conducting electrode. The electric arc, which is built up between the tip of the filler wire ... GMAWâ or GMAW/. Metal Additive Manufacturing (MAM) was created.
POSCH GERHARD1 KALCHGRUBER FERDINAND 1 HACKL HEINZ 2 CHLADIL HARALD 3
MANUFACTURING OF TURBINE BLADES BY SHAPE GIVING CMT-WELDING Abstract Turbine blades are often individual, complex shaped components made of demanding materials. In this study it is investigated, if such components can be made of special steels like Duplex or Superduplex steels by a precise build-up of single weld seams produced with a new innovative welding process called “CMT”. Although shape giving welding is well known in industrial manufacturing its application was limited due to restrictions by the welding process itself: in case of low energy welding, which is needed for manufacturing of thinner 3D components, the welding process got instable and much more spatter formation occurred. The driving force in developing of “CMT” (Cold Metal Transfer) was to create a highly automatable welding process which runs extremely stable and almost spatter-free already at very low welding energies. As this development is based on the standard Gas Metal Arc Welding (GMAW) process, almost all available filler metals from unalloyed, mid- and high alloyed steels but also nickel- and cobalt base can be used and can be combined, if metallurgical possible, to gradient structured parts. The outstanding characteristic feature of CMT is the absolute precise, periodic detachment of single droplets from the filler wire in a size of approx. 1mm diameter. In combination with an accurate movement of the CMT welding torch, done for example by a robot, very thin geometric shapes but also undercuts can be established. In contrast thicker shapes can easily be done only by increasing the welding energy and in further consequence the deposition rate. Keywords Gas Metal Arc-Welding, Cold-Metal-Transfer (CMT), Duplex Stainless Steel
1. Introduction Welding is a very well known technique which is used for permanent joining of solid components, quite often made of metals. There are two principle ways, how welding can be performed: applying heat with or without mechanical pressure to establish a permanent material connection. In case of “fusion welding” only heat is applied to melt locally the components at their contact area to form a common liquid “weld pool” which solidifies 1 2 3
FRONIUS Int. GmbH., Wels, Austria FRONIUS Int. GmbH., Thalheim, Austria Andritz AG, Graz, Austria
afterwards and forms a strong, permanent joint. As heat source a flame, the electric arc, a laser or electron beam can be used, but the most popular by far of them is the electric arc. There is also the possibility to add a so called “filler metal” to the liquid weld pool to fill up gaps and to modify the chemistry of the weld pool in respect to improve mechanical and/or corrosion resistance properties. The most important fusion welding process is Gas-Metal-Arc Welding (GMAW) which covers more than 70% of the welding tasks in industrialized regions. In this process a wireshaped filler metal is at the same time additional material for filling up the weld groove and conducting electrode. The electric arc, which is built up between the tip of the filler wire electrode and the groove, has to melt down the wire electrode, to fuse the components and to establish a common weld pool. For protecting the arc against the atmosphere a gas shielding is necessary – this could be done mainly by an external gas supply or more seldom by a special type of filler wire. In Fig. 1 the scheme of a modern, inverter-based GMAW power source is shown.
Fig. 1: Scheme of a modern inverter-based GMAW power source (© Fronius) Depending on the applied amperage and voltage, provided by the welding machine, different arc modes, e. g. short arc, spray arc or pulsed arc, can be set up. But this process cannot be used only for joining – also overlaying can be done in a very easy manner, if the filler wire melts down and fuses directly with the component surface. As GMAW is the stateof-the-art welding process, many different types of filler metals are commercial available and almost each type of weldable steel, aluminium, copper and nickel alloy can be welded with respectable weld metal properties. A further advantage of this special welding process is its high ability for automation due to the more or less “endless” wire electrode which enables a continuous welding – only a very fast, efficient and understandable communication between the welding power source and the welding robot, established by special interfaces, is mandatory. Putting the welding torch on a robot offers the possibility for a very precise movement of the wire electrode. In combination with a very stable, constant metal transfer from the wire electrode to the weld pool provided by a high quality power source the built-up of 3 dimensional objects can be done, if the filler metal is deposited layer by layer in a very exact manner – “Shape-giving GMAW” or GMAW/ Metal Additive Manufacturing (MAM) was created.
2. Shape-giving Gas Metal Arc Welding This technology, so called “MicroGuss ™”, was successfully introduced by ANDRITZ HYDRO in the early 1990s as an alternative manufacturing method for hydropower Pelton runners due to the worse availability and improvable reliability of fully-cast integral Pelton runners. Fig. 2 shows an example for this application – Pelton runners for a 1300 MW hydropower plant in Switzerland.
Fig. 2: 423 MW MicroGuss ™ Pelton runner, Bieudron power plant; Switzerland [1] For runner manufacturing the buckets are built up by a shape giving GMAW welding process on a central forged disc containing the bucket roots (Fig. 3). From the metallurgical point of view the used base metal, a softmartensitic steel X3CrNiMo13-4 was challenging. High toughness, high tensile strength and porous-free material are the main criteria for this application which is fulfilled by GMAW/MAM.
Fig. 3: “MicroGuss ™” technology for establishing Pelton runners (© Andritz Hydro; [1])
With this technique it is possible to manufacture Pelton runners in much shorter time (30% less) compared to fully cast runners and additionally the mechanical behaviour of critical areas within the runner performed much better. But also in comparison to fully forged Pelton runners this production technology has economic advantages especially in the production of smaller sized runners due to less material which has to be removed by the final shaping milling operation. Till now more than 400 runners were built using this technology without any remarkable complaints in more than total 9 billion in service running hours [1]. Nowadays a new GMAW process, “CMT” (Cold Metal Transfer), is here in test with very positive results in respect to a remarkable lower heat input, an extremely stable arc and a very precise metal transfer. 3. CMT (Cold Metal Transfer) Within the last decades many process improvements of GMAW were done by the power source suppliers to improve the arc stability, the metal transfer and the welding start- and end procedure. But one of the biggest improvements was the invention of the Cold Metal Transfer (CMT) by Fronius. The driving force for the development of this modified GMAW process was the aspiration of Fronius to weld steel and aluminium using GMAW. The success criterion therefore is, that a mixing of molten aluminium and molten steel has to be avoided. In other words, aluminium has to be melted but at the same time steel has to stay solid. To achieve this requirement the GMAW process has to run on a very low energy level. The technical solution is a high frequency forward- and backward movement (up to 130 Hz) of the wire during welding. This is achieved by a special designed welding torch which holds also a very powerful motor which is responsible for the small high frequent movements of the wire (Fig. 4). Thereby the electric arc burning time and in further consequence the heat energy can be reduced to a minimum, because the metal transfer is mainly driven by the forward and backward movement of the wire.
Fig. 4: CMT equipment on a robot (left), CMT welding torch (right) But out of this a second process improvement arises: each forward-/backward movement of the wire leads to exact one metal droplet which moves from the wire to the weld
pool – an extremely precise metal transfer from the wire electrode to the weld pool can be achieved. In combination with a precise movement of the welding torch by a robot, also tiny 3 dimensional objects can by “welded” – as shown in Fig. 5.
Fig. 5: CMT - mascot But CMT offers some additional features: “CMT Pinning” enables to weld tiny parts of the filler wire on the surface of a component. Depending on amperage and pulling-back forces different pin structures like ball, flat or cone can be realized (Fig. 6).
Fig. 6: CMT Pin structures
4. Metal Additive Manufacturing (MAM) with CMT As CMT is an improved GMAW process all its benefits, like easy to operate, to handle and easy to combine with robots can be directly transferred to CMT. Additionally the process is much easier to adjust once the adequate synergic lines are provided by the power source manufacturer. Also the huge variety of commercial available, different filler metal alloys with approved weld metal properties can be used with CMT. In combination with adequate robots Metal Additive Manufacturing seems to be a very an interesting opportunity, because it opens a very broad spectrum of possible applications for various metallic components with material
properties comparable to real weldments which are well accepted in construction and engineering industry. In Fig. 7 some metallic CMT/MAM-examples, made of aluminium, stainless steel and brass are shown. The CMT process parameters can also be varied over a huge range of settings without any detrimental influence on the “welding” characteristics – so for instance it`s also possible to do some engraving by using only the electric arc without any filler metal addition or to make overlays with a high deposition rate.
Fig. 7: Examples for CMT – Metal Additive Manufactured components made of different alloys – aluminium, stainless steels, brass; engraving of the base plate Another very interesting question concerning CMT/MAM belongs to the producible structures. The minimum achievable thickness thereby depends mainly on the diameter of the filler metal which is used – and the standard wire diameter for the CMT process in general is 1,2mm. In this combination minimum wall thicknesses of around 4-5 mm can be realized, depending on the wetting characteristics during the metal transfer from the wire tip to the weld pool. Broader cross sections can be realized by torch weaving during welding and/or putting a certain number of welds side by side – up to establishing complete overlays. Best results can be achieved when the electric arc is in a vertical position. If inclined 3 dimensional planes have to be made, only a limited sideward offset of the actual welding torch position to the previous one can be done. If the base is fixed, planes with a decline up to 15% from the vertical can be established. But if the base is mounted on a commercial turntable very complex 3D planes can be produced – nevertheless the programming effort for the robot and turntable movement increases rapidly.
Care has to be taken in respect to the heat input caused by the electric arc: the smaller the cross section, the less the heat input has to be to prevent excessive remelting and in further consequence a “burn through” of the already welded seams. This means in practice that the welding current has to be limited and after each seam a certain time for cooling down of the already piled up seams has to be given. Taking this necessary cooling into account, a realistic deposition rate for a single layer pile up by CMT/MAM using stainless steel is around 1,5 – 2 kg/h metal. The thicker the cross section, the higher the deposition rate can be – up to approximately 5 kg/h as it is for CMT joining and CMT cladding. For very thick cross sections also a CMT Twin process (2 wires) could be taken into account – then the process could go to its theoretical limit of about 10 kg/h as it is for real cladding applications.
5. CMT/MAM of impeller blades made of Duplex-Stainless Steels Based on the experience gathered in the first CMT/MAM projects a feasibility for MAM of impeller blades (Fig. 8) was initiated. These impellers are originally produced by casting. The main driving forces for considering CMTMAM are the very long delivery times for integral casted impellers made of Duplex Stainless steels, the limited flexibility in the casting process, casting mould development and the limited casting properties of the Duplex Stainless Steel.
Fig. 8: Impeller Blades
For the mechanical and technological investigation easy blade geometries of DuplexStainless Steels were made by CMT/MAM. These blades (Fig. 9) had a dimension of around 200 x 200 x 10mm. The total welding time which was needed to build up 136 layers was 87 min, including interpass cooling times. The overall deposition rate (including cooling times) was around 1,7 kg/h.
Fig. 9: CMT/MAM blade – first attempt The blades were established on a horizontal mounted base plate, the decline from the vertical was achieved by a 0,5 mm horizontal sideward adjustment of the torch after each layer. To establish a much more vertical curved plane the base plate has to be fixed on a turntable but higher programming efforts for the geometric movements of torch and turntable are needed. As filler metal for this trials a typical filler metal for joining Duplex Stainless Steels, Böhler CN 22/9 N-IG was used. The mechanical properties of the all weld metal are shown in table 1; the delta ferrite content of the all weld metal is between 30 – 60 FN. Yield strength 660 MPa Tensile strength 830 MPa Elongation 28% Toughness (20°C) 85 Table 1: mechanical properties of all weld metal Böhler CN 22/9 N-IG [3] The microstructure of a Duplex Stainless Steel blade made by CMT/MAM is shown in Fig. 11; Fig. 10 shows the position of the samples and the examined area. As it can be seen, it is a typical Duplex Stainless Steel weld microstructure. The measured delta ferrite content of 34-36 FN is within the proposed values of the filler metal supplier.
Fig. 10: location of specimen for microstructure investigation
In Fig. 11 metallographic pictures from position 2 in different viewing directions are shown. Dark spots in the etched microstructure at lower magnification could be identified at higher magnification as areas with different etching behaviour. Further investigations concerning these areas are ongoing.
Fig. 11: Duplex Stainless Steel (weld) microstructure; position 2; etched with V2A Till now, the microstructural investigation gave no evidence for pores and slag inclusions and all findings correlate with the expected weld metal properties given by the filler metal supplier. In a next step the mechanical properties will be determined and compared with the all weld metal properties of the filler metal.
6. Conclusions The electric arc has the possibility to provide the necessary energy to melt metals – an effect which is widely used in arc welding and the most common used welding process is Gas Metal Arc Welding (GMAW). To use GMAW for Metal Additive Manufacturing it is necessary to run this process on a very low energy level, to prevent a “burn through” of already deposited material. GMAW/MAM is successfully applied since more than 20 years for the built up of softmartensitic Pelton turbine runners but due to the restrictions in the minimum energy in combination with a stable metal transfer the smallest achievable wall thicknesses are limited. As Fronius developed a modified GMAW process, CMT (Cold Metal Transfer) it is now possible to reduce the minimum wall thickness to around 4-5 mm and at the same time to increase the precision of the metal transfer from the wire electrode to the melt pool. As this process operates with any available GMAW filler wire a huge variety of different metals can be handled and in combination with a precise robot guidance various CMT/MAM- components can be made. Simplified Duplex Stainless Steels impeller blades for example point out overall deposition rates of around 1,5 kg/h and microstructures compared to all weld metals knowing from state-of-the art welds. It is also expected that the mechanical properties are comparable to the very well established all weld metal data provided by the filler metal supplier.
References [1] D. Appelyard, Welding Pelton Runners, HRW-Hydro Review Worldwide (2012) [2] FRONIUS CMT-Schweisstechnologie, WEKA Media GmbH (2013) [3] Datasheet “Böhler CN 22/9N-IG”, voestalpine Böhler Welding
