Welding Technology Automatic and robotized applications of welding

Automation in welding
Welding processes can be subjected to different levels of automation, also based on the complexity of the work to be done and depending on the production.
The processes that are better suited to robotic applications are:
GMAW;
GTAW;
PAW;
laser and electron beam welding;
resistance welding.

Levels of automation in arc welding

Type of robots employed in welding
On the basis of the architecture is possible to distinguish different types of robots:
anthropomorphic (revolute)
cartesian
spherical or polar
cylindrical
SCARA
For each of these categories, is important to define:
degrees of freedom, ie the possible movements (linear or angular) of the end effector
workspace, ie the area of possible motions that can be achieved from the end effector

An interesting opportunity is the ability to use hollow wrist robots:
easier movement;
wider workspace;
easier PC simulation.
Anthropomorphic robot configuration

In the case of positions on multiple workspaces, or complex elements, is possible to use a hybrid geometry
portals with robots (also mounted in the reverse position)
sliding rails with anthropomorphic robots
Type of robots employed in welding

Programming and simulation of the process
Programming tools for the robot:
teach pendant, simple to use, but with reduced functionality;
PC with dedicated programming languages, very versatile, but requires specific knowledge of the interface (rarely "user friendly")
advanced simulation tools
The main tool to reduce the time of finalization of the program is the simulation, which allows you to identify trajectories and programs given the input characteristics of the work and the 3D model of the final product.
Advanced Programming Systems
Virtual reality systems

Virtual programming
Testing the robot motion detection with interference, use of sensors and program fixes with keyboard.
Compiling a complete program in a virtual system calibrated with the real system (no post processing):
virtual self learning programming using the mouse to carry the torch on point and save it in a single click
ways of taking a point on the workpiece with the mouse pointer
Ability to transfer programs from the virtual to the real robot (upload) for elaboration and verification.
The differences in calibration between the virtual world of simulation and the real world are corrected by the research and the pursuits made ​​with appropriate sensors.

Application to the welding processes: GMAW
The particular characteristics of the process allows a particularly effective control of the parameters:
constant voltage (generator)
constant current (self regulating system)
the position control can be carried out by managing suitably the value of the stick - out.
The arc ignition doesn’t pose particular problems, but is important to manage it in order to avoid spatters.
Arc extinction is realized with appropriate programs (eg. crater filler)

Traditional system
Lift start system
Lift-start (LS)
Lift-end (LE)
Traditional system
Lift end system
Lift-start (LS)
Lift-end (LE)
Contact
Spatters
Wire heating
moving
Spostamento
burn back
Fine saldatura
Contact control
Reduction of the cycle time
Spostamento
Arc ingnition and arc extinction systems
Courtesy Panasonic

Other devices employed:
device for the cleaning of the nozzle (and possible spraying of non-stick substances)
device for the cutting of the wire (when necessary)
antirupture systems
Application to the welding processes: GMAW

The process is suitable for robotic applications with some limitations:
arc ignition system with high voltage and high frequency;
applications with filler metal:
the welding direction is a process parameter
the presence of the wire reduces the criticality of the arc length
for applications without filler metal, are required devices for an accurate management of the arc length
Application to the welding processes: GTAW

Ciclo di saldatura con processo TIG robotizzato
Application to the welding processes: GTAW

Contact sensors for arc welding
They are primarily used for the tracking of the joint.
Can be simple probes or complex electromechanical systems, generally oscillating along the bevel.
The distance between the sensor and the arc can be a source of problems for complex paths (high distance) or for overheating (low distance).

Arc based sensors
The simplest system is based on the arc voltage (eg, submerged arc).
The most common case is that of tracking systems of the joint, based on torch movements and consequent variation of the values ​​of voltage and / or current (the movement of oscillation is adjusted in order to obtain the constant fluctuation of these parameters).
TAST system

Vision base sensors
These systems can capture an image and process it to handle:
tracking of the joint
penetration
size of the melt pool
shape of the deposit (for example for welding in position)
To make the system less sensitive to light emission of the arc, are used sensors for specific wavelengths coupled with a high power laser beam.
The complexity is related to information management systems and to the logic for the correction of the welding parameters

LASER sensors
These are the sensors that provide greater sensitivity.
Principle of operation
a light source emits a laser beam that illuminates objects;
the signal is sampled by a digital camera;
a dedicated software filters the signal and interprets it, providing information to the control system.
The system works even if the arc is off, and allows the identification of the joint

Comparison between sensors for arc welding

Robot – sensor communication
Influence the possibility to manage in real time the movements of the system (adaptive management of the welding process).
Analogic one-way connection:
a calibration is required between the robot and the generator because of the imprecision of the analog signal
Digital connection:
digital connection decreases the differences between the real and the set parameters
CPU connection (single CPU):
total process and movement control via software by eliminating the differences between set and real values

TIME Process (GMAW)
Used for processes with high productivity and exclusively in conjunction with robotic equipment, allows higher deposition rate and welding speed if compared to a traditional GMAW process:
Wire feed speed very high, up to 30 m / min and stick out up to 35 mm, resulting in high values ​​of welding current and up to 30 kg / h (1.2 mm wire)
welding speed even higher than 700 cm/min

Multiwire applications (GMAW)
Multiwire torches allow an increase in productivity, with exclusive use in automated applications
Typical areas of application:
rail;
shipbuilding;
automotive;
tanks and reservoirs;
pipeline.

MIG Brazing (CMT)
A particular system for the advancement of the wire (in the torch) allows the obtaining of pulse speed of the wire:
operation similar to the short-arc, with current values ​​lower and greater regularity
is necessary "wire buffer"

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Resistance welding
Autogenic fusion process by pressure
Applicable to all materials
Automotive
Railway vehicles and components
Microjoining
Small thickness pressure vessels
…
The technical principle can be applied to:
Resistance spot welding
Projection welding
Flash welding
Upset welding

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Welding metallurgy: the weld nugget
Heat is generated at the center of the joint
The interface between liquid phase and solid phase is solid state welded
The final microstructure has a dendritic shape, whith solidification of grains starting from periphery of the nugget

The resistance welding process is suitable for robotic application
The critical issues are:
the weight of the gripper
services (air, cooling system)
the size of the gripper, which influence the ability to reach working points in complex structures
Load capacity of the robot
up to 10 times higher than those for the arc welding (up to 80 kg)
unlike the manual application, the clamp is not counterbalanced with ceiling supports, to ensure greater mobility
Robotic applications of RSW

Sensors and control for the spot welding
The development of inverter generators makes response times in the order of 20 ms, and consequently the application of adaptive controls.
The electrical and pressure parameters are given to a control unit, which brings them back to the standard parameters (the operation is based on the control of the resistance between the toes during welding).
During the welding process, the values ​​recorded in the system will be compared, every millisecond, with the measured values​​.

Adaptive controls

Being a welding process with keyhole technique, the conductivity of the material influence to a lesser extent, as the heat is transferred to "direct contact" between the material and the source, ensuring the welding of large thickness in one pass and small HAZ.
The ability to travel great distances in the air makes it possible to weld in areas with poor access.
They are, however, necessary preparations and movements very accurate.
Main characteristics of a LASER welded joint

Optical devices for deflection and focusing of the beam:
lenses (used for low power, because of the expansion due to overheating);
mirrors in Cu, Au or Mo (more reliable when cooled by recirculation of water).
Protective devices of the melt pool:
welding in free environment;
welding in a closed room (highly reactive materials, for leak testing or special containers to seal).
Devices for the reduction of the plasma plume.
α
2 α
Equipment for LBW

The geometry of the robot is influenced by the type of laser and the transmission system (Fiber or mirrors):
anthropomorphic systems (Nd: YAG laser, fiber);
Cartesian systems (CO2);
laser Remote welding.
The key features of the systems
welding speed (also higher than 2 m/min);
accuracy (required accuracies of the order of tenths of a millimeter).
The absence of contact between the end effector and the workpiece requires the development of dedicated sensors, able to guarantee the benefits provided.
Application to the welding processes: LBW

Laser remote welding
The applications of RW (remote welding) provide welds at distances between 0.5 m and 3 m from the LASER source
reduces the problems of accessibility;
the welding can be performed only without gas protection.

Applicability of the laser process
The choice of application of a laser process, in substitution of a traditional process, must consider the following factors:
accuracy in the preparation of the parts and in its coupling;
complexity of the system, which requires the presence of dedicated tools and flexibility;
investment, referred on the installation, use, maintenance and the cost of preparations and coupling.

Applications: LASER weldbrazing
The regularity and welding speeds allow to use low-power lasers (YAG) laser for brazing activities.
Typical applications are:
braze welding of galvanized steels with copper (CuSi3), silver (Ag55Sn) and zinc (ZnAl14) based alloys
braze welding of etherogeneous steel - aluminum joint
Braze welding of galvanized steel with CuSi3 alloy
P = 1.1 kW, v = 1,0 m/min
Braze welding between C-steel and alluminium alloy with ZnAl14 alloy
P = 1.1 kW, v = 1,0 m/min

Applications: LASER welding of tailored blanks

Friction Stir Welding
Friction Stir Welding is a solid state process based on dynamic recristalization.
Patented in 1991 by the TWI.
This process employs a rotating tool characterized by a pin and a shoulder in contact with the surface of the component.
Classified, according to UNI EN ISO 4063, 43

FSW: working principles
The friction between the tool and the parent material provide the heat required for the plasticization of the joint.

Ciclo di saldatura FSW
FSW: Phases of the welding

Characteristics of the tool
As regards the geometry of the tool, the main parameters are:
pin diameter;
shoulder diameter;
pin lenght.
A big operative limitation is related to the wear of the material constituent the tool:
the hardness is one of the most important parameters.

Esempi applicativi della tecnologia Bobbin Tool®
Geometries of the tools: Bobbin tool® typology