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Welding

Tiny torches

By Geoff Shannon, Ph.D.

Above: This is an example of a sensor seam weld using a QCW fiber laser.

Choosing the right laser technology for jobs that call for micro welding

January 2017 - When it comes to knowing what constitutes a micro weld, there is no concrete definition. Reserved for small, delicate components, the rule of thumb for a micro weld is if the application calls for a penetration depth of less than 0.04 in. and in some cases much less.

Micro welding can be achieved using four different laser systems: pulsed neodymium-doped yttrium aluminum garnet (Nd:YAG), continuous wave (CW) fiber, quasi continuous wave (QCW) fiber, and nanosecond fiber. Each type offers features best suited for specific applications. But certainly, more than one option may work for the same task. A manufacturer can narrow the choice by looking at cost of ownership and serviceability of the laser system geared to micro welding. Comparing the pulsed Nd:YAG laser with the three fiber laser options can also help anyone selecting a machine to understand guidelines for choosing the right technology. 

Nd:YAG technology is especially suited for spot welding applications under 0.02-in. penetration and seam welding heat-sensitive packages. 

The laser’s active gain medium is neodymium, which is doped into a host crystal of yttrium aluminum garnet. This solid rod of material typically has a diameter of 0.1 to 0.2-in. and a length of approximately 4 in. Micro welding Nd:YAG lasers are optically pumped using flashlamps and emit light with a wavelength of 1064 nm, but can be frequency doubled (532nm, green). The laser’s optical design is relatively simple; its heart is the power supply that drives and controls the flashlamp voltage and allows precise control of peak power and pulsed width during the laser pulse using internal optical feedback.

Focusing on beam quality

A fiber laser is generated wholly within a silicon fiber, which is doped with yttrium in contrast to the pulse Nd:YAG laser, so there is no need to align the medium to cavity mirrors, nor is it necessary to maintain optics. Another significant feature of the fiber laser is that it uses diode lasers instead of flashlamps, and the diode is long lasting. This fiber laser has a small footprint, can be air cooled, and provides high wall plug efficiencies.

The fiber laser’s focusability and its range of beam qualities provide a tunability for each welding application. The two ends of the beam quality spectrum are single mode and multi-mode—single mode is defined by a beam quality or M2 less than 1.2, while multi-mode is generally above M2 of 2. The mode defines how well the laser can be focused and the power density distribution across the laser, with single mode providing high power density Gaussian distribution through the beam. Multi-mode lasers provide lower power density with a more flat-top distribution through the beam—each being useful for specific applications. 

FFJ 0117 welding image1

Cross sectional seam weld using the wobble function of a nanosecond laser in 250-micron thick titanium.

CW fiber features

For high-speed seam welding applications, a CW fiber laser is operated in continuous wave mode, which means the laser output remains on until being turned off. For spot welding either a single weld or a seam, the laser output can be pulsed or modulated—turning the laser on and off rapidly. The CW laser’s peak power is the same as its maximum average power, so focused spot sizes are generally under 100 microns to attain sufficient power density for welding with power levels under 1kW. Due to the small optical spot size, lap and fillet weld geometries are preferred. Butt welding is possible if there is good part fit-up or if a seam location vision system is used. A manufacturer can also choose to use a scan head that can create motion lateral to the weld direction, known as wobble, which can effectively widen the weld to reduce joint alignment sensitivity. 

CW fiber lasers are well suited to general seam welding up to a depth of 0.06-in. Single-mode fiber lasers provide the capability for welding conductive metals such as copper and aluminum, and for producing spot welds under 100 microns in diameter.

QCW fiber features

The quasi-continuous wave fiber laser’s peak power and pulse width characteristics are similar to those of the Nd:YAG laser, but the parameter range is not quite as broad. Similar to CW fiber lasers, the QCW lasers offer single mode to multi-mode options with spot sizes from 0.001 in. to 0.04 in. as needed for the application. These lasers also shine in small spot size applications and penetration applications, although they do provide a fairly comprehensive coverage of many micro welding applications. 

Nanosecond features

The nanosecond fiber laser is a more recent addition to the micro welding market. It is the same laser used for laser marking applications, and is a cost effective solution that can be repurposed effectively for certain welding applications. The nanosecond laser provides multi-kilowatt peak power, but with pulse widths around 60 to 250 nanoseconds that can be delivered between 20-500 kHz. This high peak power enables welding of almost any metal. The very short pulse widths permit very fine control for welding small parts, as well as the ability to weld dissimilar materials. 

Summary

While there are a number of choices for laser for micro welding, the CW fiber and nanosecond fiber have distinctive application specialties; Nd:YAG is the established source, with great all- around micro welding capability; CW fiber lasers provide excellent speed/penetration characteristics and the ability to weld conductive and dissimilar materials; QCW fiber offers similar capability to the Nd:YAG laser, with additional small spot and penetration features; and  the nanosecond laser provides great control using sub-300 nanosecond pulses for thin materials and fine spot applications, as well as some dissimilar materials bonding.

The most interesting match-up is the Nd:YAG and the QCW fiber laser, which offer similar welding capabilities. Both have high peak power and millisecond pulse widths for large diameter (>200 micron) spot and seam welding. The Nd:YAG laser is an established technology that users are familiar with while the QCW laser is a relative newcomer with some strong cost of ownership features. The QCW laser does not use flashlamps, which reduces maintenance costs. The Nd:YAG laser is fully serviceable in the field. So the choice comes down to equipment preference, driven by whether the user requires a laser that is cheaper to run or one that can be maintained 24/7. FFJ

Sources

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MTS Sensors

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Prudential Stainless & Alloys
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Barton International
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Cosen Saws Omax Corp.
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MetalForming Inc. HE&M Saw American Weldquip
Beckwood Press Co.

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Savage Saws Strong Hand Tools
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