ME 330 Manufacturing Processes WELDING PROCESSES (cont.)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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:

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

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

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

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

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

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

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

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

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

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

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