Joining-Aluminum-MMC:Is Welding Suitable?SOLUTIONS with Effective, Practical Advice
(Sponsored Links) Although various studies were dedicated to research feasibility and properties of such weldments, it is affirmed that the welding technology is not yet mature and freely applicable as the relevant information is restricted because of proprietary rights or for security reasons if military applications are involved. Aluminum matrix reinforcements are generally particles of aluminum oxide, of silicon carbide or carbon whiskers. The particulate structure is preferred to fiber reinforcements because of their improved ease of manufacturing, lower cost and almost isotropic properties. Aluminum MMC have increased stiffness, strength and wear resistance, with higher thermal conductivity and lower coefficient of thermal expansion, when compared to their respective unreinforced monolithic aluminum alloys. Joining-Aluminum-MMC is quite dissimilar from welding monolithic metal, because of the presence of the microstructurally different inclusions, but the usual cleaning procedures and inert gas shielding applied for unreinforced metal must be implemented. The reinforcement additions to the aluminum matrix modify physical and chemical properties that influence weldability and require understanding. When arc welding by classic means, viscosity of the molten puddle is increased by the solid unfused particles present. Flow and wetting are found to be lower than those of unreinforced aluminum, as heat flow is less efficient. These characteristics make welding practice more difficult to unaccustomed welders, besides affecting final microstructures and residual stress distribution. Joining-Aluminum-MMC is influenced also by chemical reactions between matrix and reinforcements. Molten aluminum contacting solid particles can bring about the formation of new and unwanted phases that reduce mechanical properties. If fusion welding is applied, it is preferred in general to use rapid thermal cycles with limited heat input. Otherwise solid state welding should be selected. Depending on the specific dispersed phase, aluminum carbide (Al4C3) could form. The amount produced of this substance should be kept low by limiting the time at matrix melting temperature because its shape in clustered or coarse form is detrimental to mechanical properties and furthermore because, if exposed to water it will dissolve, leaving porosity and loss of integrity in the weld. Alternatively increased presence of silicon would limit the formation of aluminum carbide. In aluminum-magnesium or aluminum-copper reinforced matrix materials, the formation of magnesium or copper spinel at the interface between metal and reinforcing particles is likely to appear, but those phases are not considered to affect weld mechanical properties. The solidification process in Joining-Aluminum-MMC by welding is affected by the presence of reinforcing particles in the weld pool that are pushed ahead of the solidification front. This fact locally disturbs heat flow and mass transport and seems to improve resistance to solidification cracking. A few modifications may be needed to adapt geometry and dimensions of joints to the reduced fluidity of the molten matrix material. Several welding processes have been used to perform Joining-Aluminum-MMC, including GTAW, GMAW, EBW, LBW and RW, but also friction welding (FRW), friction stir welding (FSW), transient liquid phase (TLP) bonding, and capacitor discharge welding (CDW). GTAW is applied with the same equipment and consumables used for monolithic aluminum alloys with low and controlled heat input, but the formation of secondary phases during welding should be monitored by metallographic examinations and by mechanical testing. Welders may find it difficult, at least initially, to form properly and to advance correctly the weld pool. GMAW too can be used, with special attention in preparation and cleaning of the surfaces. The application of a grooved backing bar, of back gouging and of vigorously scrubbing with a clean stainless steel wire brush between passes is recommended. Laser beam and electron beam welding were successfully employed for Joining-Aluminum-MMC sometimes using shims of filler metal preplaced in the joints. Any new application would need a thorough development program with complete review of the results. The Aluminum Metal Matrix Consortium Read more on Joining-Aluminum-MMC in the following online publication: Any questions or comments or feedback? Write them down and send them to us by e-mail. Click on the Contact Us button in the NavBar at top left of every page.
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