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As the medical industry continues to demand smaller and more intricate components and devices, so the challenge for manufacturing them increases.
Fine wire welding is used in the manufacture of a wide range of medical devices ranging from electrical connections to implantable medical devices; vascular stents and guide wires used in minimally invasive medical applications to position devices such as catheters and stents. This application insight looks in detail at butt welding of titanium, stainless steel, stents and guidewires, as well as lap and spot welding of stainless steel and titanium wires.
Fine wire geometries are typically in the range 100-300µm in diameter and include materials such as stainless steel, cobalt chrome, platinum chrome, nitinol, titanium, molybdenum, tungsten, magnesium and platinum. Applications examples include bonding parts of both identical and dissimilar geometry and material composition.
It is worth noting that when different materials are being welded the different melting points and thermal conductivities often require careful selection of the laser processing parameters and a very high level of process control in order to achieve a successful joint.
Laser welding of fine wires can be achieved in a number of ways including butt welds (when wanting to weld two wires together) and lap welds (often used to join adjacent wires). Spot welding can be also used when joining a wire to another part or component.
Examples of each of these configurations are discussed below:
Stents are generally made by either micro cutting a mesh structure from a solid metal tube or by welding together preformed sections of wire to form a composite wire mesh structure. The materials used are typically either stainless steel, or cobalt alloys for permanent stents though more recently there has also been interest in biodegradable materials such as magnesium.
One method of making a welded wire stent is shown schematically in figure (1a) below and a completed stent in figure (1b).
The preformed wire “crowns“ must be accurately positioned and pushed together before welding with a precisely controlled Laser pulse. Typical power settings are in the range 40W – 50W with pulse durations in the range 5-10mS.
Fig. 1(a) Wire welded stent construction. Fig 1(b) Example of wire welded stent.
A guidewire is essentially a long (up to several meters), straight and flexible spring and is typically made of stainless steel or nitinol with diameters ranging from 0.25 mm to several millimeters.
Wires are widely used to guide catheters or stents into place. The tips are generally machined to a special tapered profile and a surrounding coil is welded into place to enhance flexibility. The tip coil often contains at least one section of platinum alloy wire to improve visibility in fluoroscopy.
For butt welding of wires there is often a requirement to minimise joint diameter variation. This requires careful preparation of the wire ends prior to welding, the fixturing of the wires and the selection of Laser processing parameters. A cover or shield gas is sometimes required to prevent weld contamination through oxidation. A good example is the joining of titanium wires (Figure 2) – it can be seen that there is no appreciable increase in the diameter at the weld point and due to proper shielding, the weld is bright.
Figure 2: Butt welding of titanium and stainless steel wires. 20W, single 1ms pulse.
Joining dissimilar materials can also be achieved such as in figure 3 which shows a kovar pin joined to a tungsten/rhenium wire.
Figure 3: Welding tungsten wire onto kovar pin. 30W, single 20ms pulse. Figure 4: Fine guage coil welding steel wires 20W, single 5ms pulse.
The degree of process control achievable using the Fiber Laser is demonstrated using the same Laser to control the end shape fine wires. Careful control of pulse energy and good beam positioning produce highly controlled melting and separation of the wire such that surface tension produces very well rounded wire ends.
For some surgical applications where wires are required to be inserted into the body it is important that the cut ends do not have any sharp edges that could cause tissue damage.
As illustrated in Figure 5 below, careful control of the heat input can control the size of the ball such that it is not significantly larger than the wire diameter. This technique can also be used as a pre treatment for wires to be welded where additional material is required to form the weld.
Figure 5. Controlled melting of 0.33mm platinum wire.
In some instances wires can be welded together in a lap configuration. Again, the fixturing is critical when achieving a good contact between two parts. This type of weld can be achieved as a spot weld, or a series of spots generating a lap weld.
Figure 6. Lap/Spot welding of titanium wire onto titanium rod. 30W, single 1.2ms pulse.
For joining wires to other components, which can act as significant heat sinks, spot welding can be used.
Figure 6. Lap/Spot welding of titanium wire onto titanium rod. 30W, single 1.2ms pulse. Figure 7. Spot welding of stainless steel wire. 50W Single pulse of 10 ms.
The joining of wires is a highly skilled and demanding material processing application that is made easier with the high precision and process control capabilities offered by the precision beam delivery and output power control offered by the Fiber Laser.
For further information on the specific features of our range of Fiber Lasers for increased process control in medical device welding refer to the additional applications insights on the website, or talk to your sales representative.
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