1. Clad Aluminum
The clad aluminum materials are most commonly designated as Alclad materials.

2. Resistance Welding Learning Activities View Slides; Lesson Objectives
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Lesson Objectives
When you finish this lesson you will understand:
Keywords

3. Copper Containing 2xxx More-anodic unalloyed 1xxx
These clad materials are produced by a roll bonding process where the core usually made from the 2xxx or 7xxx series aluminum are coated with the more anodic 1xxx series aluminum for better corrosion protection.
Core Clad
Copper Containing 2xxx More-anodic unalloyed 1xxx
Zn, Mg, Cu (7049, 7050) Al-Zn 7072

4. Cross Sectional Areas of Spot Weld
Area G
Area F
Area A
The above slide presents the photographs of cross sectional areas of spot weld in alloy 2024 clad aluminum; the various weld-zone structures are shown at 250X magnification, photographically reduced in this illustration. Those areas include:
Area A: Heat affected “burnt” zone showing two phase field that exists
Area B: Start of fusion of cladding contact surfaces
Area C: Unaffected parent metal with a portion of the cladding
Area D: Central dendritic zone
Area E: Transition zone -- central to columnar dendritic
Area F: Columnar dendritic zone
Area G: Transition zone -- columnar dendritic to heat affected
On both sides of the weld where melting has not occurred, the clad material can bee seen. In area B right at the edge of the weld the aluminum cladding is seen. This has fully melted and flowed causing no defects. In the weld metal, the clad has completely dissolved into the melt also causing no defects. Thus, good welds are made in all of these regions.
Area E
Area B
[Reference: Welding, p.11-7,
Kaiser Aluminum & Chemical
Sales, Inc.]
Area D
Area C

5. Spot Welds in 0.102-in Alclad
Material - Two Pieces of 2024-T Alclad
Equipment - Three-Phase Direct-Energy
Machine - Using Forging Force And
Postheat
The above slide shows typical, nearly perfectly formed spot welds in two pieces of in thick 2024-T Alclad. Note that the cladding on the faying surfaces has become completely absorbed with the base metal into the weld nugget. There is an absence of severe indentation, and the outer surfaces (in contact with the electrodes) show no heat effect. The welds show no inclusions, voids or cracks, and they may be considered as the best obtainable with present knowledge of the art. The three nuggets show good consistency which is equally as important as actual weld quality. In the final analysis, how nearly any equipment or process can approach the results shown in the above slide, both in quality and consistency, will determine its real worth.
A working knowledge of the structure of the spot weld is necessary for an understanding of its physical properties from which is determined the quality of the weld. The above slide shows three macrographs of high quality spot welds in in thick 2024-T Alclad. These may be considered typical for this material. Each weld consists of a central oval core which constitutes a granular or equal-axial zone. Surrounding this zone is another made up of columnar or dendritic type of grain structure. The area of these two zones constitutes the weld nugget. Both are a cast structure, and their relative sizes are an indication of welding technique employed. The considerable columnar zone is the result of a welding technique employing a postweld heat treatment of sufficient length of time to permit grain growth and orientation as shown.[continued]
[Reference: Resistance Welding Manual, p.11-4, RWMA]

6. Spot Welds (a) Illustrating Fusing of the Two Layers of Alclad
Outside the columnar zone is a light ring or band which consists of a layer of material which has been heated close to the melting point, and in which has occurred a segregation of eutectic in the parent material. This eutectic has been forced back into adjacent parent material which results in a darker ring immediately outside the lighter ring. This has been called the heat-affected zone.
The illustration shows the pure aluminum coating on each side of the stock and also how this coating is completely absorbed in the weld structure. Figure (a) at magnification of 50X illustrates in great detail the structure of a spot weld as described above. Figure (b) shows the appearance of a typical spot weld generally similar to those in the previous slide. In this case, the section is taken parallel to the plane of the material near the clad interfaces.
(a) Illustrating Fusing of the Two
Layers of Alclad
(b) Section Taken Parallel to the
Plane of the Material Welded
- Very Near to the Clad Interfaces
[Reference: Resistance Welding Manual, p.11-8, RWMA]

7. Nugget Formation with Different Weld Time
Material T Alclad
Equipment - Stored-Energy Machine
Top Section - Normal Elliptical Shape
with Proper Timing
Center & Bottom - The Effect of Increased
and Excessive “Weld Time” and the
Development of Hourglass Shape
The formation of the weld nugget is a function of welding current, time and force applied to the electrodes. With any given set of these variables the nugget starts with a pin point in the center and grows outward in all directions. It is, therefore, the function of the controls to exactly and consistently proportion the weld nugget. As might be expected, the higher the current density, the more rapidly the nugget is formed. The longer the duration of the current the closer the boundaries of the nugget get to the electrode. The above slide shows the development of the nugget with an excess of time. The first or top weld shows a normal, properly timed weld. The hourglass shape is caused by the cooling effect of the electrode immediately over the center of the weld.
If the current density is too low, proper nugget formation will be too slow, resulting in excessive heat; consequently, it will warp the surrounding area. If it is too high, porosity and metal expulsion will result causing weakness and sheet separation. Low current density may be partially compensated by lowered welding force; likewise, high current density must be accompanied by higher force. However, the higher the force, the greater the electrode indentation and sheet separation.
[Reference: Resistance Welding Manual, p.11-5, RWMA]

8. Nugget Formation in Two Unequal Thickness
Spot Welds in 5052-H34 Alclad
Spot welds made in unequal thickness of the same alloy, generally follow the requirements for the thinner pieces. The electrode contacting the thinner piece should be of the size and contour recommended for that thickness. The electrode for the thicker piece should be the same as recommended for that thickness, or if preferred, it may be flat. A flat electrode should not be used against a thinner stock. There will be no weld nugget penetration in the thin piece.
The above slide shows typical weld nugget formation in two unequal thicknesses of the same alloy.
Material in to in H34 Alclad
Equipment - Three-Phase Direct-Current Machine
[Reference: Resistance Welding Manual, p.11-10, RWMA]

9. Nugget Formation in Two Unequal Thickness
Spot Welds in 2024-T Alclad - 7X
The above slide shows the nugget formation when welding in 2024-T Alclad to in. Note the heat-affected zone both in the center of the in plate and its outer edge. In all these welds, the shape of the nugget will be affected to some extent by the size and shapes of the electrodes used.
Material in to in T Alclad
Note - Heat effect at outer surface of in sheet
[Reference: Resistance Welding Manual, p.11-11, RWMA]

10. Nugget Formation in Three Thicknesses
Spot Welds in 5052-H34 Alclad - 8-1/2X
Spot Welds in 2024-T Alclad - 10X
Figure (a) shows the weld structure of 5052-H in to in to in. In this setup each joint had an individual weld with the center of the central sheet being virtually unaffected by the heat. The welding equipment is electrostatic stored-energy machine. Compared with Figure (b), in which the two joints became merged into a single inclusive weld nugget.
(a) in to in to in (b) in to in to in
[Reference: Resistance Welding Manual, p.11-10, RWMA]

11. Nugget Formation in Dissimilar Alloys and Unequal Thicknesses
The above slide shows a series of four welds made in in 5052-H34 to T Alclad with an electrostatic stored energy welding machine. These welds illustrate a typical nugget formation when spot welding dissimilar alloys and thickness.
Material in-5052-H34 to in-2024-T Alclad
Equipment - Electrostatic stored-energy Machine
[Reference: Resistance Welding Manual, p.11-19, RWMA]