1 Before you begin: Turn on the sound on your computer. There is audio to accompany this presentation. Chapter 30 Fundamentals of Joining MET 33800 Manufacturing Processes Materials Processing Chapters 15-17 Chapters 20-27 Chapter 30 - 2 Chapters 30-33 Chapters 11-13 Topics Introduction to Consolidation Processes. Classification of Welding and Thermal Cutting Processes. Welding Basics and Common Concerns. Types of Fusion Welds and Joints. Design Considerations. Heat Effects. Weldability or Joinability. Chapter 30 - 3
Transcript
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Before you begin: Turn on the sound on your computer. There is audio to accompany this presentation.
Chapter 30
Fundamentals of Joining
MET 33800 Manufacturing Processes
Materials Processing
Chapters 15-17
Chapters 20-27
Chapter 30 - 2
Chapters 30-33
Chapters 11-13
Topics Introduction to Consolidation Processes.
Classification of Welding and Thermal Cutting Processes.
Welding, brazing and soldering involve solidification of molten metal.
Discrete fasteners – nuts, bolts, screws, rivets.
Adhesive bonding – any material can be bonded to any other material.
Other techniques such as shrink fits, slots/tabs, and other mechanical methods.
Chapter 30 - 4
Consolidation Processes Welding is the permanent joining of two materials,
usually metals, by coalescence, which is induced by a combination of:
Temperature
Pressure
Metallurgical conditions
Welding has become the dominant method of joining in manufacturing, and a large of metal products would have to be drastically modified, or it would be far more costly if it were not available.
Chapter 30 - 5
Introduction to Welding Process Coalescence is the merging of two or more particles
into one
Coalescence between two metals requires sufficient proximity and activity between the atoms of the pieces being joined to cause the formation of common crystals.
Chapter 30 - 6
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Introduction to Welding Process
Ideal Metallurgical Bond, for which there would be no noticeable or detectable joint would require:
Perfectly smooth, flat, or matching surfaces.
Clean surfaces free from oxides, absorbed gases, grease, and other contaminants.
Metals with no internal impurities.
Two metals that are both single crystal with identical crystallographic structure and orientation.
Chapter 30 - 7
Introduction to Welding Process Ideal metallurgical conditions are difficult to achieve
under laboratory conditions and virtually impossible to achieve in normal production.
Various joining methods have been designed to overcome or compensate for the various deficiencies.
Surface roughness can be overcome either by:
Force, causing plastic deformation and flattening of the high points, or
Melting the two surfaces so that fusion occurs.
Chapter 30 - 8
Introduction to Welding ProcessMethods to overcome ideal condition deficiencies also entail different approaches to cleaning the metal surfaces prior to welding and preventing further oxidation or contamination during the joining process.
Examples:
Solid State Welding – Mechanical or chemical cleaning, or causing sufficient metal flow so that impurities are squeezed out.
Fusion Welding – Fluxing Agents.
Chapter 30 - 9
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Introduction to Welding ProcessProduction of a high-quality welds require:
Source of satisfactory heat.
Means of cleaning and protecting the metals to be joined.
Avoid and/or compensate for harmful metallurgical effects that can occur.
Chapter 30 - 10
Classification of ProcessesAmerican Welding Society (AWS) has classified various welding process and assigned short letter symbols as designators (Figures 30-1 and 2).
Chapter 31 – Gas and arc processes.
Chapter 32 – Resistance and solid state processes.
Chapter 33 – Brazing, soldering and other processes.
Chapter 30 - 11
Classification of Processes
Figure 30-1 Classification of common welding processes along with their AWS designations.
Chapter 30 - 12
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Classification of Processes
Figure 30-2 Classification of thermal cutting processes along with their AWS designations.
Chapter 30 - 13
Common Concerns Proper selection of the correct welding process
eliminates most inherent problems.
Proper joint design is critical of quality welds.
Heating and solidification can change properties of base and filler materials.
Weld properties also affected by:
Dilution of filler by melted base metal.
Vaporization of alloy elements.
Gas-metal reactions.
Chapter 30 - 14
Common ConcernsWeld Defects can include:
Cracks in various forms
Gas and shrinkage cavities
Inclusions – slag, flux and oxides
Arc strikes and weld splatter
Metallurgical changes
Excessive distortion
Unacceptable weld shape or contour
Incomplete fusion between weld and base metal
Incomplete penetration – insufficient depth
Chapter 30 - 15
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Common Concerns
Examples of cracks due to welding.
Chapter 30 - 16
Types of Fusion Welds
Figure 30-4 Four basic types of fusion welds.
Chapter 30 - 17
Types of Fusion WeldsBead Weld or Surfacing Weld
Made directly onto a flat surface.
Requires no edge preparation.
Used primarily for:
Joining thin sheets
Building up surfaces
Depositing hard-facing materials
Chapter 30 - 18
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Types of Fusion WeldsGroove Welds
Used when full-thickness strength is desired on thicker material.
Edge preparation between abutting edges is required.
Used primarily for:
When welding can occur from only one side.
Pipeline welding.
Chapter 30 - 19
Types of Fusion WeldsGroove Welds (continued)
Common edge preparation configurations include: V, double-V, U and J.
Chapter 30 - 20
Types of Fusion WeldsFillet Welds
Requires no special edge preparation.
Used primarily for:
Tee joints
Lap joints
Corner joints
Figure 30-6 Preferred shape and the method of measuring the size of fillet welds.
Chapter 30 - 21
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Types of Fusion WeldsPlug Welds
Attaches one part on top of another and are often used to replace rivets or bolts.
Chapter 30 - 22
Types of JointsJoint design depends on:
Type and amount of loading – primary consideration.
Cost and accessibility for welding:
Secondary consideration to loading.
Cost – edge preparation.
Type of equipment to be used.
Speed and ease that the weld can be performed.
Chapter 30 - 23
Types of Joints
Figure 30-7 Five basic joint designs for fusion welding.
Chapter 30 - 24
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Types of Joints
Integrating the types of fusion welds and types of joint design.
Chapter 30 - 25
Design ConsiderationsMonolithic or One Piece Structures
Welding produces monolithic structures. When two pieces of material are welded together, they become one continuous piece.
Significant complications:
Cracking can propagate great distances through a structure resulting in failure.
Large pieces of material behave differently compared to small pieces.
Welded structures becoming too rigid.
Chapter 30 - 26
Design Considerations
Figure 30-9 Effect of size on the transition temperature and energy-absorbing ability of a certain steel. While the larger structure absorbs more energy because of its size, it becomes brittle at a much higher temperature.
Chapter 30 - 27
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Design Considerations
Liberty ship failures during early WWII is a famous example of monolithic structures becoming brittle to failure.
Chapter 30 - 28
Heat Effects Heating and cooling are integral to most welding
processes.
Fusion welding causes the base material to melt, followed by rapid cooling.
Fusion welding can be thought of as a small metal casting in a large metal mold.
Pool of molten metal is subject to all of the problems and defects associated with casting.
Welding metallurgy is therefore an extensive and complex subject.
Chapter 30 - 29
Heat EffectsWeld Pool or Fusion Zone
Mixture of parent metals along with filler metal.
The ratio of materials depends on the process used, the type of joint, and the edge preparation.
Figure 30-10 Schematic of a butt weld between a plate of metal A and a plate of metal B, with a backing plate of metal C and filler of metal D. The resulting weld nugget becomes a complex alloy of all four metals.
Chapter 30 - 30
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Heat EffectsHeat-Affected Zone (HAZ):
Adjacent to the fusion zone and wholly within the base material, is the undesirable heat-affected zone.
The adjacent metal may well experience sufficient heat to bring about structure and property changes, such as phase transformations, recrystallization, grain growth, precipitation or precipitate coarsening, embrittlement or cracking.
Chapter 30 - 31
Heat EffectsHeat-Affected Zone (continued):
In steels, the structures can range from hard, brittle martensite all the way through coarse pearlite and ferrite.
The HAZ may no longer possess the desirable properties of the parent material.
Consequently, this is often the weakest area in the as-welded joint.
Most welding failures originate in the HAZ.
Chapter 30 - 32
Heat Effects
Figure 30-11 Grain structure and the various zones in a fusion weld.
Chapter 30 - 33
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Heat Effects
Figure 30-13 Schematic of a fusion weld in steel, presenting proper terminology for the various regions and interfaces. Part of the heat-affected zone has been heated above the transformation temperature and will form a new structure upon cooling. The remaining segment of the heat-affected zone experiences heat alteration of the initial structure.
Chapter 30 - 34
Heat Effects Rate of heat input significantly affects the metallurgy of
the weld.
Low rates of heat input tend to produce:
Large total heat content
Slow cooling rates
Large HAZ
Structures with lower strength and hardness.
Structures with higher ductility.
Chapter 30 - 35
Heat Effects High rates of heat input tend to produce:
Low total heat content
Fast cooling rates
Small HAZ
Methods of controlling as-welded properties:
Heat treatment after welding.
Preheat base metal prior to welding.
Chapter 30 - 36
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Heat Effects
Chapter 30 - 37
Heat EffectsThermal-Induced Residual Stresses
Residual – are the result of thermal expansion and contraction restraint.
Reaction – residual stresses can contribute to cracking or failure during use. They can have magnitudes up to the yield strength of the parent metal.
Chapter 30 - 38
Residual stresses in a steel fusion weld measured using
neutrons. The peak tensile stress is located at the weld center.
Heat Effects
Chapter 30 - 39
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Heat EffectsEffects of Thermal Stress:
Distortion or Warping
Control methods include:
Minimizing total heat input to the weld.
Oriented out of position so that distortion will move them to the desired final shape.
Deposit weld metal in a specified pattern.
Warping can be reduced by use of peening.
Chapter 30 - 40
Weldability or Joinability Not all joining processes are compatible with all
engineering materials.
Within a given process, the quality of results may vary greatly with variations in the process parameters, such as:
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