Additive manufacturing (AM) is an advanced manufacturing process that provides the opportunity to build geometrically complex and highly individualized lightweight structures. Despite its many advantages, additively manufactured components suffer from poor surface quality. To locally improve the surface quality and homogenize the microstructure, friction stir processing (FSP) technique was applied on Al-Si12 components produced by selective laser melting (SLM) using two different working media. The effect of FSP on the microstructural evolution, mechanical properties, and corrosion resistance of SLM samples was investigated. Microstructural investigation showed a considerable grain refinement in the friction stirred area, which is due to the severe plastic deformation and dynamic recrystallization of the material in the stir zone. Micro-hardness measurements revealed that the micro-hardness values of samples treated using FSP are much lower compared to SLM components in the as-built condition. This reduction of hardness values in samples treated with FSP can be explained by the dissolution of the very fine Si-phase network, being characteristic for SLM samples, during FSP. Surface topography also demonstrated that the FSP results in the reduction of surface roughness and increases the homogeneity of the SLM microstructure. Decreased surface roughness and grain size refinement in combination with the dissolved Si-phase network of the FSP treated material result in considerable changes in corrosion behavior. This work addresses the corrosion properties of surface treated additive manufactured Al-Si12 by establishing adequate microstructure-property relationships. The corrosion behavior of SLM-manufactured Al-Si12 alloys is shown to be improved by FSP-modification of the surfaces.
The degradation of pump components by corrosion and complex, simultaneous damage mechanisms, e.g., erosion-corrosion and cavitation-corrosion leads to high costs through replacement and maintenance of parts. To increase the lifetime of cost-efficient components with superior casting properties, surface welding of duplex stainless steel on gray cast iron parts was performed using inert shielding gas metal arc surface welding (GMA-SW) and plasma transferred arc surface welding (PTA-SW). The thermal conductivity of the used shielding gas and the preheating temperature influenced the dilution of the surface layers, which had a major impact on the corrosion resistance and the microstructure. Lower cooling rates enhanced diffusion and lead to precipitation of carbides. High heat input and prolonged cooling times during surface welding resulted in high dilution and a carbide network. The corrosion resistance in artificial seawater of those surface layers was substantially reduced compared to surface layers with lower heat input and higher cooling rates. The corrosion of the surface layers in the potentiodynamic polarization test was driven by selective corrosion of the phase boundary between Cr-carbides and Cr-depleted austenite. Passive behavior was observed for surface layers with low dilution, which had homogeneous chromium distribution and no carbide networks. In conclusion, the corrosion behavior of gray cast iron was improved by surface welding with duplex stainless steel. The corrosion resistance of the surface layers produced with PTA-SW with no preheating exceeded that of the surface layers produced with GMA-SW and came close to those of a commercially available duplex stainless steel used as reference material. Keywords dilution Á duplex stainless steel Á electrochemical corrosion Á gas metal arc welding Á gray cast iron Á microstructure Á plasma transferred arc welding Abbreviations GMA-SW Gas metal arc surface welding PTA-SW Plasma transferred arc surface welding SEM Scanning electron microscope EDX Energy-dispersive x-ray spectroscopy EIS Electrochemical impedance spectroscopy CPE Constant phase element List of Symbols X i Concentration of element X in the surface layer X B Concentration of element X in the base metal X W Concentration of element X in the consumable R s Solution resistance R ct Charge transfer resistance C dl Double-layer capacitance This article is an invited paper selected from presentations at the
In this study forming tools temperated at 24 °C and 350 °C were used to systematically investigate the influence of different cooling rates on the mechanical and corrosion properties of a high strength aluminum alloy AA7075 within a novel thermo‐mechanical process that combines forming and quenching simultaneously. The samples formed within heated tools reveal higher ductility and lower material strength compared to the parts processed in cold tools. In addition, the corrosion behavior changed between samples formed with 24 °C forming tools and 350 °C forming tools, respectively. Through cyclic polarization in chloride containing aqueous media a change in the hysteresis and shift of open circuit potential was observed. Metallographic investigation revealed that there was also a very different corrosion morphology for the samples formed within the heated tools. No change in average grain size could be detected but changes of the microstructure in subgrain scale that occur during the forming within the heated tools are responsible for this effect. In further research, the effect of various cooling rates on mechanical and corrosion behavior and the microstructure will be investigated by variation of the forming tool temperature.
The good capacity of gray cast iron for the manufacture of complex geometry components is widely recognized, but its low resistance to corrosion and low weldability complicate the use of this material for some industrial applications. The corrosion resistance can be improved by metallic surface layers using welding processes with low percentages of dilution between the filler and base material. However, the welding processes impose very high heat load on the base material, which in the case of cast iron could promote the formation of hard and brittle microstructures, facilitating the formation of cracks. This work deals with weld beads of duplex steel on lamellar gray cast iron made either by plasma-transferred arc powder (PTA-P) or by metal inert gas (MIG) using the cold metal transfer (CMT) technology, with emphasis on achieving low dilution, hardness and imperfections (internal porosities). Preheating was used to reduce the hardness in the heat-affected zone, while different levels of helium were added in the shielding gas to study its effect on the geometry and hardness of the weld beads. The results showed that the PTA-P process resulted in lower values of dilution and hardness because of a low cooling rate compared to that of the MIG-CMT process. In addition, it was observed that preheating the base material reduced the hardness of the heat-affected zone but increased the dilution of the weld bead.
Surface welding of duplex stainless steel on lamellar grey cast iron was performed to produce thick (several millimetres) protective surface layers with a regulated gas metal arc welding process. As dominant parameters, the influence of the composition of the shielding gas (argon-helium mixture) as well as preheating temperature on the corrosion resistance were investigated. Both the addition of helium to shielding gas and preheating of the base material increase the heat input and reduce the cooling rates and result in higher dilution. This leads to reduced hardness in the heat affected zone and prevents cracking. However, the formation and portion of carbides in the surface layers increase. The influence of these parameters on the microstructure in the surface layers as well as in the heat affected zone and the influence on the corrosion behaviour in artificial seawater were investigated with immersion tests and potentiodynamic polarization measurements. The corrosion properties of the parent lamellar grey cast iron were greatly enhanced by the surface layers, but inferior to duplex stainless steel. With additional metallographic investigations the corrosion mechanisms were investigated. The corrosion mechanisms are extensively influenced by the phase distribution in the surface layers and by the formation of carbides.Keywords: Grey cast iron / surface welding / regulated gas metal arc welding process / microstructure / corrosion Corresponding author: B. Heider, Zentrum fü r Konstruktionswerkstoffe (MPA-IfW Darmstadt), TU Darmstadt,
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