焊接是典型的"人机料法环"全占的特种工序——大多数缺陷当时看不见，等车开到客户手里才暴露。以汽车副车架的 MAG 气体保护焊为例（母材 Q235 低碳钢、板厚 3mm、角焊缝、80%Ar+20%CO2 保护气），把"起弧—熔池形成—凝固冷却"全过程常见的四种失效模式拆开讲，PFMEA 就知道该往哪使劲。
▌ 虚焊：外观正常，一撞就脱开
- 失效后果：焊缝外观正常但内部没熔透，承载截面不足，整车碰撞时焊缝脱开——安全法规项，一旦流出即批量召回；
- 失效原因：焊接电流偏低、焊速过快、焊枪角度不对导致电弧偏吹、坡口油污锈蚀未清理；最危险之处在于"看不见"——探伤抽不到的那件，可能就是虚的；
- 现行控制：焊接参数纳入机器人程序锁定、起弧前离子清洗除油、每班首件做撕裂试验。
▌ 气孔：保护气一失守，强度密封双失守
- 失效后果：焊缝内部或表面出现气孔，强度下降，密封件焊缝直接泄漏，外观件判废；
- 失效原因：保护气体流量不足或被风吹散、气体管路漏气、母材表面有油水、焊丝受潮——夏天风扇对着工位吹，气孔率能翻几倍，环境因素在焊接 PFMEA 里权重极高；
- 现行控制：气体流量每班点检、车间风速监控、焊丝领用即拆封、焊前目视确认坡口清洁。
▌ 裂纹：完工几小时后才出现的延迟杀手
- 失效后果：焊缝或热影响区开裂并扩展，属最严重失效，件件报废；疲劳寿命大幅下降，行驶中有断裂风险；
- 失效原因：拘束应力过大、结构设计焊缝密集交叉、冷却速度过快、材料碳当量偏高淬硬——延迟裂纹往往在焊完几小时后才出现，完工检查根本看不到；
- 现行控制：焊接顺序工艺固化、关键焊缝 24 小时后二次探伤、材料进厂复验碳当量。
▌ 咬边：一道沟槽，疲劳强度降两成起步
- 失效后果：焊缝边缘母材被熔掉一道沟，截面削弱，成为应力集中源头，疲劳强度降两成起步，外观判不合格；
- 失效原因：电流过大、电弧电压过高、焊速过慢、焊枪指向偏；机器人程序没随产品切换、调用错误，是批量咬边的常见根因；
- 现行控制：焊后 100% 目视加焊缝检验尺抽检、程序调用防错（扫描工件码绑定程序）、参数修改权限管控。
▌ 实操要点：原因往可控参数上挖
太多焊接 PFMEA 把失效原因写成"焊工技能不足"就交差——这不是原因，是借口。焊接的失效原因要往电流电压、保护气体、材料、应力这些可控制的参数上挖；每个失效模式至少配一条现行控制，措施写清"怎么检、检多少、发现后怎么办"。
记一句：PFMEA 的价值不在表格，在于把焊接车间的经验沉淀下来——虚焊怎么防、裂纹怎么控，写清楚，新员工少踩坑，同类问题不再重演。
想让 PFMEA 从"填表"变成车间的经验库？续创的 FMEA 失效分析与失效库系统，帮你把每条经验沉淀复用。欢迎联系续创信息技术咨询。
Welding is a classic special process involving man, machine, material, method and environment all at once — most defects are invisible at the time and only surface after the vehicle reaches the customer. Taking the MAG welding of an automotive subframe as an example (Q235 low-carbon steel, 3 mm sheet, fillet welds, 80% Ar + 20% CO2 shielding gas), let's break down the four common failure modes across "arc start – weld pool formation – solidification and cooling", so your PFMEA effort lands where it counts.
▌ Lack of Fusion: Looks Fine, Comes Apart in a Crash
- Effect: the weld looks normal but is not fused inside; the load-bearing section is insufficient and the weld can separate in a crash — a safety/regulatory item that means batch recall if it escapes;
- Causes: welding current too low, travel speed too high, wrong torch angle causing arc blow, oil and rust not cleaned from the groove; the real danger is that it is invisible — the piece your NDT sampling misses may be the cold one;
- Current controls: welding parameters locked in the robot program, ion cleaning and degreasing before arc start, and a tear test on the first piece of every shift.
▌ Porosity: When Shielding Gas Fails, Strength and Sealing Fail Together
- Effect: pores inside or on the surface of the weld reduce strength; welds on sealing parts leak outright, and appearance parts are scrapped;
- Causes: insufficient shielding gas flow or gas blown away by draughts, leaks in the gas line, oil or water on the base material, damp welding wire — in summer, a fan aimed at the workstation can multiply the porosity rate; environmental factors carry enormous weight in a welding PFMEA;
- Current controls: gas flow checked every shift, workshop air-speed monitoring, wire unsealed only on issue, and visual confirmation of groove cleanliness before welding.
▌ Cracks: The Delayed Killer That Appears Hours After Welding
- Effect: cracks in the weld or heat-affected zone propagate — the most severe failure, scrapping every affected piece; fatigue life drops sharply, with fracture risk in service;
- Causes: excessive restraint stress, dense intersecting welds in the design, cooling too fast, high carbon equivalent making the material hardenable — delayed cracks often appear hours after welding, invisible at final inspection;
- Current controls: fixed welding sequence in the process, a second NDT on critical welds 24 hours later, and carbon-equivalent re-verification of incoming material.
▌ Undercut: One Groove, Twenty Percent Off Fatigue Strength
- Effect: the base metal at the weld toe is melted away into a groove, weakening the section and creating a stress raiser — fatigue strength drops by twenty percent or more, and appearance is rejected;
- Causes: excessive current, arc voltage too high, travel speed too low, wrong torch direction; a robot program not switched with the product — calling the wrong program — is a common root cause of batch undercut;
- Current controls: 100% visual inspection after welding plus weld-gauge sampling, mistake-proofing of program calls (scanning the part code to bind the program), and access control on parameter changes.
▌ Practical Points: Dig Causes Down to Controllable Parameters
Too many welding PFMEAs write "insufficient welder skill" as the cause and call it done — that is not a cause, it is an excuse. Welding causes must be dug down to controllable parameters: current and voltage, shielding gas, material, stress. Give every failure mode at least one current control, and spell out "how to check, how many to check, and what to do when something is found".
One line to remember: the value of a PFMEA is not the form — it is the experience of the welding shop made permanent. Write down how to prevent lack of fusion and how to control cracks, and new hires step on fewer landmines while old problems stop repeating.
Want to turn PFMEA from form-filling into your shop's experience base? CINO's FMEA failure-analysis and failure-library system helps you capture and reuse every lesson. Contact CINO Information Technology Consulting.