The latest technologies are designed to better protect the welding zone from oxidation. AMS takes a look
As well as protecting the molten metal and heat-affected areas of the workpiece against the ambient atmosphere, the right welding process gas can increase the welding speed and/or improve the mechanical properties of the weld. As a consequence, selecting the right process gas is crucial and technology suppliers are working hard to develop gases which offer genuine competitive advantages.
The Laserline range of high-purity gas products from Linde, for example, is designed to provide protection from the atmosphere, thus preventing any variation in the chemical properties of the parent material and enhancing the weld’s metallurgical properties. Other important effects of shielding gas are the control of plasma formation and even the direction that the melt flows, which both influence welding process efficiency.
As part of the wider range, Linde’s Lasgon series is an alternative to pure helium for laser welding. Among recent additions to its portfolio are dedicated mixtures for the laser welding of non-alloyed and low-alloy materials, plus aluminium and aluminium alloys.
Tackling higher aluminium contentIndeed, the growth of aluminium content in the most recent vehicle designs is driving the development of many laser-welding protection technologies – and it’s not all about gas. For instance, the latest Laserdyne SmartShield welding nozzle protects against excessive oxidation in the weld area on aluminium alloys. The nozzle is suitable for use on the three- to seven-axis 795, 430BD and 430 Vera laser processing systems.
The patent-pending nozzle provides a high-velocity gas barrier that prevents metal sparks from the weld zone contaminating the protective lens cover slide. Here, the cross-jet does not contaminate or otherwise interfere with the welding shield gas. It is designed to be used with an entire range of shield-gas delivery devices, including welding shoe and coaxial gas nozzle tips.
The shielding-gas shoe provides a controlled atmosphere for the weld zone both when molten and while cooling to a temperature at which it will no longer be compromised by the ambient atmosphere. In order to assist supply-chain manufacturers, the focusing lens/shield gas assemblies for laser welding can be changed quickly to vary the focused spot size or to switch from welding to cutting or drilling. Published data shows that guarding against oxygen in the atmosphere during the laser welding of materials, which are highly sensitive to oxidation, achieves positive results.
Welding in a state of fluxOf course, one of the major joining applications in the automotive industry concerns exhausts, which are manufactured from an entirely different type of material from those found in body assemblies, namely 409 stainless steel. This is preferred to 304 stainless steel due to its low nickel content (0.5%); the sulphur content in exhaust gas is known to attack nickel.
However, all stainless steels which contain chromium, such as 409 (approximately 12%), require protection from oxidation prior to welding. This is because chromium oxidises to chromium oxide (Cr2O3), a tenacious and refractory oxide that tends to remain on the joining surface and interferes with the welding process.
According to Yehuda Baskin, president of Superior Flux & Mfg Co, and Cary Long, product application specialist at Nordson EFD, who produced a joint study on the subject, there are two principal ways to protect stainless steel exhausts from oxidation prior to welding.
The first is to use an enclosure and blanket the welding area with argon gas. This inert gas does not remove the original oxide coating, but it does help to suppress oxidation in the weld box. However, given the dynamic situation inherent in the exhaust manufacturing process, it is almost impossible to prevent some oxygen entry in the weld box.
The second is to use flux in liquid or paste form, which dissolves and removes the original oxides, prevents re-oxidation and transfers weld heat to the seam. Liquid- rather than paste-based fluxes are preferred as they can be applied using high-pressure spray guns on automated lines, reducing labour costs.
Fluxes are essential when joining metals in ambient air, whether by soldering, brazing or welding. When heated, fluxes perform four vital tasks: dissolve or react with surface oxides; protect the cleaned surfaces against re-oxidation; transfer heat from the heat source to the joint; and remove surface oxides, allowing the clean metal surfaces to join.
Reducing weld-seam splitsIn short, better, stronger welds are one of the primary benefits of flux in exhaust manufacturing using tube-mill technology. Removing the centreline oxides results in a weld with fewer voids, fewer micro-cracks and greater consistency. This is reflected in a sharp reduction of seam splits. Furthermore, when using flux, the mill is more forgiving in terms of line speed, heat and pressure. According to Yaskin and Long, reductions of up to 40% in tube reject rates have been reported.
Another factor is that, without flux, many operators use unnecessarily high welding temperatures to produce a good weld. This can cause a bulge at the seam that must be removed by scarfing. A welding flux allows operators to keep the heat at a normal level, which reduces scarfing and scarf tool consumption.
There are also potential energy-cost benefits. This is because applying flux results in cleaner joining surfaces which need less current. In some cases, when flux has been added to the process, operators have been able to increase the line speed, boosting throughput, without increasing the welding current.
Nordson EFD has been involved in exhaust manufacturing for many years, from marking bad spots on the tube to applying anti-rust lubrication to the tube exterior. Initially, attempts were made to spray paste-type flux, but to no avail. However, things changed when Superior Flux developed flux in liquid form.
Liquid flux looks and acts like water and, from an application viewpoint, processes well. When flux is not being applied – typically by accident – operators are quick to identify poor welds. The Nordson EFD 787MS MicroSpray system uses a low-volume, low-pressure (LVLP) technology to administer the fluid to each edge of the tube prior to welding. With the capability to hit a target area as small as 3.3mm, the operator has complete control over the amount of flux being applied.
Mounting of the system is simple, using an adjustable frame of half-inch rod stock that can be adjusted up and down to accommodate tubes of different size. The best mounting place for the valves is after the last set of rollers, but in some instances it has been necessary to mount the spray system a few roller sets upstream. This seems to have no impact on the effectiveness of the weld but on occasion there may be a build-up on the roller sets which requires cleaning. Set-up entails turning off the nozzle air and adjusting the drip rate for each of the valves to about one drop per second. After setting, the nozzle air is adjusted to achieve the correct spray.
Experiments have been conducted using different formulations of flux and application methods. It was found that using a liquid flux with a viscosity of 20cps in combination with the MicroSpray valve system featuring LVLP technology produced the best results, not just in terms of the most uniform, accurate and consistent spray pattern, but with regard to increased throughput and reduced costs of the tube-welding operation.
T.H.E. Machines has developed its SLT series of machines to suit the protection gas-free laser welding of thin-walled metal sheets for stamping. Welding is conducted in fully automated process steps. Depending on the materials and sheet thicknesses, pre-programmed welding parameters help to ensure repeatable quality. Typical tensile strength and elongation at the seam are generally 85% or more of the base material, says the company.
Solid-state lasers like Trumpf TruDisk do not need shielding gasThe machine’s fibre-laser source has been adapted to the process requirements of 0.5-1kW units, while on-board cooling for the laser and fully integrated production and process controls help to ensure that the equipment can be used inline as part of a 24/7 operation, offline as a standalone unit, or in a mobile execution. Furthermore, an integrated optical seam-tracking system helps to produce high process speeds; the quality of the entire seam can be checked using top and bottom cameras.
Solid-state laser welding offers the same protection gas-free benefit as fibre, which – considering the cost of helium – can amount to significant savings; some CO2 systems blow around 12-15 litres of helium onto the workpiece every minute in order to suppress metal-vapour plasma. Since solid-state lasers, such as the Trumpf TruDisk series, do not create plasma, there is no requirement for shielding gas.
