When does laser welding with a robot or cobot pay off?

A robot can repeat a programmed path very consistently. Without sensors, it does not compensate for a deviation that the individual component brings with it. In hand-held welding, this correction is made by the person at the workpiece.

This article sets out the requirements and limits of automation. It replaces neither the system manufacturer’s risk assessment nor the operator’s workplace risk assessment and does not constitute legal advice.

What changes with automation

In hand-held laser welding, the person at the machine sees the joint. They can adjust position, feed rate and angle of incidence if a gap widens or an edge is offset.

A robot, by contrast, follows the programmed path. If the seam on the next component lies slightly differently, without seam detection the beam still strikes the programmed point. Fraunhofer IWS identified this conflict of objectives early on: the unavoidable shape and dimensional tolerances of every component on one side, the limited accuracy of the machine systems on the other.

What is compensated for during the seam in manual welding should be clarified in advance in the cell:

• how accurately the components are manufactured and delivered
• how they lie in the fixture
• whether and how the seam is detected
• how the laser radiation is reliably shielded

Component tolerances and joint gap

Without filler material, laser welding is sensitive to gaps. The British welding research organisation TWI gives as a guide value that, for butt joints, gaps must be kept below around 10 per cent of the material thickness to avoid weld defects. For 2 mm sheet, that would be less than 0.2 mm.

TWI also describes ways of extending this tolerance:

• filler wire: for steels up to 6 mm thick, according to TWI, up to roughly the full sheet thickness
• beam oscillation: around 20 to 30 per cent of the thickness
• hybrid laser-arc welding with lower requirements for edge preparation
• two laser beams arranged side by side

Each of these extensions brings its own components and parameters. Harald Keller of Keller Blechtechnik GmbH puts it this way in an article by TRUMPF: anyone who wants to exploit the potential of laser welding has to think right back to the design stage — precision is needed from the very beginning. Cutting, forming and pre-machining therefore help to determine whether a cell can run without constant rework.

Clamping fixtures are part of the process

TRUMPF states that the “zero gap” is the measure of all things in laser welding — and that this requires practical knowledge of suitable clamping fixtures.

In automated laser welding, a clamping fixture has several tasks:

• position components reproducibly so that the seam lies where the program expects it
• press the joining partners together so that the gap stays within the permissible range
• hold the position during welding
• leave access for the welding head and, where applicable, the sensor
• allow safe loading and unloading

Each component variant may require its own fixture or at least its own insert. Design, construction and trials belong in the calculation before a decision is made about the robot. If the component changes, the fixture generally changes as well.

Seam tracking and sensors

Sensors can absorb part of the tolerances. The manufacturer Precitec divides process monitoring in laser welding into three phases:

1. Before the process: optical seam tracking detects the actual seam position. According to the manufacturer, this helps in dealing with component tolerances, clamping deviations and changing seam geometries.
2. During the process: process signals are recorded continuously to make deviations visible.
3. After the process: optical seam inspection checks the finished seam for geometric and near-surface defects.

One possible measuring principle is light sectioning. Fraunhofer IWS described a seam tracking sensor whose camera captures the gap by means of light sectioning and provides information on the position, course and width of the weld gap.

The limit: sensors detect where the seam lies. They do not close a gap that is too large. They can, however, provide the basis for adjusting parameters, feeding filler material in a controlled way or rejecting a component — and they cost money, space on the welding head and set-up effort.

Enclosure and laser protective walls

TROS Laserstrahlung Part 3 follows the order of priority set out in the OStrV: first avoid or minimise hazards through other processes or work equipment, then technical measures, then organisational measures, and finally personal protective equipment. It counts enclosures, with interlocking where appropriate, and devices for automatic shutdown among the technical measures. For shielding, it names laser protective walls in accordance with DIN EN 60825-4.

For robot-assisted laser systems, a technical information sheet from BG ETEM and DGUV Test states the requirement clearly: hazardous laser radiation must not escape from the laser protective enclosure. The safety concept can consist of, among other things:

• passive laser protective walls with safety-interlocked doors
• restricted or monitored robot working areas
• monitoring of the robot movement and the beam direction
• limitation of the laser on-time
• active laser protective walls, even if only in certain areas

The technical information explicitly also considers the fault case, for example a robot that leaves its programmed path. Passive laser protective walls withstand directly incident radiation for a limited time only.

Laser class, servicing and set-up mode

The laser class is determined by the manufacturer. DGUV Information 203-093 cites marking devices with complete shielding as an example of laser class 1. An enclosure can therefore determine which hazard remains outside the system.

The same DGUV Information points out something that is easily overlooked: even enclosed equipment may require open operation in special operating modes such as maintenance, servicing or setting work. The requirements for open operation then apply. It also states:

• Openings are generally monitored by a safety position switch.
• When the enclosure is open, no hazardous laser emission may be possible.
• Observation windows are generally designed in accordance with DIN EN 207 or DIN EN 60825-4; alternatively, a camera can transmit the image to the outside.

For laser equipment of classes 3R, 3B and 4, § 5 OStrV requires a laser safety officer — unless the employer has the necessary expertise themselves. How large the laser area is — including in open servicing and set-up mode — and which protective measures are needed has to be clarified in the workplace risk assessment.

A cobot still needs laser protection

DGUV Information FB HM-080 describes how collaborative robot systems operating with the “power and force limiting” function can be used without traditional safeguards such as fences and light curtains. This function concerns forces and pressures on contact between a person and the robot.

Laser radiation is a different hazard. According to the DGUV Information, the collaborative robot system also includes tools, workpieces and fixtures — in laser welding, therefore, the laser head. The IFA stresses that the risk assessment must include the intended workstation.

The laser manufacturer IPG Photonics writes that laser welding cobots must be installed in a laser-safe enclosure to protect people standing nearby. In laser welding, the benefit of a cobot therefore lies more in simpler programming and faster changeover than in doing away with shielding.

Programming by teach-in or offline

In teach-in, the path is taught on the real component. The trade journal Maschine + Werkzeug points out that production has to stop for this; for simple tasks, however, an experienced setter can still be faster than building a digital model. According to IPG Photonics, cobots can be guided by hand for teaching.

In offline programming, the cell is recreated as a digital twin on the basis of CAD data, even before the real cell has been set up. Because the model and the shop floor never match completely, calibration is necessary. For complex three-dimensional contours, for example in welding, the article sees offline programming as having the advantage.

In addition to the path, the process parameters have to be right: laser power, feed rate and, where applicable, beam oscillation are determined for the material, thickness and joint type and tested on sample parts. This effort is incurred for each component variant.

Fume, extraction and secondary radiation

The enclosure shields laser radiation. Other hazards arising from the process remain.

For laser welding, DGUV Information 203-093 cites the UV, visible and infrared radiation emitted by the plasma as an example of secondary radiation. It can lead to glare and arc eye, but also to irreversible damage.

Laser material processing also releases particulate and gaseous hazardous substances. The DGUV Information refers, among other things, to TRGS 528 “Schweißtechnische Arbeiten” and also addresses possible fire hazards in extraction systems. For a cell, this means that capture and extraction are planned in from the outset, not retrofitted.

Who is responsible for the cell

DGUV Information FB HM-080 states the following for collaborative robot systems: a single robot is regarded as partly completed machinery and receives a declaration of incorporation. Only the system consisting of robot, tools, workpieces and fixtures forms the machine, which is supplied with a declaration of conformity and CE marking.

The technical information from BG ETEM and DGUV Test states that the selection of protective measures, responsibility and liability lie with the manufacturer of the laser processing machine, and in this context names a complete risk assessment by the manufacturer and a complete workplace risk assessment by the operator.

According to IHK Halle-Dessau, the Machinery Regulation (EU) 2023/1230 applies in principle from 20 January 2027 and replaces the Machinery Directive 2006/42/EC. Anyone who makes a substantial modification then becomes the manufacturer, with the corresponding obligations. Anyone who combines the robot, laser source and enclosure themselves or converts a system should clarify in advance who takes on this role.

When automation can pay off

IPG Photonics sees conventional industrial robots mainly where large quantities of identical seams are produced on identical components, for example on a production line. According to the manufacturer, cobots offer more flexibility when new welding tasks are programmed frequently.

Favourable conditions can include:

• recurring components in sufficient quantities
• small and stable tolerances from upstream production
• easily accessible seams
• a component range that can be handled with just a few fixtures

The cost side involves more than the robot: enclosure, fixtures, sensors where applicable, extraction, programming, trials, instruction and conformity assessment. Set against this are the previous welding time, rework and scrap. The result depends on the individual business and can only be calculated reliably with its own figures.

When hand-held remains the more sensible option

Hand-held laser welding can remain the better choice for:

• individual parts, repairs and prototypes
• large or bulky components that do not fit into any cell
• changing geometries and small batch sizes, where programming and fixturing take longer than the welding
• fluctuating gap dimensions that an experienced person can still compensate for within the limits of the process

Hand-held, however, means open operation. According to DGUV Information 203-093, most hand-held laser equipment is classified by the manufacturer as class 4, and the laser area is generally accessible. The protective effort therefore shifts from the machine to the workstation, shielding, instruction and personal protective equipment.

Beamlux offers a wheeled laser welding machine in power ratings from 1,500 to 6,000 W. The product catalogue lists suitability for automated processes as one of its features. Whether and how the machine can be integrated into a specific robot cell depends on the component, the fixture and the control system, and has to be clarified in each individual case.

Sources

• TROS Laserstrahlung Part 3 “Measures for protection against hazards from laser radiation” — order of priority of protective measures, enclosures with interlocking, automatic shutdown, laser protective walls in accordance with DIN EN 60825-4.
• OStrV § 5 — Competent persons, laser safety officer (gesetze-im-internet.de) — appointment of a laser safety officer for laser equipment of classes 3R, 3B and 4.
• DGUV Information 203-093 “Handlungshilfe für die Gefährdungsbeurteilung beim Betrieb von offenen Laser-Einrichtungen zur Materialbearbeitung mit Handführung oder Handpositionierung (HLG)” — April 2019 edition. Classification of the laser class by the manufacturer, laser area, enclosure, special operating modes, observation windows, secondary radiation, hazardous substances and extraction.
• BG ETEM / DGUV Test — technical information “Hochleistungslaser in der Automobilbranche” (high-power lasers in the automotive industry) — as of 2 June 2014. Safety concepts for robot-assisted laser systems, passive and active laser protective walls, fault analysis.
• DGUV Information FB HM-080 “Kollaborierende Robotersysteme” — edition 08/2017. Power and force limiting, robot system as a machine, declaration of incorporation.
• IFA — Technical information on collaborative robots — standards for collaborative robots, end effector and risk assessment.
• IHK Halle-Dessau — Machinery Regulation (EU) 2023/1230 — date of application, replacement of the Machinery Directive, substantial modification.
• TWI — FAQ on the tolerance of laser welding to joint fit-up gaps — guide values for gaps in butt joints and ways of extending the tolerance.
• TRUMPF — “Laser trumps arc” — zero gap and clamping fixtures; statement by Harald Keller (Keller Blechtechnik GmbH) on precision from the design stage.
• Fraunhofer IWS — press release “Sensor führt den Laser” (sensor guides the laser) — optical seam tracking with light sectioning, component tolerances and machine accuracy.
• Precitec — Process monitoring in laser welding — seam tracking before the process, monitoring during the process, seam inspection afterwards.
• IPG Photonics — Laser Welding Cobots vs. Traditional Robots — enclosure for laser welding cobots, programming, areas of application for conventional robots.
• Maschine + Werkzeug — “Robot programming: teach-in vs. offline” — downtime during teach-in, digital twin and calibration in offline programming.

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