Cold Forming Of Aluminum/Iron-Based And Aluminum/Titanium Clad Pipe And Oxidation Of Aluminum Layer

The stainless steel pipe-fittings in the test blanket module of fusion engineering aredemanded to possess high properties in electrical isolation and anti-corrosion. Ceramic coatinglike Al₂O₃on the inner surface of the pipe-fittings is needed to attain this goal. However, it isdifficult to prepare Al₂O₃layer on the inner surface of the pipe joints, such as T-shapes and elbows,owing to their complex profile and irregular surface. In addition, in the fields of aeronautics,astronautics, and petrochemical industry, Al₂O₃coating in the irregular titanium pipe joints is alsoa feasible method to overcome the defects of titanium alloys products, including erosion andscouring abrasion.A new technical route was proposed to solve these problems in this study. The steps of thenew method were as follows: firstly, stainless steel or titanium pipes were metallurgical bondedwith aluminum pipes through explosive welding. Then, the forming clad pipes were processedinto clad T-shapes and elbows by means of cold push-bending and hydroforming respectively.Finally, the aluminum layer of as-prepared pipe-fittings was oxidized into Al₂O₃by micro-arcoxidation. The complex pipe-fittings with ceramic layer of Al₂O₃were obtained through the newtechnical route. The key technology of the study was including preparation of clad pipe with highquality, plastic forming mechanism of clad elbow and clad T-shapes and the oxidation of thealuminum to obtain the dense α-Al₂O₃layer.Al-316L SS, Al-Fe and Al-CLAM steel clad pipes were prepared by explosive welding whichexhibited high dimensional accuracy, inner surface flatness and bonding strength. The bondingstrength of the clad pipes at the initial explosive end, middle part, and tail end was characterizedthrough pressure-shear test, radial flattening test, and bending test. The results showed thatAl-316L, Al-Fe and Al-CLAM samples displayed bonding strength of76.0,75.6and73.8MPa,respectively. And adhered interface could keep well-bonded when the pipes were bent at an angleexceeded138°and radial flattened by33%. Therefore, the pure aluminum layer was stronglybonded with matrix alloys and the clad pipe could undergo large deformation in the later plasticdeformation.The cold push-bending process of the Al-316L SS clad pipes was simulated by FE software.The effects of relative radius, mandrel material and friction coefficient on the quality ofpush-bended parts were investigated by the numerical simulation. For the tube blanks with thesame diameters, larger relative radius of gyration at the bended part led to lower distortion ofcircular section, better material filling quality and homogeneity of thickness distribution, indicating better quality of plastic formed pipes. Using rigid material and low melting point alloyas mandrel in the forming process could apparently reduce the distortion of circular section.Among them, low melting point alloy was more favorable than rigid mandrel due to its muchlower interfacial shear stress. In addition, lower friction coefficient resulted in smaller shear stressand better homogeneity of thickness distribution. For the Al-Fe clad pipes, the shear stress,equivalent stress and strain were analyzed during the cold push-bending process. Based on thecold push-bending results of Al-CLAM pipes with rectangular section and straight segment on theends, plastic formation mechanism differed from circular pipes was proposed. The material waseasy to flow along the axial direction but difficult to flow over the rigid ridge of tube blank.Therefore, extrados with large deformation exhibited severe thinning while intrados exhibitedsuppressed thickening.The hydroforming process of the Al-316L clad pipe was also simulated by FE software.Historical curves of nodes on both the stainless steel and aluminum layer at the area of protrusionand fillet were investigated during the hydroforming process. Also, the thickness distribution oflayers at crucial sections was studied and compared with that of single layer stainless-steelT-shapes. In addition, process parameters including inner pressure, friction coefficient and feeddistance were optimized based on the analysis of their influence on the protrusion height and wallthinning rate. It indicated that proper inner pressure and lubricant with lower friction coefficientshould be selected during the forming process. Feed distance at axial direction exhibited stronginfluence on the protrusion height, but poor relevance with the thickness homogeneity. Similarly,the effects of the inner pressure and friction coefficient on the protrusion height were alsoinvestigated for the Al-CLAM clad pipe. Finally, these clad pipes were practically hydroformedinto T-shapes according to the simulation results.Same technical route was performed to prepare Al₂O₃layer on the inner surface of TA1pipe-fittings. Aluminum pipe and TA1pipe were firstly cladded by explosive welding. Themicrostructure of the Al-TA1clad pipe was characterized including morphology, elementdistribution and phase structure. The interfacial bonding strengths between the layers, radialflattening and compression properties were measured to evaluate the quality of welded pipes. Theclad pipe-fittings were successfully fabricated by cold push-bending and hydroforming process.Micro-arc oxidation technology was employed to fabricate Al₂O₃layer on the inner surfaceof the prepared clad pipe-fittings based on Al-316L SS, Al-Fe, Al-CLAM steel and Al-TA1. Wesystematically studied the influence of the electrolyte component, current density, oxidizationduration on the Al₂O₃layer's thickness, micro-hardness, surface roughness and crystal structure.Consequently, mechanism of micro-arc oxidation to obtain the high content of α-Al₂O₃was proposed. Al₂O₃layer with length of145μm and α-Al₂O₃phase proportion of67.2%wasprepared through micro-arc oxidation at current density of20A/dm²for90min in optimizedelectrolyte, containing2g/L potassium hydroxide,3g/L sodium silicate,5g/L sodiumhexametaphosphate and10g/L sodium tartrate. Al₂O₃/Cr₂O₃composite layer was fabricated usingthe electrolyte contained Cr₂O₃particles. The properties of the ceramic layer such as bondingstrength, wear resistance, erosion resistance and electrical isolation was evaluated. The resultsindicated that bonding strength between Al₂O₃and matrix, composite layer and matrix was largerthan55N and60N respectively, while the composite layer exhibited better wear resistance thansingle Al₂O₃layer. Samples with Al₂O₃layer displayed natural corrosion potential (Ecoor) of-0.26V and corrosion current (Icorr) of1.1E-7, while those with composite layer displayed Ecoorof-0.23V and Icorr of7.56E-8. Al₂O₃ceramic layer displayed good anti-erosion performance andexcellent volume resistivity of1.73×10¹² cm, much larger than the designed value of10⁴Ω m.