Best quality wire alloys for a revolutionary technology for perfect Wire Arc Additive Manufacturing
1. General information
Metal additive manufacturing is major change of mind-set technology progressively being introduced in the industrial applications. Basically, thanks to the melting of material layer by layer, such technology allows to build parts from 3dimensional model data, using a feedstock which might be powder or wire.
Regarding the applicable main processes, when powders are laying on a bed, a laser energy source selectively melts or sinters such feedstock. In such cases the process is powder bed process and called respectively Selective Laser Melting (SLM) or Selective Laser Sintering (SLS). When feedstock is directly used to deposit material, then it can be powder or wire and process is called Direct Energy Deposition (DED) (for metal printing also Direct Metal Deposition (DMD)).
In case of wires as feedstock, a further classification might be done based on the melting source as it can be electron beam, laser beam or an electric arc. In this latter case, which is derived from the well-known welding technologies the process is named Wire Arc Additive Manufacturing (WAAM). Sources of the electric arc are plasma with a side wire feeder or wire arc, where the wire electrode is the same like in the conventional gas metal arc welding process.
Fig. 1: Plasma WAAM, Fig.2: MIG-MAG WAAM
Wire Arc Additive Manufacturing is a fast and highly efficient production process, based on well-known welding and cladding know-how, applicable to a wide range of alloys from unalloyed to high alloyed steels, nickel alloys, titanium alloys, aluminum alloys. Basically anything that can be welded can also be manufactured by WAAM.
2. Comparison of WAAM with other technologies
WAAM features some clear advantages in comparison with the conventional subtractive technologies.
Some of those advantages are illustrated here below:
- quite flexible in terms of deposition rates, just varying electrical parameters, feeding and travel speeds;
- producing Near Net Shape (NNS) parts with low material loss;
- conventional machining time reduced to minimum;
- reduced lead times;
- good structural integrity.
Differences between powder based systems and WAAM can be summarized according to the following table:
only few available as standard
high (Ti, Fe, Ni), variable Al
possible with processing
Out of position deposition
coaxial - no, side feed yes
coaxial - no, side feed yes
Deposition rate per hour
3.5 kg/h ~ 70% more
Table 1.: comparison with Powder Alloys system and WAAM
3. Features of solid wires for WAAM
In comparison with welding and cladding processes, analogies with WAAM are known and evident but WAAM is for sure more challenging for filler metals; reasons are basically linked to higher demanding metallurgical and process requirements.
About metallurgical, productivity leads to the use of high heat input and low cooling rate due to the subsequent layers deposition technique, hence physical and mechanical properties of the deposit has to be soundness also in such conditions; additionally it has to be tolerant to take multiple hardening/tempering cycles by multiple layers and afford heat treatment when and if necessary,
Wire chemistry is also influencing the arc stability, the molten material fluidity as well as the silicate islands formation over the bead, which might be detrimental for the deposition of the subsequent layer. Level of impurities and Silicon is of course very important to keep this issue under control.
About process, as arc stability and uniform feeding are crucial, the perfect spooling together with the proper wire surface condition make the big difference in order to minimize the number of stops and increase the lifetime of liners and contact tips.
Therefore voestalpine Böhler Welding decided to develop a specific range of solid wires called 3DPrint AM covering the wide majority of the alloys required in this field. Specific surface finishing of the drawn wires have been settled and for some of the alloys chemistry has been fine-tuned compared with the welding wires in order to achieve the wished mechanical properties and bead appearance properties in such demanding conditions.
Endurance testing protocols with the related acceptance criteria have been settled in order to prove a high level of arc stability and feedability measuring in real-time all the electrical parameters involved including the current at the wire-feeder which is proportional to the resistance torque.
Picture 3.: U[V]/I[A] integral plot over time (blue star in the middle) of stable process which shows only minor deviations
Therefore, benefits of Böhler 3DPrint AM wires can be summarized as per the following table:
Made for low cooling rates and high heat input
High process stability for Robotic MIG or other mechanised processes
Accepts multiple hardening/tempering cycles by multiple layers
Drum and spool weights can widely be adopted to the weight of parts
Optimised for post print heat treatment
Extended quality control to ensure consistent arc and feeding behavior
Tailor-made metallurgy for complex materials
Optimised surface technology for long arc cycles, Liners stay clean, contact tips last longer
Table 2: metallurgical and process benefits of Böhler 3DPrint AM wires
The table 3 below shows the alloys which have been introduced to meet the demand of the industry segments focused in WAAM, such as aerospace, shipyard, O&G, hydropower. Materials are available in various diameters (e.g. 1.0 and 1.2 mm) and package format (spools and drums of various sizes). Special formats can also be produced on demand.
Table 3: Böhler 3DPrint AM product range
Just to give an example, as far as specific testing is concerned, the below picture is related to one of the solid wire designed for WAAM, 3Dprint AM 17-4 PH, i.e. precipitation hardening martensitic stainless steel material type 17-4. In order to characterize the material, 20 layers have been deposited free of defect and without grinding between passes, with a robotic MIG station at a travel speed of 55cm/1’ with shielding gas M12 type, simulating an additive manufacturing real condition. Heat input has been 7.2 kJ/cm.
Picture 4: printed coupon and destructive tests on 3Dprint AM 17-4 PH deposit.
Bent- test and tensile test in ‘as printed’ and after heat treatment showed results meeting the requirements. Tensile test results are according to the following table.
Table 4.: tensile test results
As another example 3DPrint AM70 low alloyed strength steel wires with the properties of a S700 material, has also been submitted to a similar testing campaign again with a robotic MIG station and M12 shielding gas. Used travel speed has been 65 cm/min. Also in this case all the deposits resulted to be free of defect and no grinding was necessary between layers. RP0,2 resulted to be always above the required 700 MPa with an elongation A5 above 17%, also after post-heat treatment while normally the conventional equivalent welding wire is mainly conceived for as welded application.
Picture 5.: deposit and macro with 3Dprint AM 70
voestalpine Böhler Welding developed a range of solid wires tailored for productive Wire-Arc Additive Manufacturing; thanks to the fine-tuned chemistry and the specific coating and surface finishing, such wires can fully meet the demanding requirements of WAAM processes, in terms of productivity, material integrity, chemical and mechanical properties.