Today
2.008 Design & Manufacturing
II
„
Ask “Dave and Pat”
„
Joining & Welding
„
Quiz 1 on March 17th, 12:30 PM
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Spring 2004
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Welding & Joining
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2.008-spring-2004
S.Kim
1
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Closed book, calculator
Lecture notes, HWs
Q&A session? March 15th, Monday, 5-6PM ?
HW# 4, due next Monday
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2.008-spring-2004 S.Kim
Joining
„
PP 771-861, Kalpakjian
Joining processes
' &
Cost:
„ Cheap, but expensive
labor
Quality: &
„ Wide range
Flexibility: ' &
„ Manual vs automated
Rate: ' &
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engine
fasteners
door
ECUs
emission controller
windshield
Very Large Crude Carrier (VLCC)
Laser welding
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Mechanical Fastening
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Body
muffler
non
door hinges
Slow in general
plastic fans
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Rivets
Any shape and material almost
Disassemblable (except Rivets, etc.)
Least expensive for low volume (standardized)
Problems: strength, seal, insertion, loosening
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Cheap, light weight, don’t get loose
Permanent, less strong than bolts
Rule of thumb
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Minimum spacing = 3 x d
Maximum spacing = 16 x thickness of the outer plate
Rivets
Threaded
2.008-spring-2004 S.Kim
Bolted
Rivets
Figures, B. Benhabib
Snap fit
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1
Mechanical fastening
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Mechanical Joining
Crimping
Embossed protrusions
„ Plastic deformation / interference
„ Appliance
„ Automotive
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Hemming, seaming
Bend edge of one component
over another
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Automobile door stampings and
trunk lids
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Lego assembly, elastic averaging
M. Culpepper, MIT
2.008-spring-2004 S.Kim
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Adhesive Bonding
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2.008-spring-2004 S.Kim
Stefan Equation
Quick and non-invasive
Most materials with high surface to volume ratio
Insulation (thermal, electrical), conducting
adhesives available
Good damping
Clean surface preparation
Long curing, hold time, low service T
Reliability, quality?
Disassembly?
„
a
y
u
r
(
)
h/2
h/2
Squeeze a drop of liquid
„
hf goes down, and
Viscosity goes up (curing)
„
Ft goes up (to separate)
„
F
p=
3µ dh 2
r − a2
h 3 dt
Ft =
3µπa4 ⎡ 1
1⎤
⎢ − ⎥
4 ⎢⎣ hf 2 hi 2 ⎥⎦
When pullin apart?
Ft =
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Solid-Liquid: Brazing and Soldering
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Weak to tensile loading, strong to shear and
compressive
Organic
„ Epoxy, acrylic, etc..
Inorganic
„ solder, cement, etc..
Do
Do not
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DFA for bonding
„
3µµπ4 ⎡ 1
1⎤
⎢ − ⎥
4 ⎣ hf 2 hi2 ⎦
Do
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„
In brazing, the filler material (silver, brass, bronze) has a melting
point above 425 ºC; and, in soldering, the filler material (lead, tin)
has a melting point well below 425 ºC.
Capillary forces for the wetting and flow of the liquid metal into the
gaps.
Proper fluxes for lowering surface tension, remove oxides, and
prevent oxidation.
Do not
γL
γS
θ
Liquid
(a) Contact angle, θ
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Solid
γSL
θ
θ
o
(b)θ < 90, wetting
B. Benhabib
o
(c) θ> 90, no-wetting
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2
Welding
Wave soldering
Exposed metal joints are passed over a wave in a continuous
motion, where liquid solder penetrates into the joint by capillary
forces:
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Enlarged area
Circuit
board
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Enlarged area
Solder
fountain
Solid-state welding
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Molten
solder
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Copper lead
wires
Fusion welding
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Solder
wave
No liquid, electrical, chemical, mechanical
Resistance, diffusion, ultrasonic
Chemical: Oxyfuel
Electrical: Arc welding
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Heater
Pump
B. Benhabib
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Fusion Welding
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Underfill and undercut
underfill
Incomplete fusion, penetration
Underfilling, undercutting, cracks
undercut
Heat affected zone (HAZ)
Feed rate
Good weld
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Heat Intensity
Oxyacetylene
Thermit
102
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Laser for Razor
A measure of radiation intensity, W/cm2
Air/Fuel
Gas Flame
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Heat source: chemical, electrical
Heat intensity
„ Control:
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Consumable electrode
Non-consumable electrode
Friction
103
Arc
Resistance Welding
(Oxygen Cutting)
104
-500 µm spots
-3 million spots per hour
Electron Beam,
Laser Beam
Welding
105
106
107
High intensity, high heat flux, the faster the melting
Automation needed to prevent overmelting, vaporizing
For a planar heat source on steel,
tm= (5000/H.I.)2
Oxy: 20-30 sec, E-beam: µ sec
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How fast the welding speed is required?
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Kalpakjian
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3
Melting front speed
2D simplification
Welding speed
„
(T − T )
The Jacob number, J a = c p melt initial .
h fs
The thermal diffusivity is given by α =
k
ρc p
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tm = (sm)2 / (2 α Ja),
Any longer, over-melt!
Weld pool size d
.
The melt front moves as : s = 2αJ a t
Vmin = d / tmax
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s
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HAZ(Heat Affected Zone)
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100
100
101
Copper,
Aluminum
10-2
10-3
10-4
Steels,
Nickels
10-5
10-6
10-7
102
Uranium,
Ceramics
103
104
105
106
107
Interaction Rate (Seconds-1)
10-2
10-1
Higher α, Ja
Interaction Time (Seconds)
102
101
10-8
103
„
10-3
10-1
Bad microstructure, course grains, weak to corrosion
Plastic vs. Metal
Base metal
HAZ
Weld metal
Molten metal
Melting point
Temp
Microstructure change temperature
108
104
105
106
107
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Weld Pool – Heat Source
Interaction Time
103
If the weld pool size is d in diameter, then you must feed at a
rate that exceeds d/tmax.
HI increases, welding speed must go up
α Ja increases, interaction time must go up
Base metal temperature
108
Heat Intensity (Watts/cm2)
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Heat Affected Zone (HAZ)
Region near the weld pool is affected by heat.
Microstructure changes.
s ~ (α t)0.5
Grain structure
Weld line
Intergranular corrosion
recrystallization
s
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The size of the heat affected zone is controlled by the
thermal diffusivity, α: Al, Cu
HI, time (speed)
Metal vs. Plastic
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10 2
10 1
10 0
higher α
Heat Affected Zone
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Heat Affected Zone Width (cm)
J. Chun
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10 -1
10 -2
10 -3
10 3
10 4
10 5
10 6
10 7
Heat Intensity (Watts/cm2)
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4
Oxyfuel Gas Welding
Arc fusion welding
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Temperature up to 30000oC
Heat travels with the electrons!
Straight polarity: workpiece +, electrode –
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reverse polarity
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Low cost, manual
Oxyfuel: Oxygen + Fuel (Acetylene, methyacetylenepropediene, etc): can reach 3300oC
CNT
Stoichiometry: neutral flame
„ C2H2+O2 ⇒2CO+H2+Heat ⇑
„ 2CO+H2+1.5O2 ⇒ 2CO2+H20+Heat ⇑
More oxygen will cause oxidization (Oxidizing flame): bad for
steels, OK for copper
More fuel will cause carburization (carburizing flame): Low heat
for brazing, soldering, flame hardening
2100oC
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Inner cone:
e-
Arrangement for
consumable electrodes
25
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Coating deoxidizes weld and
provides shielding gas
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Base metal
Multiple pass
Slag removal
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Kinetic energy
Drop across cool gas boundary
Work function(heat of condensation)
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Preventing oxidation is the most important problem.
Coated electrodes to provide shielding gas
„ Coatings: fluorides, silicate binders(clay), cellulose,
carbonates
„ Electrodes: different materials such as mild steel.
Submerged arc welding: covered by granular flux
„ Flat surfaces, high throughput, good quality
Inert gas from external source
„ Gas metal arc welding (MIG)
„ Flux-cored arc welding, etc. (very versatile with proper
fluxes)
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Non-consumable electrode welding
Gas Tungsten Arc Welding
Gas Metal Arc Welding
Gas metal arc welding is a
special variation on arc welding
in which the electrode filler
metal is fed directly through the
torch tip. As arcing occurs the
electrode instantly melts,
forming molten droplets that fall
into the weld pool. Shielding gas
is supplied through the torch tip
to prevent chemical interactions
with the surrounding
atmosphere
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Gas tungsten arc welding, formerly known as TIG welding
Tungsten electrode + filler wire.
Good for thin materials, better control, Al, Mg,Ti, etc
High quality and finish
Tungsten electrode contamination
200A DC-500A AC, 8kW-50kW
Metal transfer
- Spray transfer
- Globular transfer
- Short circuiting
2.008-spring-2004 S.Kim
Eelectron at cathode is given by
three components:
I(φ + Va + Vthomson)
Fusion Arc Welding
ctr
od
e
ele
Fil
ler
/
Weld
Heat generated at anode is
about 15% of the heat
generated at cathode.
2.008-spring-2004 S.Kim
Shielded metal arc welding
„ Cheapest and most basic process < $1,500
„ 50A-300A, <10KW
„ Workpiece thickness 3-20mm
Shielding gas
+
-
Consumable electrode processes
Slag
Deeper penetration
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1300oC
3300oC
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Shallow penetration, sheet metal, wide gaps
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Travel
Shielding gas
Filler wire
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Solidified weld
Arc metal
Molten metal
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Tungsten
electrode
30
5
Other non-consumable electrode
processes
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Plasma arc welding:
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Resistance Welding
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(33000oC),
very higher energy concentration
high feed rate
120-1000 mm/min, arc stablility, less thermal distortion,
deep penetration (keyhole technology)
Spot, seam, projection welding
High current through the weld, 3000A-100,000A (0.5 – 10 V)
Weld nugget
Laser beam welding, electron beam welding, etc
plasma gas
Electrode
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Shielding gas
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Shielding
gas
nozzle
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Extremely high heat flux (105 W/cm2)
Heat generated: H=i2Rt K (K=efficiency)
Ex: 5000 A, 5 mm dia elctrodes, two 1mm thick steel plates
Orifice
Work
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Arc plasma
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Plasma Arc Welding
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Spot welding Sequence
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Pressure on
Current on
Current off
Pressure off
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Resistance welding
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Effective resistance 200 Ω, Then H= 500 J
Nugget volume, 30 mm3, mass 0.24 g, 1400 J/g needed to melt
336 J for melting < 500 J
Solid-state: interatomic bonding
Weld nugget
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Ultrasonic
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indentation
separation
Projection welding
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Friction
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Diffusion
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Rotational part (at least one), flash
Goldsmiths bonded gold over copper
Diffusion T > 0.5 Tm
Interface is the same as the bulk.
Fuselage frames
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Weld quality and defects
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Oscillating tangential shear + static normal forces
10KHz to 75 KHz
Shear Æ plastic deformationÆ breaking up top layer
(contaminated) Æ inter-atomic bond
Weld quality and defects
Porosity
„ Trapped gases, contaminants
„ Preheat or increase rate of heat input
„ Reduce speed allowing gas to escape, cleaning
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Incomplete fusion/penetration
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Preheat and clean joint
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Clean weld area, enough shielding gas
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Change joint design or type of electrode
Slag inclusions
2.008-spring-2004 S.Kim
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Oxides, fluxes, electrode coating trapped in weld zone
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Clean weld bead during multi-weld processes
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Provide enough shielding gas
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6
DFA - Welding
Weld quality and defects
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Cracks, residual stresses
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Temperature gradients, embrittlement of grain boundaries
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Inability of weld metal to contract during cooling
Loading conditions
2.008-spring-2004 S.Kim
Kalpakjian
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2.008-spring-2004 S.Kim
Welding accessibility, DFM
DFM welding
Electrode must be held close to
45° when making these fillers
Try to avoid placing pipe joints near
wall so that one or two sides are
inaccesible. These welds must be
made with bent electrodes and mirror
Easy to draw, but the 2nd weld
will be hard to make
Easy to specify
“weld all
around” but …..
Too close to side to allow
proper electrode positioning.
May be OK for average work but
bad for leakproof welding
Pipe
Easy
Very difficult
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Wall
40
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