The welding program optimization of a S355 steel assembly used in agricultural machinery

The paper will include the realization of a weld joint line with an ABB robot. The manufacturing area contains a welding device and a robot. For the robotic welding, the methods and a complete welding program will be presented. The first stage is the design of the device that can be used for manufacturing a range of comparable size pieces with similar gripping capability. There are described the axes and the way of movement, the presentation of the speeds for the rotating joints and roto-translation. CAD design of the workpiece including WPS (welding technology that contains the shape of each joint and welding parameters: welding current, electric arc voltage, welding speed and secondary parameters). The base material, the addition material, protective gases and a list of parts from the robot that require periodic changes will be presented. It is presented the CAD design of the workpiece including WPS.


Introduction
The paper will present an optimization of the welding process of steel components S355 used in the construction of agricultural machinery. The welding installation consists of an ABB welding robot and a welding station. The welding station has 3 rotation axes ( figure 1, a), it performs a movement around the z-axis which allows access of the two welding posts to the welding torch. The station also allows rotation on the x and y axes. The welding station allows the mounting of a welding device (figure 1, b) that is specific to each assembly that could be welded in this area.
The robot has 6 degrees of motion, given from the rotation or rotating coupling from which it is formed. The robot programming is done with the FlexPendant SDK, Figure 2 [1].

Fig. 2. The ABB robot and the SDK FlexPendant
The programming of the robot is done by learning, by the tactile sweep of the welding joint not by predetermined coordinates. The coordinates results and are recorded for the following welding commands.

Motivation
In the motherboard, a badly sized channel was cut by laser. The two pieces that are part of the considered ensemble merged by embedding. The parameters used for cutting were inappropriate resulting in a larger gap in the base plate (sole) in which the top piece (core type) is mounted. An empty semi-circle and a linear space are seen laterally, figure 3. For this reason, it will be used to sweep the welding head that leads to excess material. From the calculation it results that the increase of the cord from the z5 area is 12.5 mm 2 the area of the joint at the z6 area will be 18 mm 2 , which means at 1000 mm length of the cord a volume from z5 V5 = 12500 mm 3 and at z6 V6 = 18000 mm 3 . The volume difference will be 5500mm 3 , that is 44% of the designer's requirement. As a result, the melted filler material enters between the two pieces in an amount greater than that established by the initial programming.
Following is the presentation of the piece after welding, where part of the molten bath has passed into the gap between the parts and thus the cord is narrower and with the greater possibility of breaking due to the tension concentrators, figure 4. Remedial options: -using a laser sensor to read the joint and transmitting to the robot the requirement to modify the parameters so that the cord remains the same geometric parameters or the locksmith after montage to manually fill the defect; -the welding operator will be able to lower the welding speed on the defective cord portion so that the cord's z remains constant throughout its length; The motivation for these corrections (figure 5) is necessary because the piece is part of the chassis of an agricultural machinery, which raises a large amount of ground for aeration and enters up to 600 mm in the soil.

The programming steps for welding the part
The rotation of the welding station 1 after the x position /2[ ] -the position of waiting of the assembly and programming the robot (Figure 6). At this point, the robot's zero programming is done, [2]. First, the wire cutter is sent to 10mm from the gas nozzle and the spatters are removed (Pulizia Torcia). Prior to welding, the torch is brought to the device that spray silicone to prevent weld splashes from sticking. The head was previously manually straightened and cleaned of spatters with a miller, [3]. Setting to the zero position follows with the TCP at reference pointer command, if it is not at zero, a TCP Check is performed with a laser sensor of the torch, and itself searches for the zero position of the device.
Type New Modules and the next dialog window opens. In this module you can enter a very high number of routines. Enter the part code (example R17610160 Figure 8).
Rotation along the x-axis position (0 [rad]) -In this position ( figure 9) the circular weld on the horseshoe and the corner seam between the heart and the sole are welded, [4].  The robot reads ActStationProd -sending it into production. We started the robot reading the next move.
The following command types can be programmed: MoveL -linear motion; MoveJmovement in space from one point to another point, the robot chooses the trajectory, figure 11;

Fig.11 Program for the first welding phase
In order to make the intermittent seam it is necessary to make the following movements: The robot is in the standby position, doubled with the cleaning and siliconing of the torch. The programmer will move the robot so that the torch is ready for the beginning of the welding. For this purpose, two moves are made in space MoveJ, followed by MoveL, for positioning the welding head at the beginning of the joint, Figure 12.  The programmed welding parameters on the welding source: the arc source voltage selected at 22.5 V with a correction factor of -2, for a 177A welding current. The welding current is selected by default depending on the diameter and the material of the electrode wire, Figure 13. The gas is started by 0.3 seconds earlier (purge_time) and the gas maintenance time after welding (preflow_time) is 0.2 sec. All this data was saved on the source with a number (source specification).
The welding parameters programmed on the robot are marked with weld_data (weldR17610160). For this step choose the welding speed (in the case of corner joint z5 vs=8 mm/s) and correlate with the data selected on the source (wire feed speed, spring tension, and possible correction factors). Wire feed rate (wirefeed) is 7,5 mm/s. The pendulum function of the welding head is marked with weavedata and the pendulum characteristics are programmed: length, height, width and frequency. The pendulum is used for a more efficient melting of the base material and for a better bond between the base material and the deposited material. This avoids overheating the base material.

The stage of effective welding
There are the following notations: ArcLast -the beginning of linear welding, ArcC -circular weld seam (to create an arched weld seam three points on the arc and three more on the same arc are given), ArcLend -end of the linear weld and ArcCend -at the end of circular weld seams.
If the seam is discontinuous when a gap follows, a movement is made in the lifting space of the torch, followed by moving in space at the next start of the seam and lowering to the beginning of the next seam. A weld seam follows with determined parameters and movement, then again, a movement is made for the gap in the discontinuous seam then we repeat the program on the other side and complete the workpiece by cleaning the torch. The next step is to start station 2. After each program end the welding head is taken to the cleaning and silicone device ( figure 14). We have the following welding elements: corner joint with z5 with 6x80 (50) length and for corner horseshoe z5 for a circular element, the cord is made all over the piece, figure 15.    The number of passes is calculated using the formula: So, the number of passes will be 1 t n = for both joints. Calculating the amount of loose material lost for a linear stitch meter

Conclusions
Making welded joints with robots is more and more prevalent. But for this it is necessary to process the joints with low tolerance. The presented study shows that engineers with lower welding skills can learn programming if they receive enough information or sufficient visual tools. But programming a robot performing a weld at the stage of approval shall take into account the structural changes occurring in HAZ not only defects that appears because of an improper joint.
In the study, there were defects due to the wrong dimensions of the parts obtained by machining and the welding operator's attempt to correct the program, without taking into account the structural changes.
The mounting stage of an assembly should be as precise as possible and the preassembled cords should be of a length specified by the designer.
It can be seen that a processing defect can lead to an increase of consumption by 44% of the additive material, accompanied by additional energy consumption, protective gases, a 30% reduction in welding speed, and the biggest problem is the introduction of some fissure. The price per 1 m linear welding has increased by 16,0 Euro only for materials and energy consumption.
In the company where the problem was studied, one of cause of these problems is the lack of skilled workers in both welding and machining.