In this work, a functionally graded material (FGM) part was fabricated by depositing a Cu-based alloy on top of a high strength low alloy (HSLA) steel by twin-wire and arc additive manufacturing (T-WAAM). Copper and steel parts are of interest in many industries since they can combine high thermal/electrical conductivity, wear resistance with excellent mechanical properties. However, mixing copper with steel is difficult due to mismatches in the coefficient of thermal expansion, in the melting temperature, and crystal structure. Moreover, the existence of a miscibility gap during solidification, when the melt is undercooled, causes serious phase separation and segregation during solidification which greatly affects the mechanical properties. Copper and steel control samples and the functionally graded material specimen were fabricated and investigated using optical microscopy, scanning electron microscopy, and high energy synchrotron X-ray diffraction. Retained δ-ferrite was found in a Cu matrix at the interface region due to regions with mixed composition. A smooth gradient of hardness and electric conductivity along the FGM sample height was obtained. An ultimate tensile strength of 690 MPa and an elongation at fracture of 16.6% were measured in the FGM part. 

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Steel-copper functionally graded material produced by twin-wire and arc additive manufacturing (T-WAAM)

Steel-copper functionally graded material produced by twin-wire and arc additive manufacturing

Six different composition steels were designed based on CD3MWCuN, a common industrial alloy formulated to have a microstructure consisting of 50 pct ferrite and 50 pct austenite. The steels were designed to have ± 10 pct Cr, Ni, and Mo over the nominal composition of CD3MWCuN and were obtained by remelting a keel bar of the base alloy and adjusting the composition. The + compositions variations were achieved simply by adding additional Cr, Ni, or Mo, while the − variations were obtained by adding additional amounts of the remaining major elements to the nominal composition of the keel bar. In this way it was possible to produce a series of alloys where the Cr/Ni equivalent ratio, Creq/Nieq, as calculated using the equations first employed by Schoffler varied from ≈ 1.35 to 1.65. A series of homogenization temperatures ranging from 1000 °C to 1250 °C were used to produce a uniform starting microstructure and the alloys were cooled at rates ranging from 500 °C to 1000 °C/hour from these homogenization temperatures. Air-cooled and water-quenched samples were examined as well. Based on observations of the microstructure a generalized ferrite prediction diagram was developed that takes into account homogenization temperature, cooling rate, and chromium-to-nickel equivalence ratio (Creq/Nieq) to predict the amount of ferrite that can be expected in the alloy. This diagram represents the first major evaluation of ferrite content as a function of homogenization temperature and cooling rate specifically for cast duplex alloys.

Influence of Cooling Rate on the Ferrite Prediction Diagram of Duplex Stainless Steel Castings

The influence of homogenization temperature and rapid cooling rate on the ferrite content and microstructures of CD3MWCuN was investigated using BSE-SEM images. For these experiments CD3MWCuN was homogenized at six different temperatures 1000 °C, 1050 °C, 1100 °C, 1150 °C, 1205 °C, and 1250 °C for 4 hours followed by air-cooling and water quenching. In addition to these samples two temperatures, namely 1150 °C and 1205 °C, were chosen and samples were cooled at the varying rates of 500 °C/h, 750°C/h, 850 °C/h, and 1000 °C/h. Determination of the resulting ferrite/austenite ratio revealed that homogenization temperature plays the dominant role in determining the final ferrite content. Slower cooling rates drop the amount of ferrite in a linear manner that is approximately constant for the different homogenization temperatures studied. An equation is proposed to predict the ferrite content covering the homogenization temperatures from 1000 °C to 1250 °C given that the initial ferrite amount present at the homogenization T is known.

Influence of Rapid Cooling Rate and Homogenization Temperature on the Ferrite Content and Microstructures of CD3MWCuN Castings

Development of coated electrodes for welding of Super Duplex steel.

Highly alloyed CD3MWCuN was chosen to investigate the influence of cooling rates on the ferrite content of duplex stainless steels. It is known that cooling rate can influence the overall ferrite/austenite ratio, and achieving the proper balance of phases is crucial in producing a casting with suitable mechanical and corrosion properties. Initial experiments were carried out to study the ferrite/austenite ratio as a function of cooling involved samples obtained using different cooling rates through the solidification temperature as well as different cooling rates from the homogenization temperature. A single keel bar was obtained, sectioned, and recast at six different solidification cooling rates. Each of the six castings was then sectioned into six pieces and each piece cooled from the homogenization temperature to produce six different homogenization cooling rates. Backscattered electron imaging was used to measure the phase percentages and observe the microstructures. The ferrite content was found to increase with increasing homogenization cooling rates. It was also found that the homogenization cooling rate should be higher than 200 °C/h in order to get high ferrite contents (> 40%). As the cooling rate slows, intermetallic phases such as sigma start to precipitate at the grain boundaries.

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Longlong Liao

Papers

Influence of Cooling Rate on the Ferrite Prediction Diagram of Duplex Stainless Steel Castings

Influence of cooling rate on the ferrite content and microstructures in CD3MWCuN castings

Influence of Rapid Cooling Rate and Homogenization Temperature on the Ferrite Content and Microstructures of CD3MWCuN Castings