Cladding is the bonding together of dissimilar metals. It is different from fusion welding or gluing as a method to fasten the metals together. Cladding is often achieved by extruding two metals through a die as well as pressing or rolling sheets together under high pressure.
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The United States Mint uses cladding to manufacture coins from different metals. This allows a cheaper metal to be used as a filler. For example, dimes and quarters struck since have cores made from pure copper, with a clad layer consisting of 75% copper and 25% nickel added during production. Half dollars struck from to for circulation and in for collectors also incorporated cladding, albeit in the case of those coins, the core was a mixture of 20.9% silver and 79.1% copper, and its clad layer was 80% silver and 20% copper. Half dollars struck since are produced identically to the dimes and quarters.
Laser cladding is an additive manufacturing approach for metal coatings or precise piece restorations by using high power multi-mode optical fiber laser.[1]
In roll bonding, two or more layers of different metals are thoroughly cleaned and passed through a pair of rollers under sufficient pressure to bond the layers. The pressure is high enough to deform the metals and reduce the combined thickness of the clad material. Heat may be applied, especially when metals are not ductile enough. As an example of application, bonding of the sheets can be controlled by painting a pattern on one sheet; only the bare metal surfaces bond, and the un-bonded portion can be inflated if the sheet is heated and the coating vaporizes. This is used to make heat exchangers for refrigeration equipment.[2]
In explosive welding, the pressure to bond the two layers is provided by detonation of a sheet of chemical explosive. No heat-affected zone is produced in the bond between metals. The explosion propagates across the sheet, which tends to expel impurities and oxides from between the sheets. Pieces up to 4 x 16 metres can be manufactured. The process is useful for cladding metal sheets with a corrosion-resistant layer.[2]
Laser cladding[3][4] is a method of depositing material by which a powdered or wire feedstock material is melted and consolidated by use of a laser in order to coat part of a substrate or fabricate a near-net shape part (additive manufacturing technology).
It is often used to improve mechanical properties or increase corrosion resistance, repair worn out parts,[5][6] and fabricate metal matrix composites.[7] Surface material may be laser cladded directly onto a highly stressed component, i.e. to make a self-lubricating surface. However, such a modification requires further industrialization of the cladding process to adapt it for efficient mass production. Further research on the detailed effects from surface topography, material composition of the laser cladded material and the composition of the additive package in the lubricants on the tribological properties and performance are preferably studied with tribometric testing.
A laser is used to melt metallic powder dropped on a substrate to be coated. The melted metal forms a pool on the substrate; moving the substrate allows the melt pool to solidify in a track of solid metal. Some processes involve moving the laser and powder nozzle assembly over a stationary substrate to produce solidified tracks. The motion of the substrate is guided by a CAM system which interpolates solid objects into a set of tracks, thus producing the desired part at the end of the trajectory.
Automatic laser cladding machines are the subject of ongoing research and development. Many of the process parameters must be manually set, such as laser power, laser focal point, substrate velocity, powder injection rate, etc., and thus require the attention of a specialized technician to ensure proper results. By use of sensors to monitor the deposited track height and width, metallurgical properties, and temperature, constant observation from a technician is no longer required to produce a final product. Further research has been directed to forward processing where system parameters are developed around specific metallurgical properties for user defined applications (such as microstructure, internal stresses, dilution zone gradients, and clad contact angle).
While many people can identify a cladded product by looking at it, not many people know how the process works. At Materion we have been cladding since and have a deep understanding of the process. In this article we'll dive into how clad products are made.
While there are many process variations of cladding, we specialize in cold roll bonding (CRB). In this method, cladding is achieved at room temperature using pressure at the bonding mill. This method differs from hot roll bonding (HRB) which uses heat above the metals’ recrystallization temperature along with pressure to join the two metals together. CRB's advantages over HRB include:
Figure 1. Metal alloy combinations in cold roll bond contrasted against electroplating and electron beam welding.
Materion can produce inlay clad, overlay clad, and Dovetail Clad® materials. See Table 1 for details on each version. Regardless of the cladding method, there are three key steps to cladding metals: cleaning, bonding, and sintering.
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Table 1. Review of Clad Types Offered by Materion
CLEANING
The key to getting good clad metal starts well before the bonding mill. Surface preparation is important and often needs to be done in two steps. First, a degreasing step is applied to remove any grease or debris that may be on the metal. If grease or dust gets between the two layers, it can cause delamination or blisters in subsequent steps. Once the metal is clean it then needs to be brushed. Brushing removes the oxide layer and adds texture to the metal. Both are critical to achieve a good bond.
BONDING
When the metals are clean enough, they are brought to the bonding mill which is similar to a rolling mill. Here, the material undergoes severe plastic deformation through a 60-75% thickness reduction in one pass (Fig 2). The texture applied at the brushing step reduces the overall pressure needed to achieve this reduction.
At the roll bite, two things are happening simultaneously. First, heat is being generated due to the high reductions of the metals. Second, the metal is being stretched linearly, exposing new, unoxidized metal surfaces at the interface. These conditions allow for what is known as a green bond to form between the two metals because there is enough heat and pressure generated to overcome the activation energy for the two unoxidized surfaces to adhere together.
Figure 2. Bonded material leaving the roll bite on the right shows significant reduction compared to unbonded material on the left.
This green bond is a mechanical bond formed between the two metals. Generally, the higher the reduction the metals undergo, the stronger the green bond will be. However, there is a limit to the amount of cold working a metal can undergo before it becomes too brittle to work with. These characteristics must be balanced when designing the bond parameters.
While it may seem like a simple process, there are many other parameters that must be controlled to get a perfect bond. These include speed, lubrication of the rolls, front and back tension, temperature control to avoid overheating, temper of the metals, etc.
SINTERING
The green bond created at bonding is relatively strong, but not strong enough to withstand the highly technical applications our customers use clad metals for. To achieve our customers’ requirements, the clad metal must be sintered. This drives diffusion between the two metals and creates a true metallurgical bond but keeps the individual layers intact (Fig 3). After this step, the material can then be handled like any other strip metal.
Figure 3. Fully sintered clad showing three distinct layers.
Understanding how clad metals are made is essential for appreciating their benefits. In combining two or more dissimilar metals, the resulting material can deliver properties from all those materials. Learning more about how the metals are combined could provide the solution to your materials challenges. In the next article, we will explore the numerous benefits of clad metals and their applications across various industries.
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