Let’s start with conventional welding, the most common technology for shipbuilders. Several new areas of work or research are interwoven here, primarily lasers and digitization. It is clear that with the advent of the laser, it became possible to weld parts together. The Orbita R&D project, implemented at the St. Petersburg State Marine Technical University (SPbSMTU) under the supervision of its rector Prof. Dr. Gleb Turichin, has shown potential of this area: the speed of welding two steel sheets 16 mm thick reached 85 m/h.
This very promising technology offers several advantages in comparison with the existing process. Firstly, it ensures the stability of welding process at high processing speeds. Secondly, it allows you to control the structure and properties of welded joints. Thirdly, it provides weldability of special steels and alloys. Fourth, it reduces welding distortions and ensures high quality welded joints. Finally, compared with arc welding, hybrid laser-arc welding increases the speed of the process two to three times.
It remains to pass all the necessary tests and start introducing this technology at shipyards. Technology and equipment for hybrid laser-arc welding of pipe, shipbuilding, high-strength special-purpose steels, light alloys, aluminum and titanium are already under development.
Of course, this technology needs a motion system for moving the laser welding head. A gantry, a robot arm or just a robot able to move along complex curved surfaces, an automated trolley, if it comes to welding flat sheets, or a metalworking machine can be such a system.
If we insert the welding head into the chuck of a five-axis milling machine, we get a hybrid technology that significantly expands both milling and welding capabilities. Earlier, I wrote about activities being carried out by Russian company ABAGY Robotics Systems, aimed at the economically viable use of robots in single-item and small-scale production, which is shipbuilding. I think that we’ll see, in a near future, a symbiosis of hybrid laser-arc welding and artificial intelligence able to effectively weld various, including complex curvilinear structures.
The Bauman Moscow State Technical University (BMSTU), where Academician of RAS, Dr. Nikolay Aleshin is conducting his research, might be called the center of digitalization of manual welding. The primary goal that he has set himself is to digitalize welding processes and their control. A digital platform being developed, the C60, can control welding processes to improve their quality, conduct digital monitoring of the welding process and move to maintaining as-built documentation in digitized form. The digital platform implements remote monitoring of the welding process and its compliance with the welding procedure specification (WST).
A well-established WST compliance procedure is now in place and includes: welder’s work permit check, WST and weld number selection, measurement and recording of welding process parameters, informing the welder with sound and light signals when the parameters are beyond the limits.
How will this process change in the near future? Just like in the previous technology, artificial intelligence and big data processing will come. A welding database on a digital platform is being established. Moreover, the knowledge accumulation process involves all process stages: preparation, the welding process itself, non-destructive testing, heat treatment and acceptance of as-built documentation. The use of artificial intelligence will make it possible to work actively with the established knowledge base, recognizing various welding defects in automatic mode, providing feedback for optimizing welding parameters and generating as-built documentation.
The introduction of a digital welding platform entails such prospects as enhanced individual responsibility of the welder through the introduction of a «live rating», keeping a digital welding log with digital joint data sheets and, the main thing, defect prevention at early stages.
Of particular interest to shipbuilding is the manufacture of parts or precision workpieces from metal powders. A number of pilot installations for direct laser deposition of products were developed and built, and a process for producing precision workpieces from a number of alloys was mastered in the framework of several R&D projects carried out by Gleb Turichin. Today it is necessary to carry out tests and certification activities that will allow the use of this technology in the industry. And what are the development prospects or capabilities of this technology in the future?
To begin with, I’d like to recall the essence of the process itself. Laser radiation is passed through a narrow gas-powder jet. The radiation mode and the flight time of a particle in a laser field are automatically maintained so that to get a liquid shell and a solid core of the powder grain. No burn-through occurs. As a result, the volume of the liquid phase is small, which provides very high crystallization rates. This preserves and even improves the structure. By installing the head in a robot arm, which traces a desired trajectory, we get an installation to grow products. Of course, all this is greatly simplified, but, in short, the essence of the technology is as follows.
Several questions arise in this context. Why not develop a multi-powder machine? If direct laser deposition results in a precision workpiece, is it possible to combine several technologies in one machine and immediately get a finished part or assembly? For example, direct laser deposition and metal cutting, or even hybrid laser-arc welding as an addition. And how can we improve the quality of grown workpieces?
Research studies have already yielded first results. For example, it was possible to make a demonstrator of a bimetallic product weighing 0.7 kg, 100 mm in diameter, and 30 mm in height from CuNiAl + Inc625 materials with a minimum wall thickness of 1.1 mm. That is, a workpiece consisting of two materials at once was grown in one pass. This opens up great opportunities for growing various framed, skeleton and other structures and units. If we imagine that control of feeding several powders will occur smoothly rather than discretely, then we can produce parts with a variable material composition along the length. That is, when answering the question: «What material is this part made of?» you have to ask a clarifying question: «Where is it exactly?»
Now I should like to say a few words about attempts to develop hybrid technologies. The Institute of Laser and Welding Technologies at SPbSMTU is developing a symbiosis of a direct laser deposition machine and a five-axis milling machine. The prototype will have the following characteristics: five-axis kinematics with synchronous interpolation, a powder feeder (two five-liter tanks), a milling-and-turning unit, as well as one-setup deposition and machining. The maximum dimensions of the product are 1100 mm in diameter and 400 mm in height. The unit’s capacity on Fe, Ni, Co alloys is up to 2.5 kg/h. In addition, local gas protection will be provided. I think such a hybrid machine will occupy a worthy place in the production chains of shipbuilding enterprises.
Technology and equipment for hybrid laser-arc welding of high-strength special-purpose steels, light alloys, aluminum and titanium are already under development
Work is also underway to combine direct laser deposition and hybrid laser-arc welding technologies. Direct laser deposition of workpieces and then their welding between each other are being tested
on one installation. After manufacturing, the structure has passed quite severe tests, showing the capability to work at a temperature of about 950 °C and internal pressure of about 100 atmospheres.
It is worth mentioning also about efforts to improve the quality of the articles produced. Of course, considerable insight into the physics of the direct laser deposition process has been gained in recent years. In addition, scientists began to apply not only direct non-contact laser measurements, but also computer simulations to calculate the thermal distortions that arise. Thus, we deliberately form the calculated “incorrect” geometry of the part, which, after cooling, will fall within the tolerances specified by the drawing and become suitable for use.
It is this “wrong” geometry that is incorporated into the program that controls an industrial robot for growing a suitable part. In general, the physics of the deposition process is quite complicated, and that’s why the software being developed for writing robot control programs must take into account many aspects: the physical properties of the material from which the part is grown, its behavior during the formation of a particular geometry (for example, loss of stability), deformation of the substrate material, and much more. In addition to all the above, the software should provide also its main functions: import the geometry of all common standards, build tool paths, edit and sort them by layers, set the processing order, specify process parameters (laser power, gas and powder flow), simulate robot movements taking into account the geometry of each individual installation, check the robot’s movement for collisions, write a control program and send it to a direct laser deposition installation.
At the initial technology development stages, powders were mainly imported. It was their high price that became the main constraint to the introduction of additive and hybrid technologies based on them. But now the situation is improving dramatically. Following VIAM, Ruspolimet (Kulebaki, Nizhny Novgorod Region) and several other domestic enterprises launched their installations to produce high-quality powders for additive technologies. In connection with the production of large volumes of powders and the use of new highly efficient equipment, their price will undoubtedly decrease, which will increase the use of additive technologies in various industries.
I am sure that in a short time sophisticated marine engineering products, various components of deep-submergence vehicles and many other shipbuilding products will be grown and processed using innovative equipment.