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Heavy vehicle applications of robotic MIG welding
18
Heavy vehicle applications of robotic MIG welding È M , Volvo Wheel Loaders AB, Sweden H BROSTRO
18.1 Introduction The development of robot technology has made it possible to use robotic welding of big, heavy objects with very long arc times for each object. It is possible to connect not only arc welding equipment but also other equipment to the robot station. Today it is common to see turntables, positioners, arc seam sensing and tracking, cameras, double-wire guns, etc. and twelve to fifteen axles that can work together in the robot cell with advanced control systems.
18.2 Welding of frames for heavy wheel loaders 18.2.1 The welding process Heavy construction robot arc welding (HCRAW) uses MAG welding both with solid wire 1.2 mm and metal-cored wire 1.2±1.4 mm. The shielding gas is an argon±carbon dioxide mixture. The process development during the last five years has lead to the development of the twin-arc and tandem arc welding processes. The welding speed for single wire is on average 0.45 m/min and for double wire up to approximately 1.0 m/min. The use of two metal-cored wires in the tandem arc welding process gives a welding speed of 1.5 m/min when welding a throat thickness of 5 mm in the welding position PA (most favourable position). Figure 18.1 shows a typical wheel loader.
18.2.2 Materials and preparation of the parts The steel used is low-strength steel, thickness from 6 mm to 80 mm with an average thickness of 20±25 mm. The steel plates are shot blasted before the thermal cutting operation and the parts are straightened to within 2 mm (see Fig. 18.2). The plasma cutting gives tolerances of 1 mm. Some important location points, e.g. holes for location in tack-welding fixtures or in joints are machined with tolerances of 0.2 mm. It is very important for the quality of the
286
MIG welding guide
18.1 Heavy wheel loader.
18.2 Straightening of steel plates.
Heavy vehicle applications of robotic MIG welding
287
welds and for achieving high welding speed that the tolerances of the parts are very tight. One quality problem is the variations in thickness and hardness of the steel plates that give different spring-back in bending operations. The capability of the bending machine is therefore of great importance.
18.2.3 Fixtures and robot movements The most complex fixtures are used in tack welding. After tack welding, the metal structures are fixed to the manipulators in the robot cell. Special adaptors are used in the manipulators. This gives the flexibility of welding different types and sizes of weld structure and also the possibility of welding in position PA. Most heavy robot cells are of the gantry type that gives the optimum working range. The investment for such a heavy robot arc welding cell is from US$800,000 to US$1,000,000. In order to have an acceptable payback of this investment the robot cell must be used at least for two or perhaps three shifts.
18.2.4 Welding The deep penetration when welding with tandem arc welding and metal-cored wire will be 2±3 mm (see Fig. 18.3). This gives the possibility of reducing the visible throat thickness dramatically from, e.g. 8 mm to 6 mm and still have the same strength of welded joint. The shielding gas used in this process is Ar + 10% CO2. (The gas is mixed in a mixer in the factory from argon and carbon dioxide.) The requirements on the materials for this process are clean surfaces of the joint, no gaps and welding position PA. One big advantage with this method is that the welds get a concave shape, which gives higher fatigue strength. Some welds are also TIG dressed and the weld structures are shot-blasted (steel shots) afterwards which increases the fatigue strength even more.
18.2.5 Programming It is possible to weld heavy structures in robots thanks to the gantry robot configuration (Fig. 18.4), and advanced robot programming. Robot programming was earlier a bottleneck since it was time-consuming and the robot had to be used during the programming. No production was possible for a long time. Today offline programming (OLP) is used. The utilisation of the robot is optimised and CADCAM technology makes it possible to transfer 3D-solid models from the design department to the manufacturing (see Fig. 18.5). A successful design for robotic welding requires a long experience and close cooperation between the design department and the manufacturing department.
288
MIG welding guide
18.3 Fillet weld with good external shape and penetration, position PA. The weld is welded with tandem MIG welding and two metal-cored wires diameter 1.4 mm. Shielding gas was argon with 10% carbon dioxide, gas flow 25 l/min. Welding parameters 420 A and 32 V, stick-out 25 mm and welding speed 1.2 m/min. The theoretical throat thickness is 6 mm (black), the effective throat is 8 mm (white). It is possible to reduce the actual throat as is shown by the white line. The strength is at least the same as with the large throat (black dashed line). Also note the good connection between the weld metal and the base metal that favours a good fatigue performance.
18.2.6 Transport to and from the robots The frames are loaded with a crane onto the loading wagons, then the loading and unloading to and from the robot cell is done automatically.
18.2.7 Quality control It is not possible to find defects inside the welds without using destructive testing. Such solutions are used for small objects where test samples are checked regularly. The control procedures of weld quality for heavy structures include a very strict control of the robot program, the welding parameters and the robot positioning during the welding. Welding inspection is done statistically by magnetic particle testing and ultrasonic testing. The HCRAW process must be well adopted within the welding organisation.
Heavy vehicle applications of robotic MIG welding
289
18.4 Gantry robot with tandem arc welding equipment.
18.5 3D-CAD solid model of a side-section of a rear frame, which is also used in offline programming of the robot.
290
MIG welding guide
The cooperation between design and manufacturing, support from suppliers and the deep knowledge of the robot operator are all crucial elements for the success of the robot welding.
18.3 Future trends Some trends in the future development of design and manufacturing of heavy earth-moving vehicles are: · Increased use of high-strength steel: ± Reduction of weight ± More weight in the bucket ± Less fuel consumption, less pollution, etc. · Increase of the forces on the welds means that higher quality is required of the welds. · Laser hybrid welding (laser±MIG welding): ± Less residual stresses and less deformation ± Higher welding speed ± Deeper and better control of the penetration. · Full automation of the whole robot welding line: ± Lower operation time ± Less manual handling. · Integrated quality inspection system of the welds. To reach these goals development is needed both in design and welding processes. A deeper understanding of the behaviour of the highly stressed welds under fatigue load is important and how to translate this into more precise calculation methods. With regard to processes, the laser hybrid welding is very interesting due to its high penetration.
Heavy vehicle applications of robotic MIG welding È M , Volvo Wheel Loaders AB, Sweden H BROSTRO
18.1 Introduction The development of robot technology has made it possible to use robotic welding of big, heavy objects with very long arc times for each object. It is possible to connect not only arc welding equipment but also other equipment to the robot station. Today it is common to see turntables, positioners, arc seam sensing and tracking, cameras, double-wire guns, etc. and twelve to fifteen axles that can work together in the robot cell with advanced control systems.
18.2 Welding of frames for heavy wheel loaders 18.2.1 The welding process Heavy construction robot arc welding (HCRAW) uses MAG welding both with solid wire 1.2 mm and metal-cored wire 1.2±1.4 mm. The shielding gas is an argon±carbon dioxide mixture. The process development during the last five years has lead to the development of the twin-arc and tandem arc welding processes. The welding speed for single wire is on average 0.45 m/min and for double wire up to approximately 1.0 m/min. The use of two metal-cored wires in the tandem arc welding process gives a welding speed of 1.5 m/min when welding a throat thickness of 5 mm in the welding position PA (most favourable position). Figure 18.1 shows a typical wheel loader.
18.2.2 Materials and preparation of the parts The steel used is low-strength steel, thickness from 6 mm to 80 mm with an average thickness of 20±25 mm. The steel plates are shot blasted before the thermal cutting operation and the parts are straightened to within 2 mm (see Fig. 18.2). The plasma cutting gives tolerances of 1 mm. Some important location points, e.g. holes for location in tack-welding fixtures or in joints are machined with tolerances of 0.2 mm. It is very important for the quality of the
286
MIG welding guide
18.1 Heavy wheel loader.
18.2 Straightening of steel plates.
Heavy vehicle applications of robotic MIG welding
287
welds and for achieving high welding speed that the tolerances of the parts are very tight. One quality problem is the variations in thickness and hardness of the steel plates that give different spring-back in bending operations. The capability of the bending machine is therefore of great importance.
18.2.3 Fixtures and robot movements The most complex fixtures are used in tack welding. After tack welding, the metal structures are fixed to the manipulators in the robot cell. Special adaptors are used in the manipulators. This gives the flexibility of welding different types and sizes of weld structure and also the possibility of welding in position PA. Most heavy robot cells are of the gantry type that gives the optimum working range. The investment for such a heavy robot arc welding cell is from US$800,000 to US$1,000,000. In order to have an acceptable payback of this investment the robot cell must be used at least for two or perhaps three shifts.
18.2.4 Welding The deep penetration when welding with tandem arc welding and metal-cored wire will be 2±3 mm (see Fig. 18.3). This gives the possibility of reducing the visible throat thickness dramatically from, e.g. 8 mm to 6 mm and still have the same strength of welded joint. The shielding gas used in this process is Ar + 10% CO2. (The gas is mixed in a mixer in the factory from argon and carbon dioxide.) The requirements on the materials for this process are clean surfaces of the joint, no gaps and welding position PA. One big advantage with this method is that the welds get a concave shape, which gives higher fatigue strength. Some welds are also TIG dressed and the weld structures are shot-blasted (steel shots) afterwards which increases the fatigue strength even more.
18.2.5 Programming It is possible to weld heavy structures in robots thanks to the gantry robot configuration (Fig. 18.4), and advanced robot programming. Robot programming was earlier a bottleneck since it was time-consuming and the robot had to be used during the programming. No production was possible for a long time. Today offline programming (OLP) is used. The utilisation of the robot is optimised and CADCAM technology makes it possible to transfer 3D-solid models from the design department to the manufacturing (see Fig. 18.5). A successful design for robotic welding requires a long experience and close cooperation between the design department and the manufacturing department.
288
MIG welding guide
18.3 Fillet weld with good external shape and penetration, position PA. The weld is welded with tandem MIG welding and two metal-cored wires diameter 1.4 mm. Shielding gas was argon with 10% carbon dioxide, gas flow 25 l/min. Welding parameters 420 A and 32 V, stick-out 25 mm and welding speed 1.2 m/min. The theoretical throat thickness is 6 mm (black), the effective throat is 8 mm (white). It is possible to reduce the actual throat as is shown by the white line. The strength is at least the same as with the large throat (black dashed line). Also note the good connection between the weld metal and the base metal that favours a good fatigue performance.
18.2.6 Transport to and from the robots The frames are loaded with a crane onto the loading wagons, then the loading and unloading to and from the robot cell is done automatically.
18.2.7 Quality control It is not possible to find defects inside the welds without using destructive testing. Such solutions are used for small objects where test samples are checked regularly. The control procedures of weld quality for heavy structures include a very strict control of the robot program, the welding parameters and the robot positioning during the welding. Welding inspection is done statistically by magnetic particle testing and ultrasonic testing. The HCRAW process must be well adopted within the welding organisation.
Heavy vehicle applications of robotic MIG welding
289
18.4 Gantry robot with tandem arc welding equipment.
18.5 3D-CAD solid model of a side-section of a rear frame, which is also used in offline programming of the robot.
290
MIG welding guide
The cooperation between design and manufacturing, support from suppliers and the deep knowledge of the robot operator are all crucial elements for the success of the robot welding.
18.3 Future trends Some trends in the future development of design and manufacturing of heavy earth-moving vehicles are: · Increased use of high-strength steel: ± Reduction of weight ± More weight in the bucket ± Less fuel consumption, less pollution, etc. · Increase of the forces on the welds means that higher quality is required of the welds. · Laser hybrid welding (laser±MIG welding): ± Less residual stresses and less deformation ± Higher welding speed ± Deeper and better control of the penetration. · Full automation of the whole robot welding line: ± Lower operation time ± Less manual handling. · Integrated quality inspection system of the welds. To reach these goals development is needed both in design and welding processes. A deeper understanding of the behaviour of the highly stressed welds under fatigue load is important and how to translate this into more precise calculation methods. With regard to processes, the laser hybrid welding is very interesting due to its high penetration.