Welding, Brazing And Soldering.pdf

ELEMENTS OF MECHANICAL ENGINEERING WELDING, BRAZING AND SOLDERING

Bicycle Design and Construction

Frame Components

Joining Processes for Frame Tubing

Bike Tubing plain gage tube

Better tubes are thicker at ends to give greater strength at joints

single-butted tube

double-butted tube

triple-butted tube

Important Material Properties • Weight (density) • Stiffness (elastic modulus)

• Strength (tensile strength, endurance limit) • Impact resistance (hardness)

• Corrosion resistance • Joining methods

• Recycling potential

Manufacturing processes Mould

Primary shaping Casting Molding PM methods

Granular Polymer Nozzle

Secondary shaping

Cylinder

Heater

Machining Drilling Cutting

Screw

Injection moulding No.8-CMYK-5/01

Machining

Surface Treating

Joining Welding Adhesives Fasteners

Painting Polishing Heat treating Welding

Painting

Products and Manufacturing Product Creation Cycle Design → Material Selection → Process Selection → Manufacture → Inspection → Feedback Typical product cost breakdown

Overview – Joining Processes JOINING TECHNIQUES Metallurgical Brazing P

Soldering P

P

= Permanent

NP = Non-permanent

Mechanical Screw Fasteners NP

Adhesive Natural P

Compression Joints

Elastomer

NP

P

Riveting

Thermoplastic

P

P

Sheet Metal Folding

Thermoset

P

P

Toughened P

MECHANICAL Screw Fasteners These are non-permanent methods of joining materials. There is a very wide variety of screw fasteners available they include; •Woodscrews •Self-drilling/tapping screws •Machine screws •Furniture screws •Bolts •Compression Joints

MECHANICAL Screw Fasteners Woodscrews

MECHANICAL Screw Fasteners Woodscrews

Wood Screw Pozi Head CSK

Wood Screws Round Head Slotted

MECHANICAL Screw Fasteners Furniture Screws

Furniture Screws & ‘Knock Down’ fittings

MECHANICAL Screw Fasteners Knock Down Fittings

MECHANICAL Self Drilling/Tapping Screws

MECHANICAL Machine Screws

MECHANICAL Bolts

Hexagon Flange Bolts Chrome Hexagonal Bolts Hexagon Socket Head

MECHANICAL Nuts & Washers

MECHANICAL Compression Fittings

Straight Coupling

Equal Tee

Used primarily in plumbing applications

MECHANICAL Pop Rivets

Pop Riveting Tool with Threaded Inserts This is a permanent joining process

MECHANICAL Rivets

Snap Head Rivet

Countersunk Rivet

These are used for permanent joints in steel plate

METALURGICAL Soldering Electric Cable

Replaceable Tip Insulated Handle

Electric Soldering Iron

METALURGICAL Soldering

Soldering Iron Stand

Soldering Wire

METALURGICAL Soldering a Component on A Circuit Board Component Lead

Foil

Soldering Iron

Circuit Board

The soldering iron tip is placed against the lead and the circuit board foil. Both are heated for 3 or 4 seconds

METALURGICAL Soldering a Component on A Circuit Board Cored Solder Wire

The solder is applied to the lead opposite the soldering iron. It is the heated lead and circuit board foil that melts the solder

METALURGICAL Soldering a Component on A Circuit Board

Molten Solder

As the solder melts it flows around the connection and forms a good conductive joint with the foil. The soldering wire and the soldering iron are then withdrawn and the joint is allowed to cool

METALURGICAL Brazing

Firebrick Brazing Torch

Brazing Hearth

METALURGICAL Brazing Whole of joint area is heated and it is the heated joint that melts the Spelter

Brazing Torch

Brazing Rod (Spelter)

METALURGICAL Brazing

Typical Brazing Applications

Adhesives Natural Adhesives These are made from natural ingredients rather than being fabricated synthetically from chemicals.

Sources •Animal based •Fish based •Vegetable based

A Brief History of Welding • Late 19th Century – Scientists/engineers apply advances in electricity to heat and/or join metals (Le Chatelier, Joule, etc.) • Early 20th Century – Prior to WWI welding was not trusted as a method to join two metals due to crack issues • 1930’s and 40’s – Industrial welding gains acceptance and is used extensively in the war effort to build tanks, aircraft, ships, etc. • Modern Welding – the nuclear/space age helps bring welding from an art to a science

Principle of Welding A welding is a metallurgical process in which the junction of the two parts to be joined are heated and then fused together with or without the application of pressure to produce a continuity of the homogenous material of the same composition and the characteristics of the parts which are being joined .

Types of Welding Welding processes may be classified based on the basic principles employed as ; (i) pressure welding and (ii) fusion welding. In pressure welding, the parts to be joined are heated only up to the plastic state and then fused together by applying the external pressure. The different types of pressure welding are : forge welding and resistance welding.

In fusion welding which is also known as nonpressure welding, the joint of the two parts is heated to the molten state and allowed to solidify. The different types of fusion welding are: arc welding and gas welding.

Types of Welding

Fusion Welding

Homogeneous

Gas Welding Electroslag High Energy Beam Electric Arc

Pressure Welding

Heterogeneous Brazing

Friction Welding

Soldering

MIG TIG

Shielded Metal Arc – “Stick”

Weldability of a Metal • Metallurgical Capacity – Parent metal will join with the weld metal without formation of deleterious constituents or alloys

• Mechanical Soundness – Joint will be free from discontinuities, gas porosity, shrinkage, slag, or cracks

• Serviceability – Weld is able to perform under varying conditions or service (e.g., extreme temperatures, corrosive environments, fatigue, high pressures, etc.)

Fusion Welding Principles • Base metal is melted • Filler metal may be added • Heat is supplied by various means – Oxyacetylene gas – Electric Arc – Plasma Arc – Laser

Types of Fusion Welding • • • •

Oxyacetylene Cutting/Welding Shielded Metal Arc (“Stick”) Metal Inert Gas (MIG) Tungsten Inert Gas (TIG)

Welding ELECTRODE COATING CORE WIRE WELDING ATMOSPHERE ARC STREAM ARC POOL SOLIDIFIED SLAG PENETRATION DEPTH

WELD

BASE METAL

Straight Polarity

Reverse Polarity

(–) (+) Shallow penetration (thin metal)

AC - Gives pulsing arc - used for welding thick sections

(+) (–) Deeper weld penetration

Consumable Electrode SMAW – Shielded Metal Arc Welding GMAW – Gas Metal Arc Welding SAW – Submerged Arc Welding

Non-Consumable Electrode GTAW – Gas Tungsten Arc Welding PAW – Plasma Arc Welding

High Energy Beam Electron Beam Welding Laser Beam Welding

Arc Welding: The most common fusion welding • A pool of molten metal is formed near electrode tip, and as electrode is moved along joint, molten weld pool solidifies in its wake

Weld Metal Protection • During fusion welding, the molten metal in the weld “puddle” is susceptible to oxidation • Must protect weld puddle (arc pool) from the atmosphere • Methods – Weld Fluxes – Inert Gases – Vacuum

Weld Fluxes • Typical fluxes – SiO2, TiO2, FeO, MgO, Al2O3 – Produces a gaseous shield to prevent contamination – Act as scavengers to reduce oxides – Add alloying elements to the weld – Influence shape of weld bead during solidification

Inert Gases • Argon, helium, nitrogen, and carbon dioxide • Form a protective envelope around the weld area • Used in – MIG – TIG – Shield Metal Arc

Vacuum • Produce high-quality welds • Used in electron beam welding • Nuclear/special metal applications – Zr, Hf, Ti

• Reduces impurities by a factor of 20 versus other methods • Expensive and time-consuming

Arc Welding When two conductors of an electric circuit are touched together momentarily and then instantaneously separated slightly, assuming that there is sufficient voltage in the circuit to maintain the flow of current, an electric arc is formed, Concentrated heat is produced throughout the length of the arc at a temperature of about 5000 to 6000°C.

In arc welding, usually the parts to be welded are wired as one pole of the circuit, and the electrode held by the operator forms the other pole. When the arc is produced, the intense heat quickly melts the workpiece metal which is directly under the arc, forming a small molten metal pool. At the same time the tip of the electrode at the arc also melts, and this molten metal of the electrode is carried over by the arc to the molten metal pool of the workpiece.

The molten metal in the pool is agitated by the action of the arc, thoroughly mixing the base and the filler metal. A solid joint will be formed when the molten metal cools and solidifies. The flux coating over the electrode produces an inert gaseous shield surrounding the arc and protects the molten metal from oxidizing by coming in contact with the atmosphere. Fig. illustrates the arc welding process.

2. Arc Welding Electrodes The two types of electrodes used in arc welding are (i) consumable electrodes and (ii) nonconsumable electrodes.

General Welding Procedure A Step-by-step general procedure for welding is as described below: Step 1: Cleaning: The surfaces of the parts to be welded need to be thoroughly cleaned for removal of dirt, oil, grease, etc.

Step 2: Edge Preparation: The process of preparing a contour at the edges of the pieces to be joined is called as edge preparation. This involves beveling or grooving. The idea of doing this is to get fusion or penetration through the entire thickness of the member.

Step 3: Clamping: Next, the parts to be welded are clamped suitably through jigs and fixtures so that there are no undesirable movements during welding.

Step 4: Check for safety devices : Safety devices like goggles and shields to protect the eyes, protective clothing to prevent the sparks and flying globules of molten metal, safety shoes, gloves, aprons and other safety devices must be ensured.

Step 5: The Initial weld: Initial tack welds are done at the opposite corners of the joint to secure the pieces together. Any cracks at this stage must be chipped off as the presence of these cracks causes residual stresses.

Step 6: Intermediate and final welding : The weld joint is formed through various weaving movements (of varying shapes called weld beads). During the process, filler metal and a suitable flux are used. After the intermediate run of welding, the final run is taken.

Step 7: Excess material removal: Extra material on the weld surface can be removed using tongs and chipper. The final weld is now allowed to cool and finally cleaned.

Gas welding Gas welding is a fusion method of welding, in which a strong gas flame is used to raise the temperature of the workpieces so as to melt them. As in arc welding, a filler metal is used to fill the joint. The gases that can be used for heating are; (i) oxygen and acetylene mixture and (ii) oxygen and hydrogen mixture. The oxy-acetylene gas mixture is most commonly used in gas welding.

Oxyacetylene Welding • Flame formed by burning a mix of acetylene (C2H2) and oxygen TORCH TIP

Inner Cone: 5000-6300 deg F

2300 deg F

Combustion Envelope 3800 deg F

• Fusion of metal is achieved by passing the inner cone of the flame over the metal • Oxyacetylene can also be used for cutting metals

Joint Design

BUTT JOINT FILLET JOINT STRAP JOINT

LAP JOINT

CORNER JOINT

(a) Butt joint, (b) corner joint, (c) lap joint, (d) tee joint, and (e) edge joint

Groove Welds • (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)

Fillet Welds • (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)

Standard Identification and Symbols for Welds

Generalized Welding Symbol FAR SIDE DETAILS

Field weld symbol

Weld Geometry Electrode Material

D D

ARROW SIDE DETAILS

L1-L2 L1-L2

Weld all-around for pipes, etc.

D = Weld Depth (usually equal to plate thickness) L1 = Weld Length

L2 = Distance between centers for stitched The Field Weld Symbol is a guidewelds for installation. Shipyards

normally do not use it, except in modular construction.

Example Welding Symbol

Geometry symbol for V-groove

One-sided welds are max 80% efficient Two sided are 100% efficient 1/2

1/2

1/2”

1/2”

Weld Symbols (Butt Joints)

Backing

Weld Symbol (Fillet Joints)

Weld Symbol (Corner Joints)

Typical Fusion Welded Joint

Fusion Weld Zone

Figure 29.1 Characteristics of a typical fusion weld zone in oxyfuel gas and arc welding. See also Figs. 27.16 and 28.14.

Grain Structure in Shallow and Deep Welds (a)

(b)

Grain structure in (a) a deep weld (b) a shallow weld. Note that the grains in the solidified weld metal are perpendicular to the surface of the base metal. In a good weld, the solidification line at the center in the deep weld shown in (a) has grain migration, which develops uniform strength in the weld bead.

Weld Beads (a)

(b)

Figure 29.3 (a) Weld bead (on a cold-rolled nickel strip) produced by a laser beam. (b) Microhardness profile across the weld bead. Note the lower hardness of the weld bead compared to the base metal. Source: IIT Research Institute.

Regions in a Fusion Weld Zone Schematic illustration of various regions in a fusion weld zone (and the corresponding phase diagram) for 0.30% carbon steel. Source: American Welding Society.

Corrosion Figure 29.5 Intergranular corrosion of a 310-stainlesssteel welded tube after exposure to a caustic solution. The weld line is at the center of the photograph. Scanning electron micrograph at 20 X. Source: Courtesy of B. R. Jack, Allegheny Ludlum Steel Corp.

Incomplete Fusion

Figure 29.6 Low-quality weld beads, the result of incomplete fusion. Source: American Welding Society.

Discontinuities in Fusion Welds Figure 29.7 Schematic illustration of various discontinuities in fusion welds. Source: American Welding Society.

Cracks in Welded Joints

Types of cracks (in welded joints) caused by thermal stresses that develop during solidification and contraction of the weld bead and the surrounding structure. (a) Crater cracks. (b) Various types of cracks in butt and T joints.

Distortion After Welding

Distortion of parts after welding: (a) butt joints; (b) fillet welds. Distortion is caused by differential thermal expansion and contraction of different parts of the welded assembly.

Residual Stresses Developed During Welding

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

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

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

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

Soldering Methods • Many soldering methods are 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

Soldering

Metal Joining Processes

Soldering

Solder = Filler metal • Alloys of Tin (silver, bismuth, lead) • Melt point typically below 840 F Flux used to clean joint & prevent oxidation • separate or in core of wire (rosin-core) Tinning = pre-coating with thin layer of solder Applications:

• Printed Circuit Board (PCB) manufacture • Pipe joining (copper pipe) • Jewelry manufacture • Typically non-load bearing Easy to solder: copper, silver, gold Difficult to solder: aluminum, stainless steels (can pre-plate difficult to solder metals to aid process)

PCB Soldering Manual PCB Soldering

Metal Joining Processes PTH - Pin-Through-Hole connectors

• Soldering Iron & Solder Wire

• Heating lead & placing solder

• Heat for 2-3 sec. & place wire opposite iron

• Trim excess lead

PCB Reflow Soldering Automated Reflow Soldering

Metal Joining Processes SMT = Surface Mount Technology

• Solder/Flux paste mixture applied to PCB using screen print or similar transfer method

• Solder Paste serves the following functions: – supply solder material to the soldering spot, – hold the components in place prior to soldering, – clean the solder lands and component leads – prevent further oxidation of the solder lands. Printed solder paste on a printed circuit board (PCB)

• PCB assembly then heated in “Reflow” oven to melt solder and secure connection

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

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

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

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

Brazing

Metal Joining Processes

Brazing Use of low melt point filler metal to fill thin gap between mating surfaces to be joined utilizing capillary action • Filler metals include Al, Mg & Cu alloys (melt point typically above 840 F) • Flux also used • Types of brazing classified by heating method: – Torch, Furnace, Resistance Applications: • Automotive - joining tubes • Pipe/Tubing joining (HVAC) • Electrical equipment - joining wires • Jewelry Making • Joint can possess significant strength

Brazing

Metal Joining Processes

Brazing Use of low melt point filler metal to fill thin gap between mating surfaces to be joined utilizing capillary action • Filler metals include Al, Mg & Cu alloys (melt point typically above 840 F) • Flux also used • Types of brazing classified by heating method: – Torch, Furnace, Resistance Applications: • Automotive - joining tubes • Pipe/Tubing joining (HVAC) • Electrical equipment - joining wires • Jewelry Making • Joint can possess significant strength

Brazing

Metal Joining Processes

Brazing Figuring length of lap for flat joints. X = Length of lap T = Tensile strength of weakest member W = Thickness of weakest member C = Joint integrity factor of .8 L = Shear strength of brazed filler metal Let’s see how this formula works, using an example. Problem: What length of lap do you need to join .050" annealed Monel sheet to a metal of equal or greater strength? Solution: C = .8 T = 70,000 psi (annealed Monel sheet) W = .050" L = 25,000 psi (Typical shear strength for silver brazing filler metals) X = (70,000 x .050) /(.8 x 25,000) = .18" lap length

Soldering & Brazing

Metal Joining Processes

Brazing

Figuring length of lap for tubular joints.

X = Length of lap area W = Wall thickness of weakest member D = Diameter of lap area T = Tensile strength of weakest member C = Joint integrity factor of .8 L = Shear strength of brazed filler metal Again, an example will serve to illustrate the use of this formula. Problem: What length of lap do you need to join 3/4" O.D. copper tubing (wall thickness .064") to 3/4" I.D. steel tubing? Solution: W = .064" D = .750" C= .8 T = 33,000 psi (annealed copper) L = 25,000 psi (a typical value) X = (.064 x (.75 – .064) x 33,000)/(.8 x .75 x 25,000) X = .097" (length of lap)