TPT March 2024

ARTICLE

OMNI-X

Mandrels with aluminium-bronze functional surface produced by thermal

cladding technology By Pavel Doubek, Kevin Quinn, Luboš Fišar from OMNI-X

Introduction and motivation Rotary draw bending with the use of an internal support mandrel is widely used, and considered the most accurate method of tube bending. Simply put, it is a technological process, that is produced through external bending moments induced by bending forces from the tools needed to bend the tube and to overcome all frictional resistances. Mandrels are used as an internal support for tube bending and their purpose is to mitigate defects such as flattening or crinkling of the inner radius of the bent tube. Selecting the proper mandrel type is critical to produce the desired bend. High demands are placed on the functional surfaces of the mandrels. The mandrel stress during the working process is closest to the tensile stress in bending; significant frictional forces between the tube and the mandrel are generated. In order to avoid excessive wear of the mandrel, and stretching and thinning of the tube, it is necessary to correctly select the material of the functional surface in combination with suitable lubrication for the material of the tube. With the right choice, the friction and the tensile loading will be reduced, which ultimately leads to a reduction in size of the total bending moment. The experimental campaign includes a study of the behaviour of the cladded layer during the bending process under controlled manufacturing conditions, its influence on the parameters and quality of bending and the effect on the estimated theoretical life of the tool. Bi-material mandrel shanks are made of tool steel to which thin functional layers of aluminium bronze are applied by a thermal cladding technology.

OMNI-X has developed and produced bi-material mandrels to improve its mechanical properties (such as hardness, abrasion resistance and its slide properties) while maintaining optimal mechanical properties of the steel core, leading to an extension of its lifetime. Material combinations and thickness of the different metallic layers are chosen during design or restoration of the mandrel. Also taken into consideration for the particular friction pair is the industry sector, reliable uniform production, and the ability of the surface to carry the lubricant. Figure 1 demonstrates an example of a bi-material mandrel. Cladding technology is one of the methods of manufacturing the functional surfaces of mandrels. It is an innovative approach to solving the aforementioned issues. The principle of thermal cladding technology is a process that involves the heating of a metal powder that is melted together with the base material, creating a surface layer with a strong metallurgical bond, between new the metallic surface and substrate. This method is suitable for creating new functional surfaces with specific properties and for carrying out high quality repairs as well. The advantages of this technology is a smaller heat affected area and minimal impact from temperature, reducing the risk of thermal deformation. It has excellent adhesion and cohesion properties offering a wide range of possible material combinations. The cladding technology is an effective replacement of older processes which may be described as problematic from a health or environmental point of view, for example, the electrochemical process of applying hard chrome. Experimental campaign Thermally applied cladding is suitable for mandrels used for bending tubes from OD 88.9mm and bigger, however the technological minimum is approximately OD 50mm. The five-balls close pitch mandrel for bending stainless steel tubes OD 101.6 with wall thickness 2mm was chosen for the experimental campaign. The basic material (the core) of the shank was produced of tool steel to which a thin functional surface layer of aluminium-bronze was applied. The thickness of the cladded layer varied between 1.0 and 1.2mm.

Figure1: Example of the mandrel with bi-material shank (aluminium bronze/tool steel)

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MARCH 2024