Joining Dissimilar Materials With Pulsed Nanosecond Fibre Lasers

  • Wednesday, 05 April 2017 00:00

Apart from their well-known uses of marking, cutting and even micromachining, nanosecond fibre lasers can also be used for welding and joining. By Jack Gabzdyl, vice president of the Pulsed Laser, and Daniel Capostagno, applications manager, SPI Lasers

Most applications of high-peak-power, short-pulsed lasers tend to be related to material removal, so their use for welding and joining is perhaps counterintuitive.

However, the versatility afforded by master oscillator power amplifier (MOPA)-based nanosecond fibre sources give unparalleled flexibility in terms of control of the output characteristics.

These lasers can be used, for example, with high-peak-power nanosecond pulsed output with tuneable pulse duration and high frequency-modulated quasi-continuous-wave (QCW) modes, as well as operated as a more conventional continuous-wave (CW) laser.

Energy Characteristics Are Key

Compared to CW and QCW sources, they have very low average powers (typically less than 100 W) and pulse energies of just 1 mJ, which is a thousand times less than typical diode-pumped solid-state (DPSS) and QCW pulses.

This does currently restrict the use of pulsed fibre lasers to relatively thin sections up to 0.5 mm, but recent results show that penetration depths of up to 1 mm are possible.

Operating with pulse durations often in the 100 to 500 ns range, with peak powers of up to 10 kW and pulse repetition frequencies of larger than 50 kHz, these lasers are significantly differentiated from conventional QCW sources. This ability to tailor the input energy characteristics is key to their use as a tool for joining.

Bonding Bright Metals

Examples of various metals welded to stainless steel.

When bonding thin material, there is a requirement for reliable joining processes that avoid over-penetration, distortion, and warping.

In conventional keyhole welding processes, overcoming material thresholds requires relatively high powers (usually more than 200 W). This challenge is even greater for joining bright metals, and even more so for dissimilar material combinations.

Joining dissimilar metals has long been a challenge for welding and design engineers who have often been told that it is difficult or that it could not be done.

As any metallurgist will tell you, it is the combination of physical properties and metallurgical incompatibility that governs weldability. However, there are a lot of potential applications that require such joints, particularly bright metals to steels and aluminium alloys.

The research focus in this area has yielded incremental improvements. Fundamentally fusion welding of dissimilar metals creates problems associated with the formation of brittle intermetallic phases. These can form planes of weakness, making the joints susceptible to fracture.

These intermetallics typically manifest themselves in the interface of the weld pool and form based on time and temperature. Heat input is a key factor in their formation.

Nanosecond Welding Process

An example of a section through a spiralled spot weld.

A differentiated mechanism such as SPI Laser’s nanosecond welding process has proven to be capable in joining a wide variety of dissimilar metals. The use of nanosecond lasers for joining is unlike conventional welding in that it does not generate a large melt pool, so opportunity for the formation of these unwanted intermetallic phases is inhibited.

The spot size is very small (often in the 30 µm range), so to make spot welds or linear welds, techniques such as spiralling and beam wobbling are used. The spot size can be easily changed by appropriate selection of beam expanding collimators.

The majority of the processing was conducted with SPI 70 W EP-Z laser with an M2 of less than 1.6. However, for some applications, it has been found that a broader energy distribution, as offered by the 70 W HS-H model with its M2 of 3.0, results in improved joint characteristics.

Joining Copper To Aluminium

A cross-section of copper (top) into
aluminium (bottom) weld.

The joining of copper to aluminium (with the copper on top) is a joint of significant commercial interest. It is technically challenging in that the copper is highly reflective and conductive, presenting the laser with a high threshold to enable coupling of the incident laser beam and the subsequent initiation of the weld.

The high peak power of the nanosecond pulses is crucial to overcoming this threshold and aids coupling into the material. The cross-section through a weld made through copper into aluminium shows the beam penetrating through the upper layer of copper and melts the aluminium below.

There is incomplete mixing and the aluminium is drawn through the copper to form more of a metallic bond between the two layers, resembling a rivet.

Spot Welding Using Spiralling Beam Paths

A single-pass joint gives only 30 to 50 µm of joint width, which may be insufficient for general applications. A spot weld created by a spiralling beam path is a more practical solution, where the size of the weld is dictated by the number of circular passes in the spiral.

Grid-pattern weld geometry, with a welded area ~4mm in
diameter and done in ~1 s.

These spot welds do not show the characteristics of a conventional spot weld, where there is a distinct single weld nugget. The spatial dimensions of the spiral can be varied to change the weld characteristics such that passes overlap, creating a more homogeneous joint.

A 0.8mm-diameter spot can be achieved through 150 µm copper into 400 µm aluminium in under 300 ms using a redENERGY 70W EP-Z SPI pulsed laser, and tensile testing has shown the joint strength to be more than 11.8kg (for three 0.8 mm spots
made in ~1s). Testing has also shown that use of secondary passes can improve the cosmetic appearance of these joints and can even improve strength, particularly if no shielding gas has been used.

Wobble Welding

Seam welding with a wider weld width can be achieved by using a wobble welding technique, with the wobble amplitude and frequency being optimised based on the material types and thicknesses. A wide range of material combinations have been made, but material combinations that appear to be of general interest include copper to aluminium, aluminium to copper, and stainless steel to aluminium.

A major area of current interest is the joining of battery cells. A wide range of materials are being studied, including aluminium, copper, and nickel-plated copper with a view to produce a reliable and tolerant process that generates strong welds with no burn-through or witness mark on the battery contact.

Heat input into the part can be critical and this can be accurately controlled by nanosecond welding. The material thickness of the tabs tends to be in the 250 to 500 µm range, which is generally within the capability of the nanosecond joining process range.

The nanosecond welding process offers multiple options in the join design—for example, it is possible to produce a single large spot or multi-spot arrays.

Joint Strength

Battery welding that lacks burn-through and witness

Studies have shown that although the contact area per given welding time was roughly the same, multi-spot arrays have proven to be stronger (both in tensile and peel strength) and give greater control of heat input. Using a 3 × 3 array made in approximately 2 seconds using a 100W laser, it can be seen that no burn-through or witness mark is made on the underside of the battery cap.

Peel testing has shown these joints to be incredibly strong and that the failure occurs in the heat-affected zone adjacent to the weld nuggets, leaving the weld stubs intact. Applications such as the welding of nickel-plated copper and aluminium tabs to standard lithium-ion cells is therefore possible.

APMEN Feature, Apr 2017

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