lecture 7: joining methods - ncsu
TRANSCRIPT
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NC State University Department of Materials Science and Engineering 1
MSE 440/540: Processing of Metallic Materials
Instructors: Yuntian Zhu Office: 308 RBII Ph: 513-0559
Lecture 7: Joining Methods
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Joining and Assembly
Joining - welding, brazing, soldering, and adhesive bonding
• These processes form a permanent joint between parts
Assembly - mechanical methods (usually) of fastening parts together
• Some of these methods allow for easy disassembly, while others do not
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Types of Welding Processes
Welding processes can be divided into two major categories: Fusion welding
– Arc welding (AW) – Resistance welding (RW) – Oxyfuel gas welding (OFW)
Solid state welding: Joining processes using pressure alone or a combination of heat and pressure – If heat is used, temperature is below melting point of metals
being welded – No filler metal is added in solid state welding
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Five Types of Joints
(a) Butt joint, (b) corner joint, (c) lap joint, (d) tee joint, and (e) edge joint
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• (a) Square groove weld, one side; (b) single bevel groove weld; (c) single V‑groove weld; (d) single U‑groove weld; (e) single J‑groove weld; (f) double V‑groove weld for thicker sections (dashed lines show original part edges)
Groove Welds
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• (a) Inside single fillet corner joint; (b) outside single fillet corner joint; (c) double fillet lap joint; (d) double fillet tee joint (dashed lines show the original part edges)
Fillet Welds
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Plug Weld and Slot Weld
• (a) Plug weld and (b) slot weld
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Fused section between surfaces of two sheets or plates: (a) spot weld and (b) seam weld
• Used for lap joints • Closely associated with resistance welding
Spot Weld and Seam Weld
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Typical Fusion Welded Joint
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• A pool of molten metal is formed near electrode tip, and as electrode is moved along joint, molten weld pool solidifies in its wake
Arc Welding: The most common fusion welding
http://www.youtube.com/watch?v=TeBX6cKKHWY
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Two Basic Types of Arc Welding Electrodes
• Consumable – consumed during welding process – Source of filler metal in arc welding
• Nonconsumable – not consumed during welding process (e.g. Tungsten) – Filler metal must be added separately if it is
added
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Arc Shielding
• At high temperatures in arc welding, metals are chemically reactive to O2, N2 and H2 in air – Mechanical properties of joint can be degraded by
these reactions – To protect operation, arc must be shielded from
surrounding air in AW processes • Arc shielding is accomplished by:
– Shielding gases, e.g., argon, helium, CO2 – Flux
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Flux
A substance that prevents formation of oxides and other contaminants in welding, or dissolves them and facilitates removal
• Provides protective atmosphere for welding • Stabilizes arc • Reduces spattering
Flux Application § Pouring granular flux onto welding operation § Stick electrode coated with flux material that melts
during welding to cover operation § Tubular electrodes in which flux is contained in the
core and released as electrode is consumed
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Consumable Electrode Arc Welding Processes
• Shielded Metal Arc Welding • Gas Metal Arc Welding • Flux‑Cored Arc Welding • Electrogas Welding • Submerged Arc Welding
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Shielded Metal Arc Welding (SMAW)
Uses a consumable electrode consisting of a filler metal rod coated with chemicals that provide flux and shielding
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Welding Stick in SMAW
• Composition of filler metal usually close to base metal
• Coating: powdered cellulose mixed with oxides and carbonates, and held together by a silicate binder
• Disadvantages of stick welding: – Sticks must be periodically changed – High current levels may melt coating prematurely
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SMAW Applications
• Used for steels, stainless steels, cast irons, and certain nonferrous alloys
• Not used or rarely used for aluminum and its alloys, copper alloys, and titanium
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Gas Metal Arc Welding
Uses a consumable bare metal wire as electrode with shielding by flooding arc with a gas
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GMAW Advantages over SMAW
• Better arc time because of continuous wire electrode – Sticks must be periodically changed in SMAW
• Better use of electrode filler metal than SMAW – End of stick cannot be used in SMAW
• Higher deposition rates • Eliminates problem of slag removal • Can be readily automated
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Presence or absence of externally supplied shielding gas distinguishes: (1) self‑shielded - core provides ingredients for shielding, (2) gas‑shielded - uses external shielding gases
Flux-Cored Arc Welding
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Resistance Welding
• Resistance welding, showing components in spot welding, the main process in the RW group
http://www.youtube.com/watch?v=66-RK0DPXfU
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Resistance Seam Welding
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Roll Welding
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Explosive Welding • Commonly used to bond two dissimilar metals, e.g., to
clad one metal on top of a base metal over large areas • (1) Setup in parallel configuration, and (2) during
detonation of the explosive charge
http://www.youtube.com/watch?v=HvJee_1w4tA
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• (1) Rotating part, no contact; (2) parts brought into contact to generate friction heat; (3) rotation stopped and axial pressure applied; and (4) weld created
Friction Welding
http://www.youtube.com/watch?v=5JbnDXw-0pM
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Friction Stir Welding
• (1) Rotating tool just before entering work, and (2) partially completed weld seam
http://www.youtube.com/watch?v=_qAOKR6mdTI
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Residual Stresses and Distortion
• (a) Butt welding two plates
• (b) Shrinkage • (c) Residual
stress patterns • (d) Likely warping
of weldment
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Techniques to Minimize Warpage
• Welding fixtures to physically restrain parts • Heat sinks to rapidly remove heat • Tack welding at multiple points along joint to create a
rigid structure prior to seam welding • Selection of welding conditions (speed, amount of
filler metal used, etc.) to reduce warpage • Preheating base parts • Stress relief heat treatment of welded assembly • Proper design of weldment
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Welding Defects
• Cracks • Cavities • Solid inclusions • Imperfect shape or unacceptable
contour • Incomplete fusion • Miscellaneous defects
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Brazing
Joining process in which a filler metal is melted and distributed by capillary action between faying surfaces of metal parts being joined
• No melting of base metals occurs – Only the filler melts
• Filler metal Tm is greater than 450°C (840°F) – But less than Tm of base metal(s) to be joined
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Brazing Compared to Welding
• Any metals can be joined, including dissimilar metals • Can be performed quickly and consistently, permitting
high production rates • Multiple joints can be brazed simultaneously • Less heat and power required than FW • Problems with HAZ in base metal are reduced • Joint areas that are inaccessible by many welding
processes can be brazed – Capillary action draws molten filler metal into joint
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Disadvantages and Limitations of Brazing
• Joint strength is generally less than a welded joint • Joint strength is likely to be less than the strength of
the base metals • High service temperatures may weaken a brazed
joint • Color of brazing metal may not match color of base
metal parts – A possible aesthetic disadvantage
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Brazing Applications
• Automotive (e.g., joining tubes and pipes) • Electrical equipment (e.g., joining wires and
cables) • Cutting tools (e.g., brazing cemented carbide
inserts to shanks) • Jewelry • Chemical process industry • Plumbing and heating contractors join metal
pipes and tubes by brazing • Repair and maintenance work
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Soldering Joining process in which a filler metal with Tm less than
or equal to 450C (840F) is melted and distributed by capillary action between faying surfaces of metal parts being joined
• No melting of base metals, but filler metal wets and combines with base metal to form metallurgical bond
• Filler metal called solder • Closely associated with electrical assembly
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Soldering Advantages and Disadvantages
Advantages: • Lower energy than brazing or fusion welding • Variety of heating methods available • Good electrical and thermal conductivity in joint • Easy repair and rework Disadvantages: • Low joint strength unless reinforced mechanically • Joint weakens or melts at elevated temperatures
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Solders Traditionally alloys of tin and lead (both have low Tm) • Lead is poisonous and its percentage is minimized in
most solders • Tin is chemically active at soldering temperatures and
promotes wetting action for successful joining – In soldering copper, copper and tin form intermetallic
compounds that strengthen bond
• Silver and antimony also used in soldering alloys
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Soldering Fluxes: Functions
• Be molten at soldering temperatures • Remove oxide films and tarnish from base
part surfaces • Prevent oxidation during heating • Promote wetting of surfaces • Be readily displaced by molten solder during
process • Leave residue that is non-corrosive and
nonconductive
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Soldering Methods
• Many soldering methods same as for brazing, except less heat and lower temperatures are required
• Additional methods: – Hand soldering – manually operated soldering
gun – Wave soldering – soldering of multiple lead wires
in printed circuit cards – Reflow soldering – used for surface mount
components on printed circuit cards
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HW assignment
• Reading assignment: Chapters 12, 13
• Review Questions: 22.2, 22.3, 22.4, 22.8, 22.9, 22.10, 22.15, 23.2, 23.5, 23.6, 23.8, 23.2, 23.20, 23.22, 23.23, 24.1, 24.2, 24.3, 24.2, 24.7, 24.10, 24.11, 24.15,
Department of Materials Science and Engineering 39