key: cord-0331284-bi0ohu91 authors: Yao, Gongcheng; Yuan, Jie; Pan, Shuaihang; Guan, Zeyi; Cao, Chezheng; Li, Xiaochun title: Casting In-Situ Cu/CrBx Composites via Aluminum-Assisted Reduction date: 2020-12-31 journal: Procedia Manufacturing DOI: 10.1016/j.promfg.2020.05.053 sha: 0a9c92624fa4e3cfa24493d242d9dbd2192c5a88 doc_id: 331284 cord_uid: bi0ohu91 Abstract Chromium borides (CrBx) have been considered as promising strengthening phases for copper (Cu). However, traditional manufacturing methods such as powder metallurgy greatly restrict the sample size and part complexity. Here, we report that bulk Cu containing high volume percentages of uniformly distributed CrBx were fabricated by casting via in-situ reduction assisted by aluminum (Al). Two sets of precursors, i.e., borax-CuCr and KBF4-CrF3, were used and the respective reaction mechanisms were illustrated. In addition, the resultant in-situ particle morphologies and sizes from the precursors were studied. The use of KBF4-CrF3 generated smaller in-situ particles due to the less severe coalescence of particles. The microhardness of in-situ Cu/CrBx was significantly enhanced over pure Cu. Pure copper (Cu) is soft. Incorporating a ceramic phase into Cu is effective to improve its performance [1, 2] . Cu matrix composites with strong mechanical properties and high electrical/thermal conductivities are strongly desired for applications such as rotors for electric motors, circuit breakers, electric resistance welding electrodes, heat exchangers, etc. Transition metal borides exhibit novel properties such as high hardness, high wear resistance, high thermal stability, and good electrical/thermal conductivity [3] [4] [5] . Chromium (Cr) and boron (B) can form solid-state compounds with seven different stoichiometries (CrBx) depending on their relative contents, exhibiting different properties [6] . Among them, CrB and CrB2 are the most stable phases [6] . Borides of chromium have been considered promising strengthening phases for Cu. Traditionally, Cu containing CrBx was fabricated by powder metallurgy. Cu containing CrB2 (44 μm) was produced by powder metallurgy as electrodes for electrical discharge machining [7] . Extrusion of mechanically alloyed Cu/CrB2 powders at 500 ℃ to produce bulk samples was also reported. As-extruded Cu-1.5 vol.% CrB2 showed a yield strength of 476 MPa [8] . However, the sample size and complexity were restricted by powder metallurgy. Casting is a scalable and facile method that is widely used in metal fabrication. Attempts to cast Cu composites have been made. Cu-25.7%Zn-3.58%Al with 0.64% CrB2 microparticles was fabricated by simply melting the alloy together with CrB2 [9] . However, the feasible volume percentage of boride particles by this method was very low. The wettability between the molten metal and the particles is vital to the successful incorporation and stabilization of particles during solidification processing. The wetting angle between CrB2 and Cu is 26°-15° when the temperature is 1100℃-1300℃ [10] . Taking advantage of the good wettability, a layer of CrB2, formed by the reaction of B4C and Cr (addition of 1 at.%) at the interface, promoted the wetting of B4C and Cu [11] . In this study, a new scalable method to manufacture bulk insitu Cu/CrBx via casting was developed. Aluminum was introduced as the reductant to assist the in-situ reactions, as Cu is relatively inert. Two sets of inexpensive raw materials as B Available online at www.sciencedirect.com Pure copper (Cu) is soft. Incorporating a ceramic phase into Cu is effective to improve its performance [1, 2] . Cu matrix composites with strong mechanical properties and high electrical/thermal conductivities are strongly desired for applications such as rotors for electric motors, circuit breakers, electric resistance welding electrodes, heat exchangers, etc. Transition metal borides exhibit novel properties such as high hardness, high wear resistance, high thermal stability, and good electrical/thermal conductivity [3] [4] [5] . Chromium (Cr) and boron (B) can form solid-state compounds with seven different stoichiometries (CrBx) depending on their relative contents, exhibiting different properties [6] . Among them, CrB and CrB2 are the most stable phases [6] . Borides of chromium have been considered promising strengthening phases for Cu. Traditionally, Cu containing CrBx was fabricated by powder metallurgy. Cu containing CrB2 (44 μm) was produced by powder metallurgy as electrodes for electrical discharge machining [7] . Extrusion of mechanically alloyed Cu/CrB2 powders at 500 ℃ to produce bulk samples was also reported. As-extruded Cu-1.5 vol.% CrB2 showed a yield strength of 476 MPa [8] . However, the sample size and complexity were restricted by powder metallurgy. Casting is a scalable and facile method that is widely used in metal fabrication. Attempts to cast Cu composites have been made. Cu-25.7%Zn-3.58%Al with 0.64% CrB2 microparticles was fabricated by simply melting the alloy together with CrB2 [9] . However, the feasible volume percentage of boride particles by this method was very low. The wettability between the molten metal and the particles is vital to the successful incorporation and stabilization of particles during solidification processing. The wetting angle between CrB2 and Cu is 26°-15° when the temperature is 1100℃-1300℃ [10] . Taking advantage of the good wettability, a layer of CrB2, formed by the reaction of B4C and Cr (addition of 1 at.%) at the interface, promoted the wetting of B4C and Cu [11] . In this study, a new scalable method to manufacture bulk insitu Cu/CrBx via casting was developed. Aluminum was introduced as the reductant to assist the in-situ reactions, as Cu is relatively inert. Two sets of inexpensive raw materials as B Pure copper (Cu) is soft. Incorporating a ceramic phase into Cu is effective to improve its performance [1, 2] . Cu matrix composites with strong mechanical properties and high electrical/thermal conductivities are strongly desired for applications such as rotors for electric motors, circuit breakers, electric resistance welding electrodes, heat exchangers, etc. Transition metal borides exhibit novel properties such as high hardness, high wear resistance, high thermal stability, and good electrical/thermal conductivity [3] [4] [5] . Chromium (Cr) and boron (B) can form solid-state compounds with seven different stoichiometries (CrBx) depending on their relative contents, exhibiting different properties [6] . Among them, CrB and CrB2 are the most stable phases [6] . Borides of chromium have been considered promising strengthening phases for Cu. Traditionally, Cu containing CrBx was fabricated by powder metallurgy. Cu containing CrB2 (44 μm) was produced by powder metallurgy as electrodes for electrical discharge machining [7] . Extrusion of mechanically alloyed Cu/CrB2 powders at 500 ℃ to produce bulk samples was also reported. As-extruded Cu-1.5 vol.% CrB2 showed a yield strength of 476 MPa [8] . However, the sample size and complexity were restricted by powder metallurgy. Casting is a scalable and facile method that is widely used in metal fabrication. Attempts to cast Cu composites have been made. Cu-25.7%Zn-3.58%Al with 0.64% CrB2 microparticles was fabricated by simply melting the alloy together with CrB2 [9] . However, the feasible volume percentage of boride particles by this method was very low. The wettability between the molten metal and the particles is vital to the successful incorporation and stabilization of particles during solidification processing. The wetting angle between CrB2 and Cu is 26°-15° when the temperature is 1100℃-1300℃ [10] . Taking advantage of the good wettability, a layer of CrB2, formed by the reaction of B4C and Cr (addition of 1 at.%) at the interface, promoted the wetting of B4C and Cu [11] . In this study, a new scalable method to manufacture bulk insitu Cu/CrBx via casting was developed. Aluminum was introduced as the reductant to assist the in-situ reactions, as Cu is relatively inert. Two sets of inexpensive raw materials as B 48th SME North American Manufacturing Research Conference, NAMRC 48 (Cancelled due to and Cr precursors, i.e., borax-CuCr and KBF4-CrF3, were experimented. The respective reaction mechanisms were elucidated. High loadings of uniformly distributed CrBx particles were achieved in the Cu matrix by both sets of precursors. Using the two fluoride salts, smaller particle sizes were achieved than using the borax-CuCr, possibly due to the less severe coalescence of particles. The microhardness of insitu Cu/CrBx is significantly enhanced over pure Cu. CrBx Chromium borides with different stoichiometries Borax Na2B4O7 Cu-6.5wt.% Cr (in-house alloyed, 200g) was heated to 1300℃ (temperature measured by a K-type thermocouple) with argon protection in a graphite crucible by an induction heater. Cu-6.5Cr will provide sufficient Cr source to generate 10-15 vol.% CrBx while maintaining a melting point that is still processible. Al shots (99.99%, Alfa Aesar, 13.5g) were added to molten Cu. Mixed KAlF4 (AMG Aluminum) and Na2B4O7 (99.5%, Alfa Aesar) were added on top of the melt. KAlF4, as a buffer salt, was 5 times the volume of the CuCr alloy. The ratio of Al, Na2B4O7, and Cr (in CuCr) in raw materials was decided by Reaction 1, with Na2B4O7 4 times in excess to compensate for the loss of salt evaporation during synthesis. The melt was held at 1300 ℃ for 1.5 h. It was then cooled to 900 ℃ naturally in the furnace to allow Cu to solidify, and the molten salt was poured out. In addition, instead of adding KAlF4 and Na2B4O7, pure Na2B4O7 5 times the volume of CuCr was added, with other procedures unchanged. Assuming all Cr in CuCr alloy was transformed to boride particles, Cu-6.5wt.% Cr would be converted to Cu/14 vol.% CrB2, or Cu/11 vol.% CrB. 4 + 2 4 7 + 2 → 2 2 + 2 + 2 2 3 (1) Copper (99.99%, Rotometals, 200g) was heated to 1130 ℃ with argon protection in a graphite crucible by an induction heater. Al shots (11.2g) were added to molten Cu. KBF4 (Spectrum), CrF3 (Sigma-Aldrich, CrF3·4H2O, 97%), and KAlF4 (AMG Aluminum) were mixed by a mechanical shaker (SK-O330-Pro). The KBF4-CrF3-KAlF4 powder mixture was added on top of the melt. The designed volume percentage of CrB2 in Cu was 8 vol.%, with the amounts of reactants calculated according to Reaction 2. The volume of KAlF4 (buffer salt) was 5 times of Cu. The melt was held at 1130℃ for 1.5 h. Then, the melt was cooled to 900 ℃ naturally in the furnace to allow Cu to solidify, and the molten salt was poured out. The as-cast sample was characterized to be Cu/7.6 vol.% CrB by image processing. As-cast Cu/CrBx samples were ground, polished, and ion milled at 4° and 4.5 kV for 1 h by Precision Ion Polishing System (Model PIPS 691, Gatan) to reveal the microstructures. These samples were observed via scanning electron microscopy (SEM, ZEISS Supra 40VP) equipped with energydispersive X-ray spectroscopy (EDS). X-ray diffraction (XRD) analysis was conducted on the samples to identify the phase compositions using Bruker D8 with Cu Kα radiation ( = . ) at a step size of 0.05° and a speed of 4°/min. The microhardness of the Cu/CrBx samples was measured using an LM 800AT microhardness tester with a load of 100 g and a dwell time of 10 s. Each hardness data represents 10 tests at randomly selected points across the sample at room temperature. The particle sizes and aspect ratios were measured by image processing using Image-Pro software. The average diameters of the particles were decided by the average length of diameters measured at 2-degree intervals and passing through the particle's centroid. The aspect ratio of a particle is the ratio between major axis and minor axis of ellipse equivalent to the particle. The XRD patterns of in-situ Cu/CrBx samples using borax and CuCr alloy as B and Cr sources are shown in Fig. 1a and Fig. 1b . The XRD patterns in Fig.1a were indexed as Cu and CrB2 phases, while patterns in Fig. 1b were indexed as Cu and CrB. Hence, by adjusting the ratio between borax and CuCr in the reactants, the in-situ particles can form as CrB2 or CrB. It should be noted that borax was more than the theoretically needed amount to compensate for the evaporation loss during synthesis (Na2B4O7 melting point 743℃). The in-situ reactions are shown in Reaction 3 and 4. At 1300℃, Cu-6.5wt.% Cr is in the single-phase liquid zone according to the Cu-Cr phase diagram. Al, acting as reductant, reduces B from borax to ground state (Reaction 3 [12] ). The high activity B atoms will diffuse into molten CuCr and react with Cr atoms in the CuCr solution (Reaction 4), forming chromium borides with different stoichiometries depending on the relative Cr and B contents. Fig. 1c shows the XRD patterns of the in-situ sample using KBF4 and CrF3 as B and Cr sources. The phases were identified as Cu and CrB. The underlying reactions are as follows. Al reacts with KBF4 and CrF3 at the molten salt/molten metal interface, reducing B and Cr from KBF4 and CrF3 to ground states, respectively, according to Reaction 5 and 6. The high activity Cr and B will immediately react and form CrBx via Reaction 4 at the interface, unlike in 3.1.1 where CrBx was formed in the molten metal. The formed CrBx particles will spontaneously migrate into molten Cu instead of the molten salt to reduce the interfacial energy, as CrBx has a better wettability with molten Cu than with molten salt [2, 13] . In this study, although the Cr:B ratio was designed to be 1:2, the final product was CrB due to the more severe evaporation of KBF4 than CrF3 at high temperatures. Thus, to produce in-situ CrBx with an intended stoichiometry, the KBF4 salt should be more than the theoretical amount. + → + (5) + → + (6) For in-situ Cu/11 vol.% CrB, as shown in Fig. 2a , the CrB particles are uniformly distributed in the Cu matrix. Elements that were detected by EDS mapping of Fig. 2a are shown in Fig. 2b-c. Fig. 2b clearly coincides with the Cu matrix, while Fig.2c coincides with the CrB particles. It should be noted that B was not picked up in EDS due to its light-element nature. Additionally, the residual Al formed a solid solution with Cu, as indicated by Fig.2d ( no CuAl intermetallic phase shown in XRD). No potassium was detected, indicating that salt was not mixed into the metal (or below detection limit). The CrB particles have an average aspect ratio of 2 ± 1.1. For Cu/14 vol.% CrB2, similarly, CrB2 particles are uniformly distributed in the Cu matrix. EDS mappings of Cu, Cr, and Al agree well with the phase analysis. However, the morphology of CrB2 differs from CrB (Fig. 2a) , with a larger aspect ratio (3±2.7) . The magnified image of CrB2 particles is shown in Fig. 3e . One micro-sized CrB2 particle is comprised of several nanosized particles, which can be attributed to the particle coalescence at the high processing temperature. The microstructure of Cu/7.6 vol.% CrB fabricated through reactions of Al and two fluoride salts is shown in Fig.4 . The uniformly distributed CrB particles in the Cu matrix exhibit similar appearances with those in Fig. 2a regardless of the smaller particle sizes in Fig. 4a . The aspect ratio of particles in Fig. 4a is 2.1±1.2 , which is very close to that in Fig. 2a (2 ± 1.1) . Therefore, it is concluded that, despite size differences, the morphology of the in-situ CrBx such as aspect ratios in this study depends on the stoichiometry, irrelevant to the precursors and volume fractions. In future work, the morphology of in-situ CrBx particles in this study will be compared with literature and commercial particles. It is anticipated that the morphology of the CrBx particles is related to the interfacial energy between the particles and the synthesis media. The statistics of the in-situ CrBx particles in the Cu matrix fabricated by two sets of precursors are shown in Fig. 5 . CrB and CrB2 synthesized by Na2B4O7 and CuCr show similar sizes, with the average size of CrB particles being 4.3 ± 3.1 μm, and that of the CrB2 particles being 5.9±4.1 μm. In comparison, the CrB particles, formed by two fluoride salts, have a much smaller particle size (1.7 ± 1.3 μm). The size difference can be partly attributed to the different synthesis temperatures. Using CuCr alloy as Cr precursor, the synthesis temperature has to be higher than the CuCr liquidus temperature. Otherwise, Cr will be in solid state in molten Cu, greatly restricting the reactions. The CuCr liquidus temperature is much higher than the melting point of Cu (1083℃). Meanwhile, with CrF3 as Cr source, the reaction temperature can be just over the Cu melting point, which will reduce the degree of particle coalescence. In addition, the in-situ CrBx particles in the aforementioned samples exhibit different size distribution patterns. As shown in Fig. 5 . The CrB particles synthesized by two fluoride salts show one peak in the size distribution diagram, with more than 35% of the particles smaller than 1 μm. Meanwhile, the CrBx particles generated by borax and CuCr both show two peaks in size distribution diagrams. The fractions of particles smaller than 1 μm in Fig. 5b and 5c are also smaller than those in Fig. 5a . It is thus anticipated that, with borax-CuCr as precursors, the more severe coalescence of particles (smaller than 1 μm) leads to the two-peak pattern, with increased number fractions of particles larger than 5 μm. Therefore, KBF4-CrF3 is superior to borax-CuCr as precursors, regarding producing smaller insitu CrBx particles. Systematic studies of the effects of temperature, reaction time, precursor type, and reactant concentration on particle sizes will be investigated to optimize the synthesis results and to achieve better properties. Microhardness of the in-situ Cu/CrBx is shown in Table 1 . In-situ Cu/11 vol.% CrB has a microhardness of 128.1±32 HV. For in-situ Cu/14 CrB2, the microhardness is 138.1±32 HV. Compared to pure Cu (C10100, M20) [14] , the hardness of the Cu containing in-situ CrBx particles is significantly enhanced. The Young's moduli of CrB and CrB2 are 522 GPa and 442 GPa, respectively [15] . The hardness of CrB is 23-27 GPa, and that of CrB2 is 15-18 GPa [6] . Thus, CrB is stronger than CrB2, which is expected to induce higher strengthening effects in Cu. The residual Al contents in the in-situ samples are also shown in Table 1 . The strengthening effects in the insitu Cu/CrBx arise from the solid solution strengthening of Al, Orowan Strengthening of the CrBx particles, and Hall-Petch effect due to grain refinement by CrBx particles [16] . The particle size of CrB in Cu/7.6 vol.% CrB is smaller than that of the Cu/11 vol.% CrB, which will induce higher Orowan strengthening. In the Cu/11 vol.% CrB, the microhardness roughly increased 7.1 HV over Cu per volume percent of CrB incorporated. The strengthening efficiency of CrB in the Cu/7.6 vol.% CrB sample is expected to be higher than 7.1 HV/vol.%. Thus, the hardness of Cu/7.6 vol.% CrB is estimated to be higher than 104 HV. For future work, the removal of residual Al in the Cu matrix will be investigated to recover the high electrical and thermal conductivity of the composites. [14] N/A A new scalable manufacturing method was developed to fabricate bulk Cu containing high contents of uniformly distributed in-situ CrBx particles via casting, assisted by aluminum reductions. Two sets of inexpensive raw materials as B and Cr precursors, i.e., borax-CuCr and KBF4-CrF3, were experimented and compared. In-situ CrB and CrB2 particles displayed different morphologies. It was found that the aspect ratio of the same type of in-situ CrBx in Cu was irrelevant to the precursor types. However, using KBF4-CrF3, smaller boride particle sizes in the Cu matrix were achieved, due to the less severe coalescence of particles. Moreover, the in-situ fabricated Cu/CrBx samples showed significantly enhanced mechanical property over pure Cu. Copper and Copper Alloys The authors appreciate the support from MetaLi LLC.