Laser welding
Introduction.
This site is dedicated to industrial laser technologies, namely laser welding. Nowadays all manufacturers are familiar with laser cutting; many companies possess various types of laser cutters; laser marking is becoming more popular. Laser welding, however, is mainly utilized for welding subminiature components in instrument making industry. For this reason, it is important to point out that a lot of works on development of technologies and equipment for industrial laser welding were widely carried out all over the world. These works were brought up to the level of designing real technologies along with performing a large number of trial weld joints with certification purposes; in some cases these technologies went through industrial implementation. Today, when manufacturing enterprises are faced with the fact of necessity to employ laser welding technologies, they should know that they do not have to bootstrap this technology or look for it in other countries. There are highly skilled technologists and scientists in Canada specialized in various aspects of laser welding technology.
In this article, we will not touch upon the issue of spot impulse laser welding which has been widely used in Canadian instrument-making industry for a long time. We will discuss the subject of butt-seam laser welding of thick metals – from 1 mm to 10 mm.
Physics of the process and types of laser welding
The process of laser welding consists of dissolution (melting) of metal induced by highly concentrated light energy. Laser emission is focused upon the surface of metal in the area of junction of two work-pieces. The emission is partially absorbed by metal heating it up to the temperature of dissolution and boiling. Although initial absorbing capacity of metals and alloys is low, it increases as the temperature grows. As boiling temperature is reached, layer of molten metal can be ejected by the back pressure of metal vapor stream resulting in formation of a cavity and further development of gas-vapor duct. In such circumstances laser emission is almost fully absorbed and in terms of thermal physics, the heat source has the pattern of line heat source. If the focused beam of emission moves along the junction, melting zone forms and the surfaces are welded. Unfortunately, there is a physical phenomenon that considerably complicates the process: this is plasma cloud that forms upon the surface of metal. More easily ionized metal vapor starts absorbing laser emission that results in formation of plasma flame jet.
This jet may exert various impacts on the process: negative – by blocking transmission of a peace of beam energy onto the surface of metal and the melting canal or by scattering the beam through formation of a negative optical lens, or positive – due to indirect heating of metal surface at early stages when direct emission absorption is low.
In order to eliminate unfavorable effects of plasma flame jet, plasma-suppressing gas mixtures are used. In laser welding argon is used with these purposes since it can simultaneously protect the melted metal from oxidation. Since the speed of laser welding is quite high, it may be necessary to use gas protection in the tail piece as well as the other side of the weld. Pure argon can be used here too. When performed classically, laser welding needs neither adding materials nor fluxing agents. The welding process is easily controlled and is contact-free - as opposed to arc welding, there is no need to use specialized energy sources with drooping characteristic.
Classification of some processes of laser welding
It is important to imagine more or less complete spectrum of utilized modifications of the laser welding process by laser source type. The following main types of laser are developed for industrial use:
YAG – lasers are yttrium-aluminum garnet based solid-state lasers. They differ in pumping source (tube or diode). The lasers with diode pumping source are expensive and are not often used for welding purposes. The length of emission wave of these lasers is 1.06 micron – close infrared diapason. The emission power is up to 5-6 kilowatt. On the contrary, the lasers with tube pumping source are successfully utilized in production.
Optical fiber lasers. These are currently the newest types of laser with unique structure. The actuating medium in these lasers is quartz optical fiber alloyed with rear-earth metals and pumping is carried out with laser diodes. The same fiber is used to transfer the emission to the welding head which makes it extremely comfortable. The emission power is up to 5 kilowatt.
The main modes of laser welding:
а) Continuous emission welding – the laser emission power is either time-constant or it is impulse-type with impulse frequency approximately equal to tens of kHz
b) Impulse welding or pulse-periodic welding – in this case laser impulse frequency is low (10-300 Hz) and energy of each pulse is high.
Based on welding pattern, the following welding patterns can be mentioned: butt welding, lap welding, fillet welding, and other patterns of welding differing in reciprocal position of work-pieces and laser beam.
Quality and features of laser welding.
In terms of thermo physical and metallurgical processes, the main features of laser welding are significantly short melting and crystallization times as well as very local heat-affected zone. This leads to a special pattern of metallurgical metal transformation, namely to formation of various no equilibrium structures in the weld metal. At the same time, numerous tests and certifications demonstrated that laser welding is notable for very high technological flexibility and high quality of weld joints . The properties of the weld joints are not worse than those of the parent metal for the majority of constructional materials. Conducted trials show that destruction always occurs in the parent metal, not the weld.
Technological flexibility of laser welding permits to perform butt welding even with such metals and alloys as stainless steel, copper, titanium and titanium-based alloys.
The crucial step towards utilization of laser welding was made in 1996 when the united European project dedicated to possibilities of using laser welding in shipbuilding industry was successfully completed. The engineering materials of the project were passed on to the classification institutions of the member states that developed utilization guidelines for employment of laser welding in shipbuilding industry. This fact, on a large scale, gave the go-ahead signal for broad utilization of the new technology in shipbuilding industry. Nowadays laser welding is broadly used for welding certain conventional structural fragments, such as bulky honeycombs, at a number of shipyards of the United Kingdom, Germany and Japan.
Start of utilization of laser welding for aluminum alloys in automobile and aircraft building can be considered as the second technological breakthrough. The leader of auto builders uses laser to weld about 20 meters of weld seam in the body of vehicles. Air craft companies started to employ laser welding for connecting the stringers (longitudinal load-bearing elements) with encasing during construction of the lower part of the fuselage. The latter is especially remarkable: aluminum welding was almost not used in aviation because of poor statistics on its behavior and destruction of the weld seams created with the conventional welding methods. Laser welding method allows to gain low weight of joints and it considerably accelerates the process of construction as compared with clinching. Laser welding makes it possible to connect up to eight meters of stringer with encasing within one minute.
Let us look at the main features and advantages of laser welding:
High performance of the process, typical speed of welding can reach 200-400 m/hour.
Capability of welding a very broad spectrum of steels, alloys and materials – from high-alloy, high-carbon steels through copper and titanium alloys as well as various high-melting alloys.
Capability of welding dissimilar metals. Absence of welding consumables
Possibility of butt welding of rather thick sheets of metal with a single run.
Perfect properties of the weld joint metal and heat-affected zone; in majority of cases, the mechanical properties of the weld joint metal are not worse – and sometimes are even better – than those of parent metal. Small heat-affected zone and lower grade of deformities, approximately 3-5 times lower than with arc welding.
Possibility of welding in nooks and in different spatial positions
Good controllability and flexibility of the process, possibility of full automation
Ability of laser emission to travel to considerably long distances.
Environmental purity of the process defined by absence of fluxing agents and other welding consumables.
High quality of weld joints – as per the results of impact tests it becomes evident that tears (destructions) occur in the parent metal, not the weld. Tests also demonstrate high bend ductility of the welds.
Organization of the manufacturing process
It is important for production planners to know that the laser beam can travel to considerably long distances and the beam can switch to several workstations.
Due to diversity of welding geometries, high-power laser equipment for actual production is usually designed on the basis of individual orders, as opposed to laser cutters that generally have a typical pattern (design) and are produced in a serial manner. Nevertheless, there are several typical configurations for welding certain classes of items. It is necessary, however, to point out that overwhelming majority of production tasks can be performed using much simpler technology.
Laser Surfacing
Some tool production problems related to repairs and manufacturing of production accessories, can be successfully solved using laser technologies – laser welding, surfacing. Labor content of repairing while manufacturing and restoring life-expired molds and production accessories occupies a large part of tool production man-hours. Various defects emerging in the process of manufacturing and usage of production accessories can successfully be eliminated with pulse laser surfacing technology. Among such defects are burrs, deep scratches, nicks, gashes, interstice, void, cracks. Removal of such defects with traditional surfacing methods like, for example, stick electrodes is labor-consuming and expensive since geometric dimensions after surfacing and thermal post-processing can go beyond the acceptable limits. Laser surfacing technology allows eliminating this deficiency and maintaining geometric dimensions of the surfaced item within acceptable limits, even if these limits are a few microns. Hardness in the surfaced area (deposited-metal zone) remains the same as hardness of the parent material and further mechanical post-processing of the repaired defect zone is brought to minimum. It is important to point out that the time needed for elimination of a defect with laser surfacing varies from several seconds to tens of minutes depending on geometric dimensions of the defect and the laser beam energy. The technological process of laser surfacing is simultaneous feeding of the laser beam and filler wire to the area of defect.
The filler material melts and fills the defect. To prevent oxidation of the defect area, surfacing is carried out in argon settings. Mechanical post-processing required after laser surfacing is minimal as compared with mechanical treatment carried out after traditional stick electrode surfacing. High precision beam pointing and locality of laser beam action allow repairing of rigorously defined areas of defective items. Short duration of the process (duration of the impulse is several milliseconds) as well as precise energy dosing ensures minimal heat-affected zone and absence of shrinkages. Laser surfacing makes it possible to considerably decrease labor content and the costs of repairs due to absence of preheating, thermal post-processing, depleting chromium coating or coating with chromium, and owing to decreased volume of post-machining.
The substantial limitation of the existing laser surfacing technologies until (very) recently was exclusively straight line laser beam propagation. For this reason, repairing accessories of complex geometric configuration was often impossible as it was not possible to feed the laser beam to the defect area. To eliminate this deficiency a special fiber-optic system conveying the laser beam to the area of defect was developed. The length of the light conductor permits processing of bulky molds the geometric dimensions of which are several meters.
The following tasks can be performed with the developed impulse laser surfacing technology:
а) restoration of the edges of working surfaces of molds and die tooling in case of incidental under-sizing during mechanical post-processing;
b) restoration of floors of under-sized working surface;
c) repairing superficial cracks up to 3 mm deep;
d) repairing under cuttings formed after surfacing with stick electrodes;
e) surfacing of the sites at which Rockwell hardness test was performed;
f) surfacing burrs, chips, nicks, interstices, voids;
g) restoration of life-expired surfaces of die tooling and molds (e. g. sites of adhesive bonds);
h) welding templates and complex measuring clamps (staples) that underwent preliminary carburization.
Conclusion
We hope that this reasonably general overview will help manufacturers be guided in prospects of utilization of laser welding technologies. Please contact us for more detailed information or if you have questions and we will be happy to assist you