Model qualification of an additively manufactured pressure vessel

Selecting the DED process (Direct Energy Deposition process)

The essential requirements for the DED process (also known as the wire arc additive manufacturing process) were as follows for the model qualification of the pressure vessel:

• The highest possible deposition rate
• A reduced-heat process to avoid or minimize the cooling requirement and distortion  
• No lack of fusion in the connection to the base material
• Insensitive to changes in the distance between welding torch and component
• Absolute reproducibility of the high material quality required within the qualified limitations of the deposition process
• Suitability for large components

Based on the requirements, the MIG-based CMT process was chosen for the additive build-up of the weld layers—specifically, CMT mix for the first layers and CMT Additive Pro for the subsequent wall build-up, where the use of power correction significantly influenced the heat input. 

Qualifying the process

Due to the different wall thicknesses of the transition from the base pipe to the branch, the scope of application of prEN 13445-14 required three separate process qualifications (DPQR). The resultant deposition procedure specifications (DPS) are binding for the additive welding process. The final “recipe” for metal 3D printing, the so-called additive manufacturing procedure specification (AMPS), thus comprises three deposition procedure specifications, instructions for the welding sequence, and notes on material certificates and operator qualifications.

“In addition to welding guidelines, material certificates and operator qualifications are what ensures a consistent standard of product quality for additively manufactured components,” explains Manfred Schörghuber of Fronius R&D. 

The individual test pieces were subjected to non-destructive and destructive testing as per the requirements of prEN 13445-14. Visual and dimensional tests (VT), volume tests (RT-D), and surface tests (PT) were used as non-destructive testing processes to provide evidence that the pieces were free from faults both externally and internally. 

“We demonstrated that the mechanical and technological requirements for the additively manufactured material and the hybrid connection were met by checking the chemical composition and by conducting tensile and bending tests perpendicular to the direction of the deposition layer. We then performed metallographic analyses at the start and end points, and at the hybrid connection,” explains Martin Boche from TÜV SÜD.

Planning and simulating the robot path

The path for the robotic welding task was planned using a three-dimensionally modeled pipe branch in the CAM software (computer-aided manufacturing software) developed at Fronius.

“We calculated the proposed additive build-up—the actual welding program—by entering the layer height, position, speed, and build-up strategy. The welding path was visualized in a robotic welding cell that we modeled in our software,” adds Leonhard Reiter of Fronius R&D.

Position sensing and sensor travel

The component was scanned using Fronius WireSense so that it could be perfectly positioned and corrective action taken with regard to the manufacturing tolerances. This innovative sensor technology uses the wire electrode as a tactile sensor and scans the welding contour point-by-point. If the electrode extension touches the surface, a short circuit occurs, which triggers a distance signal and transfers it to the welding robot. The system’s software compares the target value programmed offline with the actual value of the WireSense travel, and corrects the welding path for additive manufacturing as necessary. The wire then immediately moves backward and—while the robot continues moving the welding torch—forward again to the next point, where a further short circuit is triggered, a distance signal is generated, and the welding path is modified again, if necessary. This minimizes geometric deviations.

Producing the component

The various wall thicknesses in the transition to the branch were implemented via different oscillation amplitudes. A constant heat balance was required to ensure the necessary optimal weld toe from bead to bead, and the uniform weld seam flow. Layer-specific welding jobs with specific parameters were used for this.

A water supply and return were provided for the component during the welding process. The water level this produced had to be far enough away from the welding point to maintain the interpass temperature in the qualified area. This allowed continuous welding without cooling breaks. Cooling of the component minimized component distortion and increased the deposition rate. 

“To observe the build-up of the layers, we used a camera that was triggered in synchronization with the process. This allowed us to analyze the process deviations more accurately afterwards,” adds Reiter of Fronius R&D.

Monitoring parameters and documentation

The WeldCube welding data management software monitored the parameter limits stated in the AMPS (additive manufacturing procedure specification) and issued a warning as soon as the preset limits were exceeded. The sum of all the parameters yielded the “fingerprint” of the additive build-up and facilitated analyses in the event of possible discontinuities.

Final component testing and looking ahead

The final testing, including confirmation of CE conformity, was performed by notified body 0036 of TÜV SÜD Industrie Service GmbH. In the present case, a starting material with mechanical data in a future, harmonized European standard (“harmonized” design) and a component of design class DC1, the result was a component inspection scope in accordance with the table below:

“In parallel with these tests, we developed metallographic analyses covering especially those areas where imperfections had been detected. Also affected were the transition areas between the classic material and the additive welding deposition, the hybrid area. The main purpose of the metallographic analyses was to verify the data collected from parameter monitoring and from mechanical, technical, and non-destructive testing. In this case, samples were taken to gain a clear idea of the extent to which the material and the applied manufacturing process were flawless,” explains Boche.

Now that the conformity assessment procedure has been completed according to Module G of the European Pressure Equipment Directive 2014/68/EU, in the form of a “model qualification” of an additively manufactured pressure vessel with subsequent CE label to confirm compliance with the requirements of this EU directive, the pressure vessel joint working group of the German Institute for Standardization (DIN), represented by Linde Engineering, TÜV-SÜD Industrie Service GmbH, MIGAL.CO and Fronius, is looking to increase the acceptance of additive manufacturing in general, and in plant and container construction in particular.

“In conclusion, we can definitely say that decades of experience with arc-based welding processes—combined with stable and innovative processes such as Fronius CMT Additive Pro—offer compelling reasons to increase the use of metal 3D printing in plant and container construction. This creates a competitive edge, especially in view of the associated advantages such as topology optimization, just-in-time manufacturing, and supplier independence,” sums up Schörghuber.

Detailed information on model qualification can be found in the following documentary.

Fronius unleashes customers’ welding potential

If you would like to experience the complete 3D offering for yourself, you can visit Formnext in Frankfurt am Main, Stand C99, Hall 12.0 from November 19 to 22, 2024, or the ADIPEC trade fair in Abu Dhabi, November 4 to 7, 2024. Hall 14, Stand 14316. The Fronius experts are looking forward to showcasing the new offers and options in person. Anyone interested in benefiting from Fronius’ know-how in 3D printing and solving complex welding challenges is welcome to contact the specialists in the Fronius prototyping center. 

You can find out more about Fronius Additive Pro here.

Information about the prototyping center can be found here.