ME 330Manufacturing Processes

WELDING PROCESSES(cont.)

Principle of the process

Structure and configuration

Process modeling

Defects

Design For Manufacturing (DFM)

Process variation

Weld Joints: Five Types

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

(a) Inside single fillet corner joint;

(b) outside single fillet corner joint;

(c) double fillet lap joint;

(d) double fillet tee joint

Weld Types: Fillet Welds

Filler metal is used

©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

(a) square groove weld;

(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)

Weld Types: Groove Welds

Filler metal is used

Weld Types: Plug Weld and Slot Weld

(a) Plug weld and (b) slot weld

Filler metal is used

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: Spot Weld and Seam Weld

Weld Types: Flange Weld and Surfacing Weld

(a) Flange weld;

(b) 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

Cross section of a typical fusion welded joint:

(a) principal zones in the joint, and (b) typical grain structure

Typical Fusion Welded Joint

Weld Quality

Motivation:

Obtaining a weld joint that is strong and absent of defects

Topics:

1. Residual stresses and distortion

2. Welding defects

3. Inspection and testing methods

Residual Stresses and Distortion

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

Butt welding

Shrinkage

Residual stress warping of weldment

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

Welding cracks: On weld or near weld Very serious because strength is reduced significantly Caused by low of weld and/or base metal combined with high

stresses during contraction

Weld Defects: Welding Cracks

Welding Defects: Cavities

Two defect types:

1. 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

2. Shrinkage voids - cavities formed by shrinkage during solidification

Welding Defects: Solid Inclusions

Nonmetallic material entrapped in weld metal:

1. Most common form is generated during arc welding processes that use flux. Instead of floating to top of weld pool, globules of slag become encased during solidification

2. Other forms: formation of metallic oxides, such as Al2O3

when welding aluminum

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

Welding Defects: Incomplete Fusion

(a) Desired profile for single V-groove weld joint,

(b) undercut - portion of base metal melted away,

(c) underfill - depression in weld below adjacent base metal surface, and

(d) overlap - weld metal spills beyond joint onto part surface, but no fusion occurs

Weld Profile in Arc Welds

Inspection and Testing Methods: Visual Inspection

Most widely used welding inspection method is manual examination for:

1. Conformance to dimensions, warpage 2. Surface defects, such as cracks, cavities,

incomplete fusion

Limitations: Only surface defects are detectable

Inspection and Testing Methods:Nondestructive Evaluation (NDE) Tests

1. Ultrasonic testing - high frequency sound waves through specimen to detect cracks and inclusions

2. Radiographic testing - x‑rays or gamma radiation provide photographs of internal flaws

3. Dye‑penetrant and fluorescent‑penetrant tests - to detect small cracks and cavities at part surface

4. 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

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

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

1. Ease with which welding is accomplished

2. Absence of weld defects

3. 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

Weldability Factors: Base Metal

1. Some metals melt too easily; e.g., aluminum

2. Metals with high thermal conductivity transfer heat away

from weld, which causes problems; e.g., copper

3. High thermal expansion and contraction in metal causes

distortion problems

4. Dissimilar metals pose problems in welding when their

physical and/or mechanical properties are substantially

different

Other Factors Affecting Weldability

Filler metal. Must be compatible with base metal(s)

Surface conditions (how clean)

1. Moisture can result in porosity in fusion zone

2. 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