--- title: "Joining processes; welded vs mechanical joints" subject: "Manufacturing and Production Processes" chapter: 12 key: "semester_4/manufacturing_and_production_processes:joining-processes-welded-vs-mechanical-joints" source: https://www.bctengineer.com/q/semester_4/manufacturing_and_production_processes/joining-processes-welded-vs-mechanical-joints site: BCT Engineering --- # Joining processes; welded vs mechanical joints Manufacturing and Production Processes · Chapter 12: Material Joining Processes - Asked in: year not given, 5 marks - Video: [6 Most Common Metal Joining Methods](https://www.youtube.com/watch?v=RtIKRrmHcPg) — Voelabs - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/manufacturing_and_production_processes/joining-processes-welded-vs-mechanical-joints - Whole chapter: https://www.bctengineer.com/md/semester_4/manufacturing_and_production_processes/chapter-12 ## Question Classify material joining processes. Compare welded joints with mechanical (riveted/bolted) connections and state when brazing and soldering are preferred over welding. ## Short answer Joining classified as welding, brazing/soldering, adhesive bonding and mechanical fastening. Welded joints have higher efficiency (up to 100%), lighter weight and leak tightness, but are permanent and need skilled welding; bolted joints allow disassembly and simpler inspection. Brazing and soldering suit thin, heat-sensitive, dissimilar or small parts and electrical joints. ## Answer **Joining** is the bringing together of two or more parts to make a single assembly, either permanently or so that it can be taken apart. #### Classification | Class | Principle | Processes | |---|---|---| | Welding | Parent metals melted (with or without filler) or forced together to form an atomic bond | Fusion: arc, gas, laser; solid state: friction, resistance, forge | | Brazing and soldering | Filler metal melts, parent metal does not; filler flows by capillarity | Brazing (above 450 degree C); soldering (below 450 degree C) | | Adhesive bonding | Polymer adhesive bonds surfaces | Epoxy, cyanoacrylate | | Mechanical fastening | Parts held by fasteners or interference | Bolts, rivets, screws, press fits, seaming | #### Welded and mechanical connections | Point | Welded joint | Riveted / bolted joint | |---|---|---| | Joint efficiency | High, up to 100 % of parent metal | 60-80 % (holes weaken plate) | | Weight | Lighter; no plates or heads | Heavier; cover plates, fasteners | | Leak tightness | Good | Needs caulking, sealing | | Disassembly | Not possible (permanent) | Possible (bolts) | | Cost and speed | Lower labour, faster | More labour, drilling/punching | | Residual stress, distortion | Present, HAZ changes properties | None | | Skill, inspection | Skilled welder; NDT needed | Simple inspection | | Fatigue | Notch at weld toe | Stress concentration at holes | #### Brazing and soldering They are preferred when parts are thin or heat-sensitive, when dissimilar metals (steel to copper, carbide tip to steel shank) or very small assemblies are joined, when low distortion and a neat appearance are needed, and for electrical joints (solder). Welding is preferred where high strength and heavy sections are required. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What distinguishes brazing and soldering from welding? A. The parent metal melts but the filler does not B. Both parent metal and filler melt completely C. Only the filler melts and flows by capillarity; the parent metal does not melt D. Pressure alone bonds the surfaces with no filler Answer: C. In brazing and soldering the filler melts and is drawn into the joint by capillary action while the parent metal stays solid; brazing is above 450 degree C, soldering below. (Key point: Brazing and soldering principle) 2. Why is the efficiency of riveted or bolted joints lower than that of welded joints? A. Welds are always thicker than the plate B. Holes weaken the plate, giving 60-80 % efficiency versus up to 100 % for welds C. Bolts melt the parent metal D. Welded joints contain no residual stress Answer: B. Drilled or punched holes reduce plate strength, so mechanical joints reach only 60-80 % while a good weld can approach 100 %. (Key point: Joint efficiency) 3. Which is an advantage of a bolted joint over a welded joint? A. Lighter weight B. Better leak tightness without sealing C. It can be disassembled and has no residual stress or HAZ D. Faster with lower labour Answer: C. Bolts can be removed and cause no distortion or HAZ; welds are lighter, leak-tight and cheaper in labour but permanent. (Key point: Welded vs mechanical joints) 4. In which case are brazing or soldering preferred over welding? A. Heavy sections needing very high strength B. Thin or heat-sensitive parts and dissimilar metals such as a carbide tip on a steel shank C. Joining thick steel plates for a pressure vessel D. When the joint must be 100 % as strong as the parent metal Answer: B. Low heat suits thin, heat-sensitive or dissimilar-metal assemblies with low distortion; welding is preferred for high strength and heavy sections. (Key point: When to prefer brazing/soldering) ## Related in Manufacturing and Production Processes - [Gas flames, arc welding and heat input](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Aoxyacetylene-flames-smaw-heat-input-numerical) — Manufacturing and Production Processes, chapter 12 - [Classification of manufacturing processes; evolution](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Aclassify-manufacturing-processes-evolution) — Manufacturing and Production Processes, chapter 1 - [Weld metallurgy, HAZ and heat sources](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Aweld-metallurgy-haz-defects-energy-sources) — Manufacturing and Production Processes, chapter 12 ## Related in other subjects - [Temporary and permanent joints; soldering, brazing, welding](https://www.bctengineer.com/question.md?key=semester_2%2Fworkshop_technology%3Ajoints-soldering-brazing-welding-compare) — Workshop Technology, chapter 10 - [Types of failures on riveted joints](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Ariveted-joint-failure-types) — Design of Steel and Timber Structure, chapter 4 - [Bolted joints in timber](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Atimber-bolted-joints) — Design of Steel and Timber Structure, chapter 10 - [Behaviour and detailing of bolted and nailed joints](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Abolted-nailed-joint-behavior-detailing) — Design of Timber and Masonry Structures, chapter 2