A Glimpse of Laser Welding in Additive Manufacturing.
Author: Ng Ci Xuan – R&D Intern
Editor: Bryan Ng – Marketing Manager
1. What is Additive Manufacturing?
Additive manufacturing, otherwise known as 3D printing or rapid prototyping, is the process of building a three-dimensional object from a computer-aided design (CAD) model. A layer-upon-layer of material is deposited, and this works regardless of material, shape, and size.
This procedure is a technological advancement that transforms the approach to industrial production by supporting the construction of lighter and stronger parts. Additive manufacturing can also be used to coat and repair parts with high material requirements.
There are no restrictions when it comes to design. Functions that are not practical using conventional manufacturing can be achieved with additive manufacturing.
Additive manufacturing also comes with multiple benefits in helping to shape a better outlook for the manufacturing industry.
Sustainability and customizability are also some advantages of additive manufacturing. The amount of material used to fabricate objects is very specific as the objects are produced precisely, thus amounting to little or no material wastage.
Additive manufacturing also creates products that are highly customizable and can be personalized to suit one’s needs with flexibility.
Thus, it is a promising technology that can construct complicated geometrical objects with less physical interference and high material efficiency.
2. Laser Welding in Additive Manufacturing
The process of additive manufacturing is also usually done by a system known as a laser welding system.
As the application of a heat source for the construction of metallic parts using additive manufacturing became increasingly common, laser welding proves to be the perfect tool for it. Raw materials either in powder or wire form are melted or sintered using a concentrated heat source to produce parts.
Powder systems are more mainstream than wire systems and are more commonly used within industries. The former also offers higher geometrical accuracy but lower deposition rates as compared to the latter.
Additive manufacturing using laser welding can be split into two technologies – laser metal deposition and laser metal fusion. Laser metal deposition works by depositing metal powder via a nozzle and using a laser to produce a weld pool on the surface, the mixture of both results in structures after cooling.
Whereas laser metal fusion builds a product up layer-by-layer in a powder bed as the laser melts the metal powder at positions specified by a CAD model.
3. Pros and Cons of Additive Manufacturing Using Laser Welding
The advantages of using laser welding equipment for additive manufacturing are that the demand for customized components is reduced and the system is also flexible and modularity in design. However, major issues that come with it are system control and automation work.
Till now, trying to deposit wire in a layer-upon-layer structure still poses an unstable and difficult-to-handle procedure. This calls for the control system to be more stable in maintaining the proper wire feed rate, proper laser intensity, and proper positioning of the head to the surface.
A stable flow of feedstock material to the surface is desired to ensure that the material melts smoothly and forms an even path when solidified.
So now that we have introduced how laser welding can be used for additive manufacturing, read on to find out more about what laser welding is and also the products we offer.
4. What is Laser Welding
Laser welding is the process of joining metal or thermoplastics together using a laser beam. The beam provides a concentrated heat source at certain points which causes a filler material to melt and fuse to the surface. A strong weld between the two sections is then formed after cooling.
As mentioned, common types of feedstock material include powder feeding and wire feeding.
Powder feeding uses a powdered metal alloy that is delivered directly from the processing head to the focal point of the beam path. The powder then liquefies at the position due to the high thermal energy and produces a small pool of molten material (filler).
When the filler is cooled, a joint is formed, and the excess powder is drawn away for reuse by a suction system that comes equipped with the machine.
Wire feeding works similarly, using metal alloy wire instead of powder. The wire is directed to the interaction point between the laser beam and the surface. Likewise, the wire melts from the high temperature and forms a joint.
Apart from the type of material used, the position in which it is placed is also crucial. There are two types of configuration for its positioning and that is trailing feed and leading feed.
Trailing feed refers to when the filler material is placed behind the laser beam where the molten pool is already fully developed while the leading feed is when the filler material is located in front of the laser beam and fed into the weld pool’s front edge.
Standard practice is feeding the material at the front as trailing feed causes incomplete mixing of the material with the already-developed pool.
The angle at which the material is delivered also plays a vital role in ensuring that the weld is successfully built. Normal practice would be to feed it at 45° from vertical however, angles between 30° and 60° can also be used.
Angles smaller than 30° will cause the material to overlap with a large section of the laser beam, which in turn causes the material to melt and vaporize without combining with the pool while angles above 60° make positioning the wire with the beam centreline difficult.
Thus, 45° helps to simplify the complications that may be involved.
4.1 Working Principle
Beams from the laser source are passed through the collimating lens, which outputs a parallel aligned beam. It then reaches the dichroic mirror, a contact point for two parts – the viewing port and collimated laser source.
The mirror features a thin film filter that reflects and/or transmits light depending on its wavelength.
In the case of the image, the mirror reflects light that is intended for output at the viewing port, while allowing transmission of the laser for its intended use at the working surface.
The focusing lens then concentrates the laser onto its focal length, which is at the same level as the working surface. The beam then melts the filler material onto the working surface and causes both to fuse.
The adjustment dials allow for subtle changes to the height and focus of the focusing lens, or to increase and decrease the collimation factor.
A connector helps to attach the laser to the welding head and multiple connector types such as Quartz Block Head (QBH), D80, LLK-B, and SMA905 are available in the market.
Debris and welding slag are common by-products of welding. To prevent these from entering the welding head, a glass window acts as a stop-gap between the optical parts of the welding head and the working surface. The window features a drawer-style design that allows for easy replacement.
A laser welding head also comprises several sensors that provide desired feedback to close the process control loop.
Temperature sensors help to monitor the temperature of the laser welding head and ensure that it does not exceed the working temperature range.
Laser power sensors provide the option to verify the laser output power at any time while the machine is in use and compare it with an acceptable value. This check can highlight any problems that exist within the system.
The use of a camera sensor will be further explained in the next section where we will highlight the parts used in a laser welding system.
A purging system, as well as a cooling system, also exists within the welding head. The former comes as a separate attachment that shoots air to the welding site, ensuring that no impurities are mixed with the surface while the latter helps to cool the system down.
Certain metals and alloys are sensitive to the presence of gases and vapors. The combination of both may result in unfavorable compounds that may reduce the quality of the weld and thus, it is important to have a proper purging system.
Lastly, water cooling is used to cool the system down as it may heat up relatively fast due to the welding process being conducted at high temperatures.
5. Parts Used in a Laser Welding System
The main components of a laser welding system include the laser, a motorized guide, and an imaging system.
5.1 Laser Source
A variety of lasers can be used in a welding system, but the more common ones are – gas lasers, solid-state lasers, and fiber lasers.
Gas lasers utilize a mixture of gases such as helium (He), nitrogen (N), and carbon dioxide (CO2) as their lasing medium. These lasers stimulate the gas mixture by supplying energy from high-voltage and low-current power sources. They can also operate in both pulsed and continuous mode.
Solid-state lasers use solid media in a host material as its lasing medium. The more common solid media used in solid-state lasers that are suitable for laser welding are synthetic ruby crystal (chromium, Cr, in aluminum oxide, Al2O3), neodymium in glass (Nd:glass) and the most popular one, neodymium in yttrium aluminum garnet (Nd:YAG). The first two types can only operate in pulsed mode, however, Nd:YAG can work in both pulsed and continuous mode.
The lasing medium used in a fiber laser is the optical fiber itself and it is doped with rare-earth elements. The light is produced in the optical fiber and guided to the surface by a flexible delivery fiber, known as a ‘light guide’.
Fiber lasers are becoming increasingly popular in laser welding as it provides benefits that gas and solid-state lasers are not able to. CO2 lasers have limited accuracy and also produce undesirable heat that is too high into the weld while Nd:YAG lasers do not have the most optimal welding speed, spot size, and electrical energy consumption. Fiber lasers, on the other hand, can satisfy these elements and on top of its flexibility, is what makes it a better choice.
5.2 Motorized Guide
The motorized guide combines the laser head with the computers for the welding process through computer-aided manufacturing (CAM) system based on computer-aided design (CAD). Although laser welding can be done manually, most systems are now automated for increased efficiency.
5.3 Imaging System
Most laser welding head comes equipped with imaging devices, such as a CCD camera and CCTV lens. This can be attached to the viewport to allow a camera to view the same optical path as that covered by the laser. The camera then provides monitoring and inspection of the welding effects in real-time. We also offer a variety of high-speed cameras.
6. Types of Laser Welding
Laser welding can be done in two ways – conduction and keyhole welding.
6.1 Conduction Welding
In this method, the laser beam has a power density that can heat the surface of the material but not to the extent that it vaporizes and penetrates it. Thus, conduction welding often displays a high width-to-depth ratio.
6.2 Keyhole Welding
This type of weld is usually formed by a welding head known as single spot welding head and using a technology known as laser spot welding, which welds at a single point to create a single weld spot using a laser.
The laser beam in keyhole welding typically has higher power densities as it needs to be focused to a spot small enough to cause the surface of the material to not only melt but also vaporize.
The beam then penetrates the material, forming a void known as a ‘keyhole’. The hole is sealed by the molten material trailing behind the laser, which creates a small spot weld.
This method also produces deep and narrow welds, where the power of the laser is proportional to the depth of the weld produced. Thus, resulting in welds with a high depth-to-width ratio.
Another type of welding head is known as an area scan welding head which welds at the area where one wishes to work on the desired workpiece.
It makes use of a galvo scan head with mirrors positioned in it, and a scan lens (usually f-theta scan lens) to deflect and project the laser beam to the desired area.
7. Laser Welding Applications
Laser welding is often used in a variety of applications and industries. It ranges from the jewelry industry to the automotive industry, from fixing pressure vessels to railroad equipment.
7.1 Automotive Industry
In the automotive industry, laser welding allows manufacturers to weld modules such as solenoids, engine parts, fuel injectors, transmission parts, air-conditioning equipment as well as many other products.
Its capability to weld parts with limited heat and insignificant distortion makes laser welding a popular tool.
7.2 Jewelry Industry
Laser welding is frequently used in many areas such as to re-tip gold or platinum prongs without the need to remove diamonds from its position, repair costume jewelry without having to remove and reattach the stones on it, repair stainless steel watch bands without changing parts that had come off, amend manufacturing defects and many more.
7.3 Photonics Industry
The photonics industry benefits from laser welding for the packaging of photonic devices, such as laser diodes, solar and photovoltaic cells, and light-emitting diodes, and it uses Nd:YAG laser source.
These devices are commonly used in telecommunications, which requires them to have a long operational lifespan under unfavorable environmental conditions.
Hence, the photonic devices that are encased inside metal hybrid housings require strong joints and hermetic sealings which can be achieved using laser welding.
7.4 Electronic Industry
The technique of laser seam and spot welding is a popular choice as its precision enables tiny spots and narrow seams in miniature electrical components to be adjoined. Common applications include industrial assemblies that require high accuracy and hermetic seals that are sensitive to pressure.
7.5 Medical Industry
And lastly, laser welding is commonly used for the production of medical devices and it often uses a fiber laser. Medical aids are usually made of several metals that are welded together.
These metals may have different properties which makes it a challenge to join, but fiber lasers have the ability to ensure that a strong weld joint is formed. Some devices that have been invented include defibrillators, orthodontic appliances, catheters, pacemakers, hearing aids, prosthetics, and surgical tools.
Single spot welding head is mainly used in the jewelry industry to repair pieces made of gold and silver, dental industry to repair dentures, and also in plastic welding.
Area scan welding head is used in the electronics industry for the construction of mobile phones and other electrical metal pieces, and electronic components, and in the medical industry for the production of medical devices, plastic, and instrumentation.
8. Buying a Reliable Laser Welding Head
Now that you know what a laser welding head is and its applications, you should know where to buy a quality one. Of course, you can purchase quality laser welding heads from us.
Wavelength Opto-Electronic offers welding heads in several designs that can be used for a variety of applications. Products can also be customized to suit your technical needs.