the determined process parameters to more complex, industry-related buildup welding applications was tested on the basis of the cladding of components and the alternative structure of 3D geometries. For example, so-called half hollows wear out in turbine housings. By cladding these, a service life extension or regeneration could be achieved. In the area of generating mold elements, the production of a part of a turbine blade would be conceivable. Turbine blades made of duplex steel are used in hydroelectric power generation. In summary, the following target parameters are formulated, also in terms of sustainability: - Validation of the potential of buildup welding technology from a process engineering point of view - Characterization of the technological properties of the overall system when processing solid and cored wires made of duplex steel to evaluate process reliability - Definition of process windows for deriving suitable operating points for industrial applications as an aid for the industrial user and for process optimization The entire experimental investigations were accompanied by a comprehensive materials engineering analysis. This ranges from the determination of the mechanical-technological parameters to microstructural analysis and the resulting corrosion resistance. The preparation technique derived in a previous project ensures that the determination of corrosion resistance is carried out with the greatest possible efficiency. The usability of metal powder cored wires in laser cladding was demonstrated in this project. The investigations revealed possibilities, but also limitations, in the comparison of solid and cored wires. Based on the material characterization, good models could be generated despite fluctuating results (especially pores and unmelted particles). Based on the developed models, optimized process parameters for the build-up of layers were derived, implemented and evaluated. All process parameters derived in this way showed good mechanical technological properties. Despite the limitations of the corrosion tests based on ASTM 150, resistance to pitting corrosion was demonstrable and comparable to conventional welds. Based on the results obtained, it could be shown that by using metal powder cored wires, similar good results could be obtained as with solid wires.

Target market

Wherever duplex steels are used, they impress with their performance and good price-performance ratio. For example, the Kwandong Hockey Center for the Winter Olympics in Pyeongchang 2018 could only be built with the available budget because a switch to duplex steel as a construction material was made at short notice. However, as their high strength values limit formability, they have so far only been able to hold their own in niches and currently only contribute a low single-digit percentage to the overall steel market. This is where the surface coating of low-cost structural steels with duplex steel, as envisaged in the planned project, comes in. The wire-based process developed competes with both powder buildup welding and thermal coating. The advantage over powder buildup welding in the use of axially fed wire lies in the greater geometrical freedom and better material utilization. In addition, powder feed is significantly more complex and cost-intensive than wire feed. Among other reasons, this is due to the fact that coaxial feeding concepts currently dominate in industrial systems. These require a division of the powder flow into partial flows with subsequent technical reunification in the powder focus. In addition, the handling of powder is generally significantly more complex than that of wire. Necessary cleaning processes, as well as the professional disposal of excess powder residues, also have a disadvantageous effect. In addition to the lower material efficiency compared to wire buildup welding, the occurrence of pores in the applied layer continues to be a challenge with powder buildup welding. The difference to thermal coating is that in this process (in contrast to buildup welding), the base material is generally not melted. Thus, this is an adhesive bond that can be infiltrated (unlike the cohesive one of laser buildup welding). In addition, buildup-welded protective coatings of duplex steel provide much greater protection against mechanical stress. An exception to the thermal spray processes is laser spraying. This is more or less a laser buildup welding process with powder, since the laser melts the base material and a metallurgical bond is formed. Accordingly, the properties known from powder buildup welding apply here. In addition to these process engineering advantages, the process developed has the advantage that it uses a welding head in which the laser beam sources (as direct diode emitters) are already integrated. This means that neither a separate location for the laser source nor light guidance via optical fibers (including beam coupling) or mirrors is necessary. The footprint of the system is thus significantly smaller and the elimination of fiber lasers means elimination of transport fibers, which in total means more resource-efficient use of the available raw materials. Germany currently has 66 waste incineration plants with a total capacity of 20.6 million tons and 32 refuse-derived fuel power plants with a total capacity of 5.8 million tons. More than 70% of the shutdowns at these plants are caused by chloride-induced corrosion. In conjunction with erosion triggered by flue gas particles, the corrosion risk increases further. Not only at these plants could the resistance be increased by the developed process, but also the coating of pipes laid in corrosive media and (with the appropriate nominal diameter) the internal coating of pipes carrying corrosive media are conceivable. One application example for such pipes is desalination plants. Another conceivable application is in the offshore sector, where thick-walled steel structures are used as support elements (e.g. on drilling platforms or wind turbines). If, instead of duplex steel, a structural steel could be used here which is provided with a protective layer of duplex steel by means of the process to be developed, this would open up considerable cost reduction potential. Since laser buildup welding is an additive manufacturing process, it is also possible to create complex three-dimensional structures. The market for additive manufacturing is currently characterized by very high growth and is forecast to be worth $22.4 billion by 2020.