Is the transition to wire additive technologies a trend or a necessity?
Metal 3D printing is increasingly used in the aerospace industry and the automotive industry, in power industry, shipbuilding, medicine, as well as in jewelry, research, in the creation of art objects, and in a few other areas. Almost always, metal printing is chosen when the requirements of the developer, technologist, surgeon, dentist, scientist or designer go beyond the capabilities of traditional types of production. This can be, for example, a topologically optimized design, or a complex product with the integration of several elements of an assembly unit, or an individual implant or prosthesis.
Another good reason for switching to additive technology may be the lack of the shop space to accommodate the entire range of machining equipment or the inability to use extremely expensive and harmful foundry production.
Today, metal-powder technologies: Laser Beam Melting (LBM), Electron Beam Melting (EBM), gas-powder surfacing with Direct Energy Deposition, DED – are already associated with the standard for printing high-quality metal workpieces. Of course, the details obtained by these technologies may need to be refined: removal of supports, separation from the base, improvement of surface roughness, as well as the internal structure of the material, which sometimes takes a considerable time. But it is hardly necessary to consider this as a significant drawback, because in general, with the help of metal 3D printing, it is possible to significantly reduce the cycle time, the number of personnel involved, and significantly increase the structural complexity of manufactured products. And it is often necessary to modify only those surfaces or the structure of bulk bodies that need a tight fit to other parts of the assembly, improving the aesthetic quality, or in order to minimize stress and porosity with the help of mechanical processing, vibro-grinding, sandblasting, thermal or gas-static treatment.
Types of wire-feed surfacing technologies
Recently, additive technologies are gaining popularity, where metal wire is used instead of powder. These technologies receive thermal radiation from various energy sources. For example, Electron-Beam freeform Fabrication (EBF3) uses an electron beam, Laser Wire Cladding (LWC) uses a laser, Wire Arc Additive Manufacturing (WAAM) – an electric arc, and Rapid Plasma Deposition (RPD) – plasma (Fig.1).
As you can see from the picture, the most popular technology is WAAM. It is based on the same process of electric arc welding (MIG/MAG), which was used by our compatriot N. G. Slavyanov in 1888 for welding the shaft of a steam engine.
Thus, we can say that the process of electric arc welding is studied so thoroughly today that the modes, processes, metals, welding fluxes, heads for automatic welding, wire diameter, protective gas are easily selected in full accordance with the task in order to obtain a guaranteed high quality of the seam. Usually, blade machining technologies are used to give the responsible surfaces of the parts shape and roughness according to the drawing are used in the production cycle using wire surfacing (Fig. 2).
Why did developers and users of 3D printing become interested in using wire? Types of wire surfacing systems would be more reasonable to compare with traditional blade processing and casting technologies, as well as their impact on the full production cycle. But in this review, we will try to systematize the current situation in the context of metal additive technologies. (Fig. 3) We will also give a comparative assessment of the main technical characteristics of the devices and their capabilities. You can find examples of successful applications of technologies in this segment on the Internet.
Types of system
The layout diagram of wire surfacing equipment is currently represented by three main types (Fig. 3). One, two, or more robots can be used as part of a robotic system (Fig. 4). Each of the robots can perform the same type of surfacing operations, thereby increasing the overall performance and processing dimensions. It is also possible to use one robot for surfacing, and the second – for non-power milling.
When using a classic machine tool scheme in the form of a portal or console, it becomes possible to implement non-destructive testing, full-fledged milling, as well as running-in and forging. Among the advantages in this case are higher safety, positioning accuracy, quality of surfacing, convenience and intuitive programming, no need for calibration and uneven melting at the corners and edges of the part, as when using robots.
The cost of robotic system will be somewhat cheaper than surfacing using machine-type systems with an equal number of working bodies: burners, robots, laser heads, plasma torches, electron beam guns.
Users of metal-powder additive systems will appreciate the fact that the cost of welding wire, depending on the metal, is cheaper than the powder in 2-10 times. In addition, a wide range of wire materials is available, which includes hundreds of items available in the markets of Europe, the United States, China and Russia. For example, it will not be difficult to find high-quality wire made of titanium and titanium alloys, heat-resistant alloys, tantalum, tungsten, niobium, molybdenum, stainless, low-carbon, tool, martensitic-aging steel, invar, aluminum alloys, zirconium, bronze, copper, copper-nickel, magnesium alloys, etc.
With special industrial wire, things are somewhat more complicated, it is more expensive than welding, but can be made to meet the customer's requirements using fairly simple technological methods. In other words, for many decades, manufacturers of raw materials have saturated the world of welding with all possible brands of wire with uniform mechanical and physical properties, which, in contrast to metal-powder technologies, allows you not to spend time selecting or obtaining a material with the necessary properties. It is also extremely important that the material uses almost for100% and store it easily, as opposed to powder. There is a great potential for manufacturing bimetallic and polymetallic products in one production cycle.
If the classic additive technologies have already used mainly medium-and high-power lasers, then electric arc energy sources are the most widely used in wire surfacing. These are welding machines of well-known manufacturers, for example, Fronius International GmbH (uses Gefertec GmbH), a little less often-laser (fiber, disk, diode) with a power of up to 4 kW (uses Fraunhofer Institute for Material and Beam Technology IWS, Germany) and plasma (uses Norsk Titanium AS, Norway). The list ends with electron beam installations with a capacity of up to 42 kW (replaced by Sciaky Inc., USA), as well as combined sources (for example, plasma and a melting electrode with a plasma torch of reverse polarity, as in the Russian company "Hybrid additive manufacturing").
When using WAAM technology, the temperature reaches 4000°C in the anode region, and 7000-10000°C in the arc gap (Fig. 5). When using plasma and electron beam, the welding bath can reach a temperature of 12000°C. A higher temperature increases the fluidity of the molten metal, which is a definite advantage of EBF3 and RPD technologies. However, it is important to understand that it may take longer to cool down. If the product is not very large, the deposited layer may not have time to cool down for applying a new layer, which is required by any technology of wire surfacing. As a result, all the advantages of a higher speed can be balanced by the cooling process.
In contrast to laser and electron beam additive technologies that work with metal powder, RTK and wire surfacing machines significantly benefit in the size of the resulting parts. Usually the useful working volume exceeds 1 m3, which cannot but inspire both active users and potential consumers of additive technologies. Among the mass-produced equipment, Sciaky Inc. can boast of the "giant" size of the working chamber. with the EBAM 300 installation with a working area of 5791x1219x1219 mm. Enertech GmbH offers the arc603 installation in the 3-axis version with a construction zone size of 1200x1500x1600 mm and a product weight of up to 3000 kg (in the 5-axis version, the maximum product size is 900x700 mm).
However, at the request of the customer, the company can produce equipment of any size. RTCs usually have no restrictions, especially for 3-axis versions (Fig.6), but in this case, the robot must move along the guides with a deterioration in accuracy characteristics (Fig. 7). The use of 5-axis processing is only possible with the use of globe turntables, and this greatly reduces the working volume.
The wire feed to the melting zone can be either coaxial or lateral, and for Sciaky Inc. systems. with EBAM-double technology (Fig. 8). When using a double wire feed, the performance increases almost twice, but at the same time the quality may deteriorate slightly. Using a coaxial wire feed, you can fully use the five-working axis of the device, which is no longer possible while a side feed, because the freedom of rotation of the energy source is limited when describing complex trajectories.
The steel quality of the products obtained by wire surfacing is significantly higher than all the currently popular additive technologies. The wire, in contrast to the powder, has a homogeneous structure without remaining gases during its production, inclusions of particles with unsatisfactory morphology and shape, problems with insufficient or excessive fusion temperature, etc. The high-Temperature melt bath is large enough and homogeneous (Fig. 9). Young companies manage to achieve a relative porosity of the deposited metal at the level of 0.9-2%. Serious market players use the most advanced technologies, native software, which together allows you to get surfacing without cracks and non-melts in the zone of thermal influence and with single pores.
However, in wire-feed technologies, you should carefully monitor the quality of the protective welding mixture or inert gas (argon, helium and their mixtures are the preferred process gases for additive production, and the active components CO2, O2, N2 or H2 can be added to fine-tune the properties of the material), since their composition and the coating of the fusion zone affects the quality of the result and the presence of oxides on the surface of the part. Higher surface quality of the parts is provided by the electron-beam surfacing technology, since the process takes place in a vacuum. However, with EBF3 technology, a cavity may appear at the root of the seam, so quality control should be carried out. Cathodic cleaning of the combined "plasma plus melting electrode" technology also makes it possible to obtain good quality due to the removal of the oxide film. In the classic WAAM technology, the surface of the part is exposed to a sufficiently high thermal effect. This increases the degree of thermal deformation of the workpiece (the degree of deformation is also affected by the quality of the atmosphere, the amount of hydrocarbons and moisture). To reduce warping, the most advanced companies, for example, the center for welding technology and laser processing at Cranfield University and Gefertec GmbH, use the latest developments of the market leader, Fronius International GmbH, in their WAAM and 3DMP technologies, respectively, the cold Metal Transfer (CMT) process with reduced heat input (Fig. 10).
There are known cases of using power supplies from other companies, for example, EWM AG with the technology of limited heat input EWM-coldArc (Fig. 11). These processes allow to transfer the metal from the melting electrode to the molten pool at low temperatures due to short circuit and reciprocating motion (with a frequency of 70-130 Hz). This results in a stable and controlled arc, high speed (up to 30% increase in productivity when welding steel and up to 10-fold increase in productivity when welding aluminum), no splashes and the ability to weld hard-to-weld materials.
Some manufacturers, such as the Cranfield University center for welding technology and laser processing and the Hybrid additive manufacturing group of companies, combine surfacing with layer-by-layer running-in. This allows you to obtain mechanical properties at or above the basic material with an isotropic structure, reduce the grain size and remove residual stresses in the part (Fig. 12).
This run-in can be performed using a pressure roller on the top or side, two rollers, as well as ultrasonic or pneumatic shock treatment (Fig. 13). Forging is effective for removing residual stress in places that are inaccessible to the roller. Along with the advantages of this method of hardening, it is worth noting the increase in cycle time and the inability to use 5-axis processing, so this method of reducing porosity, internal residual stresses and increasing fatigue characteristics has not yet been widely used.
In general, safety issues when working with common WAAS, RD, EB F3 systems in machines with lockable doors or roller shutters are resolved at a high level (Fig. 14), but this is achieved by various means.
An important condition for ensuring work safety is not only the prohibition of personnel being in the immediate vicinity of the surfacing zone and the radiation source, but also access by authorized persons to power, electronic, filter elements, cooling systems, gas cylinders, etc. For these purposes, both technical devices (rotary cranes, lockable sectional switches, fire extinguishers, sensors, lights) and organizational solutions are used. In the EBF3 technology, in addition to the above, in connection with the use of the electron beam, the user company must have a radiation protection specialist and carry out regular radiation safety checks (at high capacities, part of the electron energy is converted to x-ray radiation, which is dangerous for the health of the personnel). Equipment with a potential risk of injury can include almost all RTCs, except for their closed versions, as in the Australian company AML Technologies (Fig. 15).
High wire consumption in wire surfacing technologies requires either frequent replacement of coils with the material, or the use of special reels for welding. The first ones have a reserve for 2-8 hours depending on the surfacing speed (the weight of the coils with the material is 5-18 kg), the second ones can feed the equipment for 50-250 hours with a reel capacity of 250-500 kg. Using of reels saves time for replacing 13-26 coils, allows using equipment with a high degree of automation (Fig.16) and build massive parts without the need for the constant presence of maintenance personnel.
Productivity and accuracy
All wire technologies are many times faster than the popular metal additive technologies implemented in metal-powder 3D printers (Fig.17). For example, WAM/3DM technology can reach a capacity of up to 5 kg/h or 450-1000 cm3/h depending on the material. Norsk Titanium's RPD plasma technology works confidently at speeds of 5-10 kg/h. EBAY's Electron beam melting uses a typical performance of 3-9 kg/h, and also has a potential of up to 11.34 kg/h. But the use of combined sources, such as a melting electrode and plasma surfacing, according to the developer, can raise the production level to 15 kg/h!
On the other hand, the geometric accuracy and repeatability of wire technologies is significantly lower than that of DED gas-powder surfacing and LBM selective fusion. But if we consider the production of products as blanks for machining, the precision characteristics of surfacing do not play such a big role.
Efficiency of wire surfacing technologies
As it was already mentioned at the beginning of the article, wire surfacing technologies are most appropriate to compare with traditional technologies, most often with milling. The most noticeable effect in this case will be manifested when working with hard-to-process materials: corrosion-resistant and heat-resistant steels and alloys; materials with high hardness and strength; alloys based on titanium and refractory metals (tungsten, molybdenum, niobium). The processing features of these materials cause the cutting speed to decrease and contribute to a decrease in tool life. Solid titanium oxides and nitrides that occur during blade processing, as well as the low thermal conductivity of titanium alloys, affect the cost and time of obtaining the finished product. Refractory metals have high hardness and high wear capacity. Add to this the need to process medium-sized and large workpieces with 60-90% of the material going into the chip, and the choice of wire additive technologies becomes obvious, because the material utilization factor can reach 90-100% (Fig.18). Under certain conditions, it is more convenient and much more profitable to use the Pro - haul as a replacement for metal-powder additive technologies.
Of great importance when choosing a wire surfacing technology or developing your own solution is the software that will allow you to program numerous parameters of the working body of the machine or robot:
- importing CAD data,
- support for a hybrid module for milling or impact processing,
- the creation of trajectories,
- temperature control in the zone of the melt a protective atmosphere,
- collision management,
- updated database,
- accounting for deformations, etc.
It is also important to have non-destructive testing, visual analysis using cameras, and 3D scanning. In other words, it is important to manage the technology in real time — this will avoid problems and improve the quality, reliability and speed of the process.
Wire surfacing allows you to produce parts with an allowance of 1-3mm for machining, which reduces the number of chips by 10 or more times. For example, the ratio of purchased and used material in the traditional production of the 2-wing rib can be 37 units: from a billet of 657 kg, a finished product weighing 18 kg is obtained! And the transition from traditional production methods to the use of wire surfacing can reduce production costs by 60-70% (Fig. 19).
Mass production with benefits already on the first part, maintenance and repair, easy adjustment of changes, using the principles of shape optimization — another reason to think about choosing in favor of wire surfacing.
Today, technologies with the additive principle of forming wire products go through a difficult path from popularization, testing, and economic justification to attempts by companies to contribute to the development of devices that meet or exceed the capabilities of leaders. While there are only a few companies in the world that produce equipment in series, these are, for example, Gefertec GmbH, Sciaky Inc. after all, corporations with large investments and solid experience can achieve high quality, reliability and ensure compliance with standards. Therefore, many foreign manufacturers prefer to provide services using their own production equipment, while Russian inventors and universities use their experience to implement projects inside their factories and laboratories.
For the application of wire additive technologies, a lot of models with different energy sources and different costs are offered. Another way is to develop your own technology based on Russian and foreign experience, which will inevitably take time.
To the question of whether wire technologies are a fashion trend or a necessity, each company will have its own answer, based on the urgent need, technological interest, and opportunity to invest.
For ourselves, we chose WAAM/3DMP electric arc surfacing technology as the most studied, reliable, flexible, and affordable. In our opinion, mass-produced Gefertec equipment is the Golden mean of choice for both researchers and mass-produced plants, as well as maintenance and Repair companies.