In hermetic and seam sealing applications utilizing pulsed laser welding, it is critically important that weld spots are evenly spaced, overlapping to form a continuous welded seam. Traditional pulsed laser welding approaches attempt to do this by firing the laser at a constant repetition rate. While this can be made to work along straight lines or other paths that can be traversed at constant speed, the result is sub-optimal and the approach really falls short when welding along irregular contours. For example, it does not work well for hermetically sealed packages, such as implantable medical devices, aerospace sensors or electronics modules.
A better alternative is to use what we call “position-based firing,” where the laser fires at particular positions on the seam – placing weld spots in predictable locations regardless of the speed of the laser beam along the weld path. This approach improves both the weld quality and process efficiency of seam sealing laser welding processes. Here’s how it works.
First a little more background on the challenges of traditional laser welding
It all has to do with changes in the speed of the laser beam along the weld contour on the part. Even with straight line or smooth curve paths, constant repetition rate laser firing leads to unequal spacing of weld spots due to finite acceleration and deceleration rates of the motion system components that move the laser beam along the weld contour. Laser pulses that occur at a constant rate produce weld spots that are correctly spaced only when the relative motion of the laser beam with respect to the part is at its target speed. The spots are spaced more closely together during acceleration and deceleration of the motion system components, because the speed of the laser beam along the weld contour is less than the target speed. The close spacing of weld spots can lead to overheating in these areas because more heat energy is applied to a smaller segment of the weld path. And this overheating can affect weld quality or even damage internal components in the device being welded.
It may be possible to strike a balance between firing rate and part speed to create a weld that is “close enough” when welding straight lines or simple curves. With other shapes, striking this balance is much more difficult. To stay in focus, the laser focus head must constantly be at a fixed distance from the part that is being welded. Also, in many cases the laser beam must strike the surface of the part at a constant angle of incidence. These constraints can put a serious burden on the motion system components by requiring individual components (motion stages) to accelerate or decelerate at the limits of their capabilities. For example, even in the simple case of seam welding the edge of a rectangular package with rounded corners, one or more stages must attempt “instant” change of direction at certain critical points while maintaining constant speed. Physical stages cannot do this.
The traditional solution is for the multi-axis motion system to slow down automatically at these points, and then speed up along more benign segments of the weld contour. Unfortunately, a laser firing at a constant repetition rate causes the weld spots to bunch together at these locations, just as they do during other periods of acceleration and deceleration.
In some cases, operators choose to slow the entire process down to avoid the negative consequences of variations in speed of the laser beam along the weld contour, but this can greatly extend the overall welding time and reduce manufacturing efficiency.
Position-based firing to the rescue
Experts at AMADA MIYACHI AMERICA are always up for developing innovative solutions to real-world welding challenges. We've developed a new position-based firing approach to eliminate the issues faced with constant repetition rate laser firing.
Instead of simply setting a constant rate at which the laser fires, the laser is commanded to place weld spots at pre-determined positions along the weld path. In other words, the laser fires in predictable locations regardless of speed of the laser beam along the weld contour; there is therefore no need to coordinate the laser firing rate with the speed of the laser beam along the weld contour to balance efficiency and quality.
Whereas constant repetition rate firing requires that the entire process be slowed down to minimize overheating, position-based firing allows the laser welding process to occur rapidly along contour segments that can be traversed at constant speed, and only slow down where necessary to accommodate motion stage limitations – all the while delivering consistent spot-to-spot spacing.
Position-based firing is especially valuable in situations where the operator wants to speed up or slow down the entire welding process. If they choose to do so, operators simply change the motion system feed-rate to ensure that the laser continues to fire at each appropriate location. By contrast, if operators used constant repetition rate firing, they would have to coordinate the laser firing rate with the new speed of the laser beam along the weld contour, compromising as best they can at the critical points where one or more motion system components must accelerate or decelerate at the limits of its capability. Compromised laser-motion coordination yields compromised weld quality results.
Key benefits of position-based firing are: 1) The technique works with any arrangement of motion stages, any combination of rotary or linear stages, and in full 3D space; and 2) No special programming is required for different weld paths; just set the desired spot to spot spacing at the beginning of the NC program and position-based firing does the rest, regardless of the programmed weld path or machine feed-rate.
Transitioning laser welding systems used for hermetic and seam sealing applications from a traditional approach to a position-based firing approach increases overall efficiency and weld quality, and improves final product quality.