1. ME 330 Manufacturing Processes WELDING PROCESSES (cont.)

2. Principle of the process
Structure and configuration
Process modeling
Defects
Design For Manufacturing (DFM)
Process variation
Welded parts assembly design

3. Weld Joints: Five Types
(a) Butt joint, (b) corner joint, (c) lap joint, (d) tee joint, and (e) edge joint

4. Weld Types: Fillet Welds
(a) Inside single fillet corner joint;
(b) outside single fillet corner joint;
(c) double fillet lap joint;
(d) double fillet tee joint
- Used to fill in the edges of plates
Filler metal is used

5. Weld Types: Groove Welds
square groove weld;
single bevel groove weld;
single V‑groove weld;
single U‑groove weld;
single J‑groove weld;
double V‑groove weld (for thicker sections)
Filler metal is used
Common with butt joints
Except for (a), a groove is required for good weld penetration
Edge preparation increases cost of parts fabrication
©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

6. Weld Types: Plug Weld and Slot Weld
(a) Plug weld and (b) slot weld
- Used with lap joints
Filler metal is used

7. Weld Types: Spot Weld and Seam Weld
Fused section between surfaces of two sheets or plates:
(a) spot weld and (b) seam weld
Used for lap joints
Used commonly with resistance welding
No filler metal

8. Weld Types: Flange Weld and Surfacing Weld
Surfacing weld used not to join parts but to deposit filler metal onto surface of a base part

9. Principle of the process
Structure and configuration
Process modeling
Defects / quality control
Design For Manufacturing (DFM)
Process variation

10. Typical Fusion Welded Joint
Cross section of a typical fusion welded joint:
(a) principal zones in the joint, and (b) typical grain structure
Regarding the HAZ region:
- Chemical composition same as base metal, but this region has been heat treated:
- longer cold-worked grains are transformed to finer/shorter grains
- This has negative effect on mechanical properties in HAZ region
- Hence: HAZ region is where welding failures often occur

11. Weld Quality Motivation:
Obtaining a weld joint that is strong and absent of defects
Topics:
Residual stresses and distortion
Welding defects
Inspection and testing methods

12. Residual Stresses and Distortion
Shrinkage
Butt welding
Residual stress
warping of weldment
- Shrinkage is shown in (b)
- Residual stress patterns are shown in (c)  ‘+’ stresses = tension (due to shrinkage), ‘-’ stresses = compression (in reaction to the shrinkage)
- Typical warping shown in (d)
Principle: Rapid heating and cooling in localized regions during fusion welding result in thermal expansion and contraction that cause residual stresses, and stresses cause distortion and warpage

13. 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
- Clamping work to a metal table

14. Weld Defects: Welding Cracks
Welding cracks: On weld or near weld
Very serious because strength is reduced significantly
Caused by low ductility of weld and/or base metal combined with high stresses during contraction

15. Welding Defects: Cavities
Two types of defects:
Porosity - small voids in weld metal formed by gases entrapped during solidification, caused by inclusion of atmospheric gases, sulfur in weld metal, or surface contaminants
Shrinkage voids - cavities formed by shrinkage during solidification

16. Welding Defects: Solid Inclusions
Nonmetallic material entrapped in weld metal:
Most common form is slag inclusions generated during arc welding processes that use flux. Instead of floating to top of weld pool, globules of slag become encased during solidification
Other forms: formation of metallic oxides, such as Al2O3 when welding aluminum
- normally Al2O3 is on the surface of Al

17. Welding Defects: Incomplete Fusion
A weld bead in which fusion has not occurred throughout entire cross section of joint. Forms of incomplete fusion are shown below:

18. Weld Profile in Arc Welds
Desired profile for single V-groove weld joint,
undercut - portion of base metal melted away,
underfill - depression in weld below adjacent base metal surface, and
overlap - weld metal spills beyond joint onto part surface, but no fusion occurs

19. Inspection and Testing Methods: Visual Inspection
Most widely used welding inspection method is manual examination for:
Conformance to dimensions, warpage
Surface defects, such as cracks, cavities, incomplete fusion
Limitations: Only surface defects are detectable

20. Inspection and Testing Methods: Nondestructive Evaluation (NDE) Tests
Ultrasonic testing - high frequency sound waves through specimen to detect cracks and inclusions
Radiographic testing - x‑rays or gamma radiation provide photographs of internal flaws
Dye‑penetrant and fluorescent‑penetrant tests - to detect small cracks and cavities at part surface
Magnetic particle testing – iron filings sprinkled on surface reveal subsurface defects that distort the magnetic field

21. Inspection and Testing Methods: Destructive Testing -- Mechanical Tests
(a) Tension-shear test,
(b) fillet break test,
(c) tension-shear of spot weld, and
(d) peel test for spot weld
Mechanical tests: destroy the weld to test the weld strength

22. Inspection and Testing Methods: Destructive Testing -- Metallurgical Tests
Metallurgical tests – to examine metallic structure, defects, extent and condition of heat affected zone, presence of other elements, etc. of the weldment
Microscopy is an example of this
Metallurgical tests: weld is destroyed to obtain and prepare test specimen

23. Weldability
Capacity of a metal or combination of metals to be welded into a suitable structure, and
For the resulting weld joint(s) to possess the required metallurgical properties to perform satisfactorily in intended service
Good weldability characterized by
Ease with which welding is accomplished
Absence of weld defects
Strength, ductility, and toughness in welded joint

24. Weldability Factors: Welding Process
Some metals or metal combinations can be readily welded by one process but are difficult to weld by others
Example: stainless steel readily welded by most arc welding and resistance welding processes, but difficult to weld by oxyfuel welding
Welding process also affects weldability

25. Weldability Factors: Base Metal
Some metals melt too easily; e.g., aluminum
Metals with high thermal conductivity transfer heat away from weld, which causes problems; e.g., copper
High thermal expansion and contraction in metal causes distortion problems
Dissimilar metals pose problems in welding when their physical and/or mechanical properties are substantially different
Dissimilar metals: different melting points, different thermal expansion coefficients

26. Other Factors Affecting Weldability
Filler metal. Must be compatible with base metal(s)
Surface conditions (how clean)
Moisture can result in porosity in fusion zone
Oxides and other films on metal surfaces can prevent adequate contact and fusion

27. Principle of the process
Structure and configuration
Process modeling
Defects
Design For Manufacturing (DFM)
Process variation

28. Design Considerations in Welding
Minimum parts ‑ welded assemblies should consist of fewest number of parts possible
Example: usually more cost efficient to perform simple bending operations on a part than to weld an assembly from flat plates and sheets
Design assembly to allow for welding gun access
Assembly should allow for welding from above
- Design of assembly should allow flat welding (welding from above) to be performed as much as possible, since this is the fastest and most convenient welding position