How to Run a Comparative Laser-Welding Test With and Without Filler Wire ART-019

Start with the question the test must answer

A supplier demonstration may show two neat welds: one without filler and one with wire. If you look only at the photograph, both may appear convincing. That is not enough for production.

The purpose of a comparative test is not to decide which weld looks “better.” It is to determine which process produces an acceptable result more consistently on your real part, how sensitive it is to the actual variation in joint fit-up, and how much time the full route to an accepted part requires.

Before the test starts, define the decision that must be made afterward. For example:

• can this part be welded in series without filler;
• is filler wire needed only for part of the product mix or for specific joints;
• does filler provide enough stability margin to justify the more complex feed system;
• is the real problem not welding but unstable part preparation;
• which option produces an accepted part in less total time once preparation and rework are included.

Without a defined question, the test can easily turn into a demonstration of machine capability. An investment decision needs a different format: one part, one acceptance framework, and a clear selection rule defined in advance.

Keep the part conditions common, but do not force identical process settings

A fair comparison starts with both variants solving the same production task.

Record the material and grade, thickness, joint type, edge geometry, locating method, fixture, surface condition, and finished-part requirements. If the test represents serial production, use blanks produced by the same cutting, bending, and assembly route that will be used in production.

At the same time, do not require wire and no-wire welding to use exactly the same laser parameters. They are different process regimes. Adding filler metal changes the weld pool and introduces its own controlled variables — wire feed rate, wire position and angle, material, and diameter. Holding every setting artificially identical can put one process at a deliberate disadvantage.

The fair rule is different: the production task and acceptance criteria must be the same, while each process must first be stabilized in its own working regime.

That lets you compare two genuinely viable ways of making the same part rather than two arbitrary parameter combinations.

Define quality criteria before the first weld

If the criterion is introduced after the test, it is easy to unconsciously adapt it to the result that already looks preferable.

Before comparison begins, define what counts as an acceptable part. For one product, the visible surface may be critical; for another, post-weld geometry, penetration, absence of particular imperfection types, or the mechanical function of the joint may matter more.

Laser-welded joints have dedicated imperfection-quality standards, including ISO 13919-1 for steel, nickel, titanium, and their alloys, and ISO 13919-2 for aluminium, magnesium, and pure copper. The important boundary is that these documents define production-quality levels for imperfections, but do not by themselves prove fitness for a particular load or function. The required level and inspection methods must follow the application, design requirements, and the responsible party.

For a production trial, prepare your own acceptance list in advance: what is checked on every part, what is checked selectively, and which defects immediately mean a failed result.

An attractive weld can be one criterion, but it should not automatically become the only one.

Test the real spread of part preparation, not one perfect joint

The meaningful difference between processes often appears not on a perfectly fitted pair of blanks, but when joint geometry begins to vary within the range seen in real production.

Before testing, measure actual joint variation on representative parts: gap, edge offset, angle, bending deformation, and repeatability of locating in the fixture. Do not manufacture an artificially extreme “worst” joint. Reproduce the deviations that really arrive at welding from upstream operations.

The test can then be built around several representative states rather than one specimen:

• a well-fitted part near the center of the actual distribution;
• a typical deviation that occurs regularly in series production;
• a worse geometry that still remains acceptable under the company’s internal requirements.

Do not borrow numerical limits from somebody else’s brochure. Manufacturers may publish gap-bridging capabilities for specific systems, but those values belong to a particular optics/head/material/regime/wire-feed combination. TRUMPF FusionLine and Laserline cold-wire/multi-spot solutions, for example, show that filler wire can extend the ability to work with imperfect fit-up, but that is a property of the complete process system rather than a universal property of “laser welding with wire.”

The practical objective is to establish your own process window: the range of real variation over which the process remains acceptable and repeatable.

Do not compare a first trial weld with an already tuned process

Another common mistake is to make a few passes without wire, switch immediately to the feeder, and declare the process that first produces a good-looking weld the winner.

Before comparison, each process needs its own short stabilization stage. For the no-filler variant, establish a stable parameter combination for the defined joint. For the wire variant, stabilize both the laser process and the filler-wire feed.

The goal is not to find one “magic” recipe. Check whether there is margin around the working point. If a small real deviation in geometry, wire position, or locating immediately destroys the result, one attractive specimen does not yet represent a production-capable process.

A logical sequence therefore has two stages:

1. establish a working regime for each method; 2. only then compare them on the same part matrix and acceptance criteria.

This separates the technologist’s ability to tune the equipment from the process’s ability to tolerate normal production variation.

Serial production tests repeatability, not the best specimen

One weld can turn out excellent by chance. Production needs to know whether the next part will behave the same way.

Important matrix points therefore need repeated samples. There is no universal correct sample count for every product: it depends on joint responsibility, test cost, expected variation, and company requirements. But one specimen cannot distinguish a stable process from a favorable coincidence of conditions.

It can also be useful not to weld every specimen of one regime first and every specimen of the other afterward if that sequence could hide warm-up, fixture-condition changes, or operator effects. The test order should make drift, restart behavior, and return to a previously established regime visible.

For a manual system, part of the spread may come from the operator. In a robotic process, positioning, fixturing, trajectory, and wire-feed stability may dominate instead. That does not make either equipment type automatically better; the sources of variation are simply different.

Record not only “pass/fail,” but the reason for failure: weld collapse, unstable fill, geometry change, excessive rework, appearance violation, or another criterion defined before the test.

Inspect in layers: from the surface to what the product actually requires

The simplest inspection layer is visual examination and finished-part geometry. It is needed in most cases, but for some products it is not sufficient.

The next layer can be internal geometry and joint character on a sectioned sample or by another justified method. For safety-critical or otherwise responsible products, nondestructive or mechanical testing may be required by design requirements, a standard, or company procedure.

Do not turn an equipment trial into an improvised certification procedure. ISO 15614-11:2025 separately governs qualification testing of welding procedure specifications for electron-beam and laser-beam welding. If the product requires formal WPS qualification or another regulated confirmation, the internal comparative test does not replace it.

For choosing between filler wire and autogenous laser welding, the principle is simpler: the same acceptance set must be applied to both variants, and inspection depth must match the real responsibility of the product.

If one process receives only visual inspection while the other is additionally checked by section or mechanical testing, the comparison is no longer equivalent.

Welding time is only one part of the comparison

Wire feeding can make the welding operation more complex while simplifying part preparation or reducing rework. Autogenous welding may be extremely fast on an accurate joint but lose that advantage if excessive manual fitting is required upstream.

Measure separately:

• part preparation and cleaning;
• placement and locating in the fixture;
• tack welding when it is part of the real route;
• welding itself;
• wire-feed operations and consumable changes where applicable;
• unloading and inspection;
• defect correction and post-weld rework;
• the share of parts that fail the acceptance criteria.

Compare the time per accepted part, not only the travel speed of the head along the seam.

This matters especially when the difference between processes shifts work from one operation to another. An extra second in welding can be beneficial if it removes minutes of fitting or grinding. Conversely, filler wire has no automatic economic advantage when an accurate autogenous process already solves the task consistently without rework.

The result should become a concrete application rule

The test does not have to select one process for the entire company.

Several conclusions are possible.

Autogenous welding is stable across the full real joint range. In that case, adding wire has no obvious process function and only increases the number of variables.

Autogenous welding works well on accurate parts, while wire saves recurring edge-of-range deviations. Then the decision must include upstream operations: is it cheaper to improve cutting, bending, and fixturing accuracy, or to use filler in a controlled way?

Both processes meet the criteria, but one has a wider process window. The advantage may lie not in the best individual weld, but in lower sensitivity to normal production variation.

Filler wire is required by the geometry or properties of a specific joint. Then it is no longer “insurance”; it is a full process variable that must be documented and reproduced consistently.

Neither regime is stable. That is also a useful result. The cause may be the joint itself, the fixture, surface preparation, trajectory, material, or an unsuitable system configuration. Buying a more complex feeder merely to mask an unidentified cause is a weak investment decision.

This approach produces not a binary “wire yes/no” answer, but a process rule for a specific product range.

A good test ends with a protocol that can be reproduced a month later

If the result exists only in the technologist’s memory or in a video from the demonstration center, part of the work will have to be repeated after the equipment is installed.

At minimum, record:

• part and material identification;
• actual joint geometry for each sample;
• surface preparation and locating method;
• machine, head, and optics configuration;
• shielding gas and delivery method;
• laser-process parameters;
• for the filler variant: wire material and diameter, feed parameters, and feed geometry;
• sample number and test sequence;
• inspection results and recorded defects;
• actual operation times;
• photographs and, when used, results of additional inspection.

The purpose is not bureaucracy. A protocol separates the process from one particular day, operator, and demonstration center. After FAT, transport, installation, or a material-batch change, there is a baseline against which the new result can be compared.

If a supplier cannot explain what conditions produced a good test weld, that specimen has much less value for serial production.

What to prepare for a comparative test with L-SEL

The most useful trial starts before arriving at the demonstration center. Prepare drawings or real parts, material grade and thickness, joint type, the actual spread of gap and edge offset after upstream operations, appearance and functional requirements for the weld, and an approximate production volume.

From that information, a common matrix can be built for both processes: stabilize the autogenous regime, separately stabilize the filler-wire regime, and then run the same part variants through the same acceptance criteria.