白皮书
A Medical Device Challenge: Welding Thin Wires of Dissimilar Metals
概述
Stainless steel or nitinol are popular materials in medical devices. Application experts from Coherent have developed a process to safely weld thin wires made of such difficult-to-weld materials. This process can even be undertaken without destroying any plastic coating
Micro-welding for medical devices
Laser welding is widely used in manufacturing medical devices like implants or endoscopic devices, often to join delicate parts. This becomes challenging if the joining partners are very thin wires made of difficult-to-weld materials. Typical materials are stainless steel, nitinol, or titanium because they are biocompatible, corrosion resistant, and easily sterilized. The challenge becomes even more difficult if a connection of dissimilar materials is intended. The laser is the preferred (if not the only) tool for this task. Welding such materials must not compromise their desirable physical properties, e.g., memory properties of nitinol.
When joining thin wires, a zero gap is required; they must be placed precisely against each other, since no filler is used and even a small gap can be too large for a reliable connection. Accordingly, the laser beam has to be precisely focused, too. While the first problem is solved with a fixture, the latter task requires the alignment of the laser focus position by means of an automated vision system.
图1：线材焊接装置 ：焊接工件固定在管子上。工艺气体通过8毫米黄铜管引入。
使用ExactWeld 430进行焊接
Welding thin wires with diameters as small as 25 µm (0.001”) requires a precise application of energy into the welding spot. The small pulse energy must be delivered in a controlled and reproducible way. The duration and temporal shape of such a laser pulse plays a crucial role.
The ExactWeld 430 machine from Coherent is a turnkey solution for precision laser welding that meets all these requirements. The workstation contains an integrated fiber laser source (Coherent StarFiber). Typical laser configurations for the ExactWeld range between 100 W and 600 W of output power. With a reproducible pulse modulation width, millijoule-energy laser pulses can be created. The laser beam is delivered by a galvoscanner or a fixed welding optics system. The workstation also includes a system for the control and supply of process gases. Just as important, the Class 1 laser housing complies with the latest ergonomic guidelines.
在处理尺寸如此微小的零件时，视觉系统和照明起着至关重要的作用。当视觉系统经过正确优化后，便能实现自动化加工——例如，当焊缝间隙过大时，间隙控制系统会自动停止焊接过程。
加工前，电线需放置于专用夹具上。该夹具在焊接过程中对保持电线精确定位至关重要。工作站提供多种机器人部件夹持方案，支持多达四个自由度（x、y、z轴及旋转）。 系统由用户友好型软件控制，并具备工厂自动化能力（支持工业4.0/物联网），可提升小型金属零件焊接的质量、良品率及生产效率。所有加工零件均可记录以实现可追溯性：数据可能包含零件编号、时间戳及特定图像。
当视觉系统经过正确优化后，即可实现自动化处理。例如，当焊缝间隙过大时，间隙控制功能会自动停止焊接过程。
Laser welding of 25 µm nitinol and stainless-steel wires
In a recent study in the Coherent application labs, the ExactWeld 430 was used to join thin wires made of nitinol and stainless steel. Several material combinations were chosen:
1) 镍钛合金/镍钛合金
2) 316L不锈钢/316L不锈钢
3) 316L不锈钢/镍钛合金
两根不同规格的导线被固定成线圈状，缠绕在衬管上，并由聚四氟乙烯（PTFE）套管保护。关键目标是在连接导线时避免损伤PTFE套管。焊接对象的直径相等，测试中使用了直径为25.4微米的导线以及部分更大直径的导线。所有情况下均成功完成焊接任务，且未穿透PTFE套管。
部分关键工艺细节如下：激光器采用2倍光束扩展器，焦距为100毫米。工作距离为0.5毫米，理论光斑直径为12.5微米。对于25.4微米的直径，焊接激光功率根据材料组合在30瓦至50瓦之间变化。 脉冲持续时间调整在0.035至0.05毫秒之间，对应焊接脉冲能量为1毫焦耳，切除凸出线材时则为2.5毫焦耳。 所有实验均采用氩气作为工艺气体，流量为15 l/min，黄铜管直径8 mm，BAK 4喷嘴距工件1.4 mm（见图2）。
For welding thicker wires the pulse duration was increased while the actual laser power was kept at the same level as it was for welding thin wires. After welding the wires, the obstructing wire ends were cut off with a series of stronger pulses.
图2：镍钛合金与镍钛合金焊接 （左）及镍钛合金与不锈钢焊接（右）。
研究结果
应用试验表明，100瓦StarFiber激光器足以焊接不同线材组合。所有类型线材的工艺窗口均较为狭窄：峰值功率需低于10瓦，脉冲持续时间需小于10微秒。 更高参数会导致电线断裂，而更低参数则无法实现连接。所采用的脉冲波形有助于工艺稳定性。关键要求是电线必须接触且间隙不可桥接。电线切割还存在将线端与相邻电线焊接的风险，因此定位精度至关重要。
| 峰值功率 [W] | 脉冲持续时间 [毫秒] | 重复频率 [Hz] | 速度 [毫米/分钟] | 备注 |
| 30 | 0.035 | 30 | 20 | 焊接 |
| 40 | 0.05 | 30 | - | 切割 |
Turnkey optimized solutions
The ExactWeld workstations are factory customized by applications experts from Coherent according to the desired applications. This customization includes not only the lasers, process control, and automated handling systems but also special fixtures, if necessary. Customers thus gain tremendous value from the collective knowledge and experience of the application engineers.
In this way, ExactWeld can be optimized for a wide variety of weld types and geometries. For contact welding, minimum welding spots with diameters down to 10 µm can be realized. For seam welding, small seam geometries can be achieved at high welding speeds. Free-form welding can also be programed. Thanks to the rotary stage, seal welding can be performed with no porosities and crack-free seams to provide for leakproof welding. For all systems, worldwide local service and spare parts supply are always readily available.
客户因此从应用工程师的集体知识和经验中获得了巨大价值。
"近期在可靠性、性能和易用性方面取得的实际改进，有望使其在经济上更具吸引力。"
Coherent is a vertically integrated manufacturer unmatched in our ability to also offer superior products at several different levels of integration. For example, the new ExactWeld 430 combines the industry-leading ExactSeries workstation platform, already proven in many applications, with a state-of-the-art Coherent StarFiber 100 to 600 laser sub-system. For traceability, every part can be logged with part and process details.
Figure 3: The ExactWeld 430 from Coherent is a class 1 laser device for automated welding tasks.
摘要
In conclusion, one of the most challenging welding applications in medical device manufacturing – the welding of wires as thin as 25.4 µm (0.001”) from nitinol and stainless steel – has been successfully accomplished. A special material such as nitinol requires a small process window; the pulse power, duration, and temporal shape must be generated reliably and repeatably. The small dimensions also demand precise positioning of the workpiece and the focused laser. The Coherent ExactWeld 430 has already established itself in a variety of other demanding welding applications in the medical device industry. It is now proved able to deliver the best solution for this difficult wire welding task. Recent practical improvements in terms of reliability, performance, and ease of use, promise to make it more economically attractive as well.