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On the mechanical and electrical properties of copper-silver and copper-silver-zirconium alloys deposits manufactured by cold spray
Author’s Accepted Manuscript On the mechanical and electrical properties of Copper-Silver and Copper-Silver-Zirconium alloys deposits manufactured by cold spray Pierre CODDET, Christophe VERDY, Christian CODDET, François DEBRAY www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(16)30252-0 http://dx.doi.org/10.1016/j.msea.2016.03.049 MSA33444
To appear in: Materials Science & Engineering A Received date: 15 December 2015 Revised date: 8 March 2016 Accepted date: 11 March 2016 Cite this article as: Pierre CODDET, Christophe VERDY, Christian CODDET and François DEBRAY, On the mechanical and electrical properties of CopperSilver and Copper-Silver-Zirconium alloys deposits manufactured by cold spray, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2016.03.049 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
On the mechanical and electrical properties of Copper-Silver and Copper-Silver-Zirconium alloys deposits manufactured by cold spray
Pierre CODDETa*, Christophe VERDYb, Christian CODDETb, François DEBRAYa
a
Laboratoire National des Champs Magnétiques Intenses (LNCMI – CNRS-UPS-INSA-UJF),
25 rue des martyrs 38042 Grenoble, France b
UTBM, Site de Sévenans 90010 Belfort Cedex, France
* Corresponding author: e-mail : [email protected] Tel: +33 4 76 88 12 44 Fax: +33 4 76 88 10 01
Abstract In this work, several copper alloy deposits were manufactured by cold spray with helium as accelerating and carrier gas. Electrical conductivity was measured to establish the potential of cold spray as a manufacturing process for high strength (> 500 MPa) and high conductivity (> 90 % IACS) copper alloys. The deposits which are characterized by a low oxygen content (< 200 ppm) and a low porosity level (< 0.1 %) present yield strength values up to about 700 MPa and electrical conductivity values up to 58.2 MS/m (100.3 % IACS). Results show that, even if a compromise has to be made between the properties according to the objectives of the application, this additive manufacturing route appears suitable for the production of large copper alloys parts with high mechanical properties and high electrical and thermal conductivity. The role of alloy composition and post heat treatments on the strength and
conductivity of the deposits was especially considered in this work. Cold spray deposits properties were finally compared with those obtained with other manufacturing routes.
Keywords: Cold spray, Cu alloys, Second phase precipitation, Work hardening, Yield strength, Electrical conductivity
1. Introduction Availability of high strength copper base alloys with high electrical or thermal conductivity is a key point for the industrial development of a large number of applications. Cooling of electronic products, lead frames, connectors [1], wiring for a bullet train, sheet and wire conductors [2], combustion chambers [3], first wall and diverter of tokomaks, rocket and space shuttle engines [4], high magnetic field magnets [2,5,6] can be cited among others. Pure copper presents high electrical (58 MS/m) and thermal (λ = 380 W.m-1.K-1) conductivity but rather poor mechanical properties (tensile strength limited to 225 MPa [7]). Several alloying elements like Ag, Cr, Fe, Nb, Zr, Be, … allow reinforcing the mechanical properties of the material thanks to precipitation hardening. Thus, copper alloys composition has been widely studied in the past to improve the strength of copper alloys by precipitation hardening, phase boundary hardening and solid solution hardening. However, the nature, the percentage and the distribution of those elements often limits the conductivity. Generally, the amount of additional elements has to remain below 5 wt.% to maintain a good conductivity. Silver which also presents as a pure metal a high conductivity (about 106 % IACS) is often used as a precipitation hardener to enhance the mechanical properties of copper as its solubility in this metal is very low. In this case, discontinuous precipitation (i.e. selective grain boundary precipitation) occurs that can be clearly distinguished in the micrographs [8]. It is generally considered that discontinuous precipitation is mainly linked to
the difference between the atomic radii of elements in the considered alloys [9,10]. In the case of Cu-Ag alloys, already widely studied in the past [8,11], precipitation mechanisms were mainly attributed to structural transformation and related to the Fournelle and Clark’s mechanism [12]. Other elements like zirconium can also be added to promote a homogeneous precipitation of the Ag rich phases which results in higher mechanical properties [13]. Indeed, zirconium acts as a grain refiner [6] and inhibits the discontinuous precipitation thus enhancing the continuous precipitation mode [5]. Another well-known way to enhance the hardening of copper alloys is to introduce a large number of dislocations into the grains. Several processes such as forging [14], Equal Channel Angular Pressing (ECAP) [7,15] and also cold rolling, drawing [6] or even spraying [16-18] have already been used to obtain the cold working hardening effect of the matrix. In summary, the requirements to produce parts with high mechanical properties and high conductivity can be listed as follow: - Precise management of the precipitation mechanisms to ensure high thermal and electrical conductivities, - Strong cold work effect to enhance the mechanical properties, - Moderate temperature during manufacturing operations to avoid oxygen contamination, - High productivity to fit with industrial requirements, - Ability to work at industrial scale and to produce larges items. Considering these requirements, an additive manufacturing process working below the melting point of the material such as Cold Spray appears as a good candidate to reach these goals. Cold Spray [19] is a technique consisting in spraying small particles (10 to 100 µm) at temperature well below the melting point of the material thanks to a high velocity gas jet obtained via the discharge of a compressed gas inside a De Laval nozzle. Indeed, ductile
materials like copper and copper alloys are particularly suitable for deposition by cold spray as the critical velocity (corresponding to the minimal velocity of the particles requested to obtain adhesion on a substrate or previously deposited layers) is relatively low (between 350 and 650 m/s) versus that requested by other materials [17,20-23]. Deposition efficiency is also a factor depending on the particles velocity but may reach very high values, close to 100 %, when the velocity is situated in the upper half of the deposition window. In addition, a cold work effect (i.e. dislocations pile-up) also results from the high plastic deformation of the powder particle impacting the substrate. Indeed, each particle sprayed at high velocity is severally deformed and then cold work hardened by the following impinging particles. Previous works concerning copper have demonstrated that the dislocations stacking effect associated with a fine microstructure (average grain sizes of few hundreds nm) allows obtaining fairly high mechanical properties [24]. It is well-known that the yield strength increases as the mean grain size decreases, following the classical Hall-Petch relation. The rate increase of the yield strength relative to the grain boundary strengthening can be evaluated by dividing the Hall-Petch constant of the considered alloy by the square root of the mean grain size [25]. Generally cold spray is performed with air or nitrogen, but gases with lower atomic weight like helium can be used favorably to enhance the velocity of the gas stream and thus the particle velocity [26]. Meanwhile, as a small penalty, with helium the drag force exerted on the particles will be lower. Therefore, the distance required for the particles to reach a given fraction of the velocity of the driving gas (which is the limiting velocity) will be greater for helium [21]. Another problem may arise from the cost of the gas but recycling allows solving that problem. Some researchers also considered the increase of the gas temperature [17] in order to enhance the deposition rate (gas speed increases due to the thermal expansion). In addition, working at
higher temperature leads to thermal softening of particles that causes a decrease in critical velocity for many materials [27]. Higher particles temperature at impact can be reached by several means: preheating the powder particles, increasing the gas temperature or increasing the distance between the particle injection and the nozzle throat using an elongated chamber [17]. Meanwhile, increasing particles temperature induces a high risk of oxygen contamination. With the use of helium gas, the system can be operated at lower temperature with high particles velocity and thus oxidation occurring during the quenching of the particles from high temperature to ambient [28] can be lowered. A great advantage of this low temperature is also to reduce the residual stresses usually encountered for deposits obtained with conventional thermal spray processes working at temperature higher than the melting point of the deposited material (1083.4°C for copper and 961.9°C for silver), given the high thermal gradients. In the same line, Schmidt et al. reported a lower hardness value (125 HV0.3) for copper deposited at 900°C when compared to deposits performed at 800°C (137 HV0.3) due to the thermal treatment of the deposit by the hot gas stream as well as by the higher particle impact temperature [29]. In this work, a Cold Spray system developed in the laboratory allowed to work at relatively low temperature (below 600°C) with high particles velocities (> 700 m/s) thanks to the use of helium gas. Deposition was performed in a chamber equipped with recycling loop to avoid costly helium losses and also possible oxygen contamination. Copper alloys were deposited and characterized before and after heat treatment regarding their mechanical properties and their conductivity. The influence of alloys composition and heat treatments was especially considered to seek for high mechanical properties without degrading conductivity. Finally, the potential of the cold spray process to produce large parts consistent with industrial needs was evaluated.
2. Experimental details 2.1 Feedstock powder In this work, we studied the influence of the Ag and Zr alloying content. The initial feedstock materials were copper, silver and zirconium granules with a purity of 99.99, 99.95 and 99.2 % respectively. The alloys powders were obtained in the laboratory by high pressure argon atomization (Nanoval process). The raw atomized powder was sorted between + 10 µm and - 63 µm using first a Walther&Cie Type 150 classifier machine operated with argon and then a sieving mesh. The particle’s size distribution was measured by laser light scattering (Malvern Mastersizer particle size analyzer). The exact composition and the oxygen content of the deposits were then determined by Atomic Emission Spectrometry (ICP-AES) and with an oxygen analyzer (LECO TC436).
2.2 Cold spray parameters The cold spray deposition process allows the manufacture of large parts with complex shapes thanks to the use of a removable mandrel. Given the use of a spray chamber, the size of the part is of course limited by the size of the chamber but nevertheless it can reach in our system a size comprised in an envelope of about 1×1×1 m3. In this work, Cu alloys deposits were produced using a CGT K-2000 Cold Spray gun equipped with a MOC 24 accelerating nozzle. The cold spray system is equipped with a closed loop circulating device which drastically limits the amount of helium necessary to operate the gun (about 0.5 m3 for 1 hour of operation). This device as well as deposition conditions were reported in previous papers [18,30].
An electrical heater is used to preheat the main gas at about 600°C to further enhance the gas expansion and thus the deposition efficiency. The distance from the particles injector exit to the nozzle throat is about 15 mm ensuring a low temperature of the deposited particles [17]. The temperature of the gun (about 550°C) and in the region close to the deposited particles on the removable mandrel (about 200°C) stays well below the melting point of copper. Temperatures were monitored by a thermocouple and by a thermal camera (FLIR Systems SC3000). This low temperature was chosen firstly to avoid the softening of the deposited particles which would lead to a limited cold work effect and thus to unsufficient mechanical properties and secondly to avoid oxygen pick-up by the deposited alloy because, even if the oxygen level in the deposition chamber is low (lower than 1000 ppm), a higher temperature could enhance the reactivity with residual oxygen. Thus, high purity copper alloys with oxygen contents lower than 200 ppm were obtained during this work with deposition parameters summarized in Table 1. A fairly high deposition rate (> 140 g/min) was also achieved without significant degradation of deposits properties [31]. During the course of the experiments, different nozzle pressures were selected for a sake of comparisons. But in fact the effect of this parameter in the selected range being limited, the deposition velocity remained in all cases in the middle of the deposition window (about 750 m/s as determined with the Kinetic Spray Software of KSS GmbH for particles of 30 µm in diameter). Therefore we assume that the different nozzle pressures used during the experiments presented here had no significant effect on the particles deformation rate at impact. Thus, in the following sections, comparisons will be made between the several deposited alloys not taking into account the pressure parameter of the gun during spray deposition.
2.3 Samples preparation and heat treating conditions
Deposits were prepared on AISI 4130 steel cylindrical mandrels (49.7 mm in diameter x 150 mm long). Before deposition, these mandrels were degreased and grit-blasted. After deposition, the steel mandrels were removed by machining thus leaving copper alloys cylinders. To characterize the tensile properties, Naval Ordnance Laboratory (NOL) rings samples [32], which are easy and cheap to manufacture, were machined from the sprayed cylinders. First, the cylinders (between 3 and 4 mm thick) were machined from the bulk. Then 10 mm high rings were cut and then milled on both sides. A schematic of the tensile test specimens was displayed in a previous paper [33]. Some tensile test specimens were kept in the as-sprayed condition while others were heat treated at several temperatures between 180 and 500°C. Heat treatments were carried out in an electrical furnace (SCFEB) under laboratory atmosphere and with a temperature accuracy of ± 5°C. Specimens were held at treatment temperature for 4 h before cooling under quiet air. To characterize the conductivity, rectangular pieces (about 30 × 20 × 3.5 mm3) were cut from the as deposited cylinders. Then, these pieces were polished (using P1000 grit paper) to obtain a plane surface suitable for conductivity measurements. Concerning alloys composition, pure copper was used as the base material (and also the reference). Then, two-phase binary Cu-Ag [8,34,35] and ternary (Cu-Ag-Zr) copper alloys were manufactured, the Ag and Zr contents of which were respectively varied between 0.1 and 23.7 wt.% and 0.05 and 0.5 wt.%.
2.4 Measurements Microstructure observations were carried out on samples cross sections by scanning electron microscopy (JEOL JSM 7800 F, equipped with X-ray energy dispersive spectroscopy, EDS). Image analysis techniques (NIH Image J software) were used to quantify the porosity level.
The accuracy of this technique is certainly questionable as it depends often on the skill of the operator but considering the objective of the measurements and our experience of the type of material it was considered of sufficient performance. Tensile tests on NOL Ring specimens were performed at room temperature using a Lloyd Instrument testing machine (LR50K) with a constant traverse speed of 2 mm/min. Electrical conductivity which easily gives a good insight on the thermal conductivity was measured using a Verimet M4900C (Testwell) conductivity meter. The probe was applied on a large polished (P1000) surface to avoid deleterious edge effect. Meanwhile, due to the limited size of the samples, a correction factor was applied on the measured values. Thus, the precision of the measurements made at room temperature is about ± 0.5 %. Some measurements were also performed using an eddy current-based coating conductivity gauge (Sigmatest 2.069, Forester, USA) for a cross check of the results. Properties of the deposits were determined in some cases along two directions on deposits made at a larger scale (cylinders about 50 mm high, internal and external diameters of 100 and 150 mm respectively) in order to consider a possible anisotropy in the structure.
3. Results and discussion 3.1. Microstructure A typical cross section of the atomized powders particles is shown in Figure 1. The rapidly solidified copper alloy powder particles show a typical two phase microstructure: the Cu-rich solid solution (black area) and the eutectic phase consisting of the Cu solid solution and Agrich solid solution lamellas (white area, Figure 1 a and b). The deposits cross sections (Figure 1 c) show that during the cold spray process, the powder particles do not undergo melting and quenching and hence no visible change in the phase repartition occurs. Heavily deformed particles and dense microstructure (porosity measured below 0.1 % per image analysis) can be
observed. However, following heat treatment at high temperature, a change in the Ag repartition in the matrix (Figure 1d) is observed as a spheroidisation of the second phase alloy occurs. The trend was already presented in more details previously for the binary Cu-Ag alloy [18] and the ternary Cu-3Ag-0.5Zr alloy [36]. Quite similar observations related to phase separation and decompositional process between Ag and Cu were also reported by Chen et al. in case of Ag-Cu alloys deposited by magnetron sputtering [37] and by Labisz et al. for long term aged Ag-Cu alloys [38].
3.2. Mechanical properties and electrical conductivity of the as-deposited samples 3.2.1. Mechanical properties As stated before, for cold spray deposits, cohesion results from adiabatic shear instabilities caused by the high strain rate deformation during impact [22,23,29]. The high hardness is due to the high density of dislocations formed during the high velocity impact of particles [20]. At first, the influence of the alloy composition on the mechanical properties was evaluated on as-sprayed samples manufactured with similar cold spray parameters (as reported in Table 1). Yield strength, ultimate tensile strength and elongation measurements made on the asdeposited samples are presented in Table 2. Among the various contributions to the increase in strength of the material, precipitation hardening is the most significant one directly linked to the alloy composition. In the same way, solid solution hardening occurs when solute atoms of different sizes are introduced in the matrix. The results presented in Table 2 clearly indicates that when the Ag content is increased (from 0.1 to 5.7 wt.%), the yield strength is increased (from 438 to 643 MPa) while the elongation at break is decreased (from 4.04 to 1.25 %). Ag-rich phase precipitation acts strongly as reinforcing phase for the Ag depleted copper matrix. However, this effect reaches
a plateau since there is no further increase of the yield strength when the silver content reaches 23.7 wt.%. On the contrary, ductility is further decreased coming down to nearly zero. The ternary alloy Cu-0.1Ag-0.1Zr (wt.%), presents strength values similar to the binary Cu0.1Ag (wt.%), however, the elongation at break is multiplied by 1.7 and reaches a value close to 7%. As could be expected, the Cu-3Ag-0.5Zr (wt.%) alloy exhibits a yield strength (576 MPa) situated between the results obtained with the Cu-0.1Ag and the Cu-5.7Ag alloys. However, the elongation at break obtained for this alloy manufactured with higher zirconium content is nil. Thus, there is a strong and combined effect of the Ag and Zr contents on ductility, with optimal values that remain to be determined. Those preliminary results thus indicate that even if the increase of the silver content in the copper matrix enhances the mechanical properties by hardening precipitation, there is a limit. Introduction of Zr in the matrix seems beneficial to the strength when the amount remains low (in the range of 0.1 wt.%) because Zr acts as a grain refiner and thus the yield strength is increased. Meanwhile, over this approximate value ductility is severely affected.
3.2.2. Electrical conductivity The electrical conductivity of the as-deposited samples is presented in Table 2. The high conductivity of those alloys can be explained by the extremely low solubility of the additional elements (i.e. Ag or Zr) in copper [39]. Meanwhile, despite the high electrical conductivity of Ag, the electrical conductivity of Cu-Ag alloys decreases when the percentage of silver increases. This is clearly observed here as the electrical conductivity decrease from 95.4 ± 0.5 to 62.4 ± 0.5 % IACS when the silver content is increased from 0.1 to 23.7 wt.%. When Zr is added, the continuous precipitation mode is enhanced and the electrical conductivity is further decreased. For example, as reported by Gaganov et al. in the case of Cu-7Ag (wt.%), the addition of 0.02 wt.% Zr, reduce the corresponding electrical
conductivity of 2.5 % IACS. When the Zr addition reaches 0.05 wt.% Zr, the electrical conductivity is reduced by 10.5 % IACS [5]. In this work, an addition of 0.1 wt.% of zirconium to the Cu-0.1Ag (wt.%) alloy is observed to reduce the conductivity by about 7.6 % IACS. A calculation based on reasonable estimations indicates that an electrical conductivity of about 81 % IACS could be expected for a Cu-3Ag alloys manufactured with our cold spray system conditions. Thus, the decrease of the electrical conductivity could be of about 16.6 % IACS for a 0.5 wt.% zirconium addition, confirming the severe deleterious effect of this element.
3.3. Mechanical properties and electrical conductivity of the heat-treated samples 3.3.1. Mechanical properties Numerous studies on copper alloys manufacturing are concerned with the use of ageing treatments to modify the mechanical properties [13,34,35,40]. Concerning this work, typical ultimate tensile strength, yield strength and corresponding elongation values at break as a function of aging treatments carried out at several temperatures ranging from 180 to about 500°C are presented in Figure 2, 3 and 4 respectively. Loss in strength associated with elongation increase normally occurs during the ageing process. Three different steps can be observed here. First, the strength level practically does not change after annealing at low temperature (i.e. below 200°C). The higher the Ag content, the higher the heat treating temperature necessary to see a change in the mechanical properties (i.e. heat treatment during 4 hours at about 230°C for the Cu-0.1Ag (wt.%), 300 °C for the Cu-5.7Ag (wt.%) and 325°C for the Cu-23.7Ag (wt.%)). This effect can be related to the diffusion rate which depends directly on concentration [41].
Then, annealing above the previously mentioned temperatures causes a decrease of the strength level which becomes more and more pronounced. This drop with the temperature increase can be related to the spheroidisation of the second phase and some changes at the grain boundaries (Figure 1 c). Hamana et al. have also observed the rearrangement and formation of new grain boundaries when annealing is performed on highly cold worked materials [41]. This phenomenon can be explained by the rapid atom transport path provided by the grain boundaries [42]. Diffusion along grain boundaries leads to mass transport and spheroidisation of the second phase. Thus, following heat treatment, cell formation occurred at both side of the grain boundary. This mechanism is consistent with the one proposed by Fournelle [43]. Finally, heat treatments at higher temperature (i.e. above 250°C for the Cu-0.1Ag (wt.%) and 350 °C for the Cu-5.7Ag (wt.%) and for the Cu-23.7Ag (wt.%)) dramatically decrease the strength level due to the further coarsening of the silver rich precipitates as well as the recrystallisation of the Cu matrix. Evolution of the precipitates shape and composition as a function of the aging time has also been widely studied in the past. On a macroscopic scale, heat treatment acts mainly on the reorganization of the matrix and thus mainly affects the gain in mechanical properties previously obtained by the cold work effect. When 0.1 wt% of Zr is added to the Cu-0.1Ag (wt.%), the strength is increased but also the material is more resistant to heat treating transformation. Between 300 and 375°C, the ternary Cu-3Ag-0.5Zr (wt.%) presents similar strength with the binary Cu-5.7Ag (wt.%). However, even if the ductile behavior is more prevalent for copper alloys with a low content of additional elements, the Cu-5.7Ag alloy present a higher ductility (about 9 % instead of 3 % for Cu-3Ag-0.5Zr (wt.%)). It can also be noticed again that a higher temperature is required to change the microstructure of the ternary alloy.
3.3.2. Electrical conductivity Electrical conductivity is also strongly affected by thermal treatments. As shown in Figure 5, the electrical conductivity of the deposited copper alloys tends to increased with the heat treatment temperature. This observation is more prevalent when the amount of second phase is higher. This behavior can also be related to the spherodisation of the precipitates. Indeed, due to the spheroidisation, the precipitates also have a lower surface to volume ratio. At higher temperature, the second phase formation is promoted and consequently the depletion in additional element (i.e. Ag) of the Cu solid solution in the matrix leads to a further increase of the electrical conductivity.
3.4. Isotropic properties In the case of sheet-conductors manufactured by cold-rolling, Sakay et al. reported anisotropy in strength with respect to the rolling direction with a decrease of 10% but no anisotropy regarding the electrical conductivity [2]. Typically, for cold rolling the strength in the perpendicular direction is higher than in the rolling direction and the rate of anisotropy is increased when increasing the reduction ratio. In the case of cold spray deposits, homogeneous properties concerning strength were observed and reported in previous paper [31] for the Cu-0.1Ag alloy. The electrical conductivity measured along the 3 direction gives values closed to 95.5 % IACS (i.e. 95.8, 95.5 and 95.1 %IACS) for the as-sprayed Cu-0.1Ag alloy. Following heat treatment at 225°C, the electrical conductivity stays about 96.1 ± 0.5 % IACS for any direction. Measurements performed on large parts revealed that no anisotropy in electrical conductivity existed within an experimental error of 0.5 % IACS. Isotropic properties (both in strength and conductivity) measured on large parts confirmed that cold spray can be advantageously used
for the manufacturing of large items. However, strength following the deposition direction was not yet measured but will be evaluated in further works.
3.5 Comparison with literature data To evaluate the work hardening effect obtained by cold spray, our results were compared with other data reported in literature. Table 3 presents respectively the ultimate tensile strength and the electrical conductivity of copper alloys manufactured using several processes. Generally, the ultimate tensile strength (UTS) is higher for the samples manufactured with high percentage of alloying elements. As example, in case of Cu-7Ag (wt.%), a UTS value higher than 1000 MPa could be obtained by hot forging, rotary swaging and drawing [6]. The work hardening effect could be very high on the severely deformed material obtained by drawing, ECAP or cold rolling (see Table 3). However, the electrical conductivity is lower than 80 % IACS and the size of the manufactured part is generally limited. In case of cold rolled and heat treated Cu-Cr alloys with and without Zr and Ag, UTS values close to 500 MPa and electrical conductivity close to 90 % IACS are reported [40]. In this case, a high electrical conductivity is maintained thanks to the low percentage of alloying elements. Table 4 focuses on results obtained for copper alloys deposited by cold spray. The best performances reported by Schmidt et al. is about 90-95 % IACS for pure copper. Calculated particles velocities corresponding to spraying with nitrogen at pressure of 30 and 40 bars and temperatures of 800 and 900°C respectively indicate that the impact velocity is exceeding in that case the critical velocity by more than 200 m/s [29]. Indeed, an increase in particle impact velocity or particle impact temperature results in significant increase of the metallurgically bonded areas. Increasing temperature seems more effective for particles of larger size which retain higher temperature upon impact [29].
Watanabe et al. presented results where cold spray copper deposits sprayed with nitrogen (gas temperature of 800°C and pressure of 30 bars), exhibited an electrical resistivity much higher (about 1.9 Ω.m) than that of bulk copper [16]. This effect was attributed to the micro texture. In the present work, the electrical conductivity of pure copper deposits in the as-sprayed condition reaches 96.9 % IACS. This obviously shows that the main effect regarding electrical conductivity is rather linked to the bonding between lamellas and to the porosity level as in our work the porosity level was below 0.1 % and the particles deformation rate ensuring cohesion was higher.
4. Conclusion In this work, a specific cold spray system allowing the use of helium without significant losses was used to manufacture several copper alloys. Helium gas was used to enhance the kinetic energy of the particles, necessary to produce a solid state bond, and avoid high temperature that could have an effect on the oxygen content. High velocity (about 750 m/s) i.e. in the middle range of the deposition window, and low temperature (impact temperature below 200°C) of the particles induced a high cold work effect. The results show that a fine microstructure, a low porosity rate (< 0.1 %) and a low oxygen content (about 150 ppm) of the deposits could thus be obtained using the cold spray manufacturing route. Mechanical properties and electrical conductivity were measured on numerous cold sprayed copper alloys samples. In the as sprayed condition, the copper alloys exhibit relatively high mechanical properties when compared with forged available copper alloys. The hardening effect resulting from the process was clearly observed with two phenomena: - a fine second phase precipitation initiated in the particles manufactured by inert gas atomization - a significant cold work effect with a flattening ratio of the particles close to 0.5.
The nature and distribution of the additional elements in the copper matrix play an important role on the deposit performances. An increase of the silver content enhances the strength due to the larger amount of second phase but reduces the conductivity. Heat treating (aging) may afterwards be used to precisely tailor the mechanical properties (strength versus ductility) selecting adequately the temperature. Changes in microstructure lead to a progressive decrease in strength linked with an increase in ductility and conductivity. An optimal composition and heat treating process could thus be defined to reach a good compromise between mechanical properties conductivity and cost of copper alloys. Such properties are interesting for applications demanding both high mechanical properties and high electrical or thermal conductivity. Moreover, cold spray allows high freedom concerning free form manufacturing and the production of large parts consistent with the industrial demand.
Acknowledgments Authors are grateful to Lucas Dembinski (UTBM) for assistance in powder atomization.
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Figure 1 : Cu-5.7 Ag (wt.%) cross sections SEM micrographs of powder particles (a) and (b), of deposit as-sprayed (c) and following heat treating at 600°C for 4 hours (d) - “ white “ particle ” in picture a) is only an artifact ” Figure 2 : Ultimate tensile strength (± 10 MPa) versus heat treating temperature (4 hours) for cold sprayed copper alloys (full symbol for binary alloys Cu-0.1 Ag (♦), Cu-5.7 Ag and (♦) Cu-23.7 Ag (♦) and open symbol for ternary alloys Cu-0.1Ag-0.1Zr (□) Cu-3Ag-0.5Zr (Δ))
Figure 3 : Yield strength (± 10 MPa) versus heat treating temperature (4 hours) for cold sprayed copper alloys (full symbol for binary alloys Cu-0.1 Ag (♦), Cu-5.7 Ag and (♦) Cu23.7 Ag (♦) and open symbol for ternary alloys Cu-0.1Ag-0.1Zr (□) Cu-3Ag-0.5Zr (Δ)) Figure 4 : Elongation (± 0.5 %) versus heat treating temperature (4 hours) for cold sprayed copper alloys (full symbol for binary alloys Cu-0.1 Ag (♦), Cu-5.7 Ag and (♦) Cu-23.7 Ag (♦) and open symbol for ternary alloys Cu-0.1Ag-0.1Zr (□) Cu-3Ag-0.5Zr (Δ)) Figure 5 : Electrical conductivity (± 0.5 % IACS) versus heat treating temperature (4 hours) for cold sprayed copper alloys (full symbol for binary alloys Cu-0.1 Ag (♦), Cu-5.7 Ag and (♦) Cu-23.7 Ag (♦) and open symbol for ternary alloys Cu-0.1Ag-0.1Zr (□) Cu-3Ag-0.5Zr (Δ))
Table 1 : Main deposition parameters for the cold-sprayed copper alloys Table 2 : Properties of the copper alloys deposits in the as-sprayed conditions Table 3 : Comparison of tensile strength and electrical conductivity obtained with copper alloys manufactured by different processes Table 4 : Comparison of tensile strength and electrical conductivity for pure copper manufactured by the cold spray process
Table 1 Chamber pressure (mbars) 476-945
Nozzle pressure (bars) 16.0-29.5
Nozzle temperature (°C) 488-540
Carrier gas Powder feed Spray flow rate distance (L/min) (g/min) (mm) 53-108 35-110 18-25
Table 2 Copper alloys
Oxygen content (wt.%) (ppm) Cold sprayed (This work)
Yield Strength (MPa)
Ultimate Strength (MPa)
Tensile Elongation Electrical conductivity (%) (% IACS)
Pure copper Cu-0.1Ag Cu-5.7Ag Cu-23.7Ag Cu-0.1Ag-0.1Zr Cu-3Ag-0.5Zr
<200ppm <200ppm <200ppm <200ppm <200ppm <200ppm
306 ± 10 438 ± 10 643 ± 10 646 ± 10 442 ± 10 576 ± 10
320 ± 5 466 ± 5 701 ± 5 646 ± 5 483 ± 5 576 ± 5
3.0 ± 0.5 4.04 ± 0.5 1.25 ± 0.5 0 ± 0.5 7.03 ± 0.5 0 ± 0.5
96.9 ± 0.5 95.4 ± 0.5 74.3 ± 0.5 62.4 ± 0.5 87.8 ± 0.5 64.4 ± 0.5
Table 3 Material
[5]
[6]
[44] [2]
(wt.%) Cu-7Ag-xZr Draw wire pieces Cu-7Ag Cu-24Ag Cu-7Ag-0.05Zr Cu-0.5Cr
[45]
Cu-6Ag Cu-8Ag Cu-12Ag Cu-24Ag Sheet-conductor Cu-Ag Wires Cu-0.44Cr-0.2Zr
[7]
Cu-0.5Cr
[40] [40]
Cu-0.5Cr-0.15Zr0.1Ag Cu-0.5Cr-0.03Zr
[40]
Cu-0.5Cr-0.1Ag
[16]
Bulk copper C1020
[35]
Deformation process Hot forging, profile rolling and drawing Hot forging, rotary swaging and drawing Equal channel angular pressing Cold rolling cold drawing and heat treatment Drawing and heat treatment Equal channel angular pressing Equal channel angular pressing and cold rolling Cold rolling and heat treatment Cold rolling and heat treatment Cold rolling and heat treatment /
Ultimate Tensile Strength (MPa) 1416
Electrical conductivity (% IACS) 59.9
1080 ≈ 1100
68.7 68
620
74.9
912 947 989 1050
/ / / 75
1000
80
700
81
570 – 579 554
35 84
570
86
490
88
500
95
225
98,5 ± 4
Table 4 Material
[46] [47] [17] [29] [48] [29] [49] [17] [29] [29] [16] [This work]
Pure Copper Pure copper Pure copper Pure copper Pure copper Pure copper Pure copper Pure copper Pure copper Pure copper Pure copper Pure copper
Gas
Gas temperature
Gas pressure
Electrical conductivity
(bars) 20
Ultimate Tensile Strength (MPa) /
Air
(°C) 450
Air
450
22
/
35 ± 3
Nitrogen
350
30
57
50
Nitrogen
300
30
90
60
Nitrogen
305
30
/
63
Nitrogen
600
30
240
80
Nitrogen
600
27
325
/
Nitrogen
800
30
391
80
Nitrogen
800
30
375
80 - 90
Nitrogen
900
40
450
90 - 95
Nitrogen
800
30
360
90.7 ± 5
Helium
600
30
300
96.9
(% IACS) ≈ 31
PII: DOI: Reference:
S0921-5093(16)30252-0 http://dx.doi.org/10.1016/j.msea.2016.03.049 MSA33444
To appear in: Materials Science & Engineering A Received date: 15 December 2015 Revised date: 8 March 2016 Accepted date: 11 March 2016 Cite this article as: Pierre CODDET, Christophe VERDY, Christian CODDET and François DEBRAY, On the mechanical and electrical properties of CopperSilver and Copper-Silver-Zirconium alloys deposits manufactured by cold spray, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2016.03.049 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
On the mechanical and electrical properties of Copper-Silver and Copper-Silver-Zirconium alloys deposits manufactured by cold spray
Pierre CODDETa*, Christophe VERDYb, Christian CODDETb, François DEBRAYa
a
Laboratoire National des Champs Magnétiques Intenses (LNCMI – CNRS-UPS-INSA-UJF),
25 rue des martyrs 38042 Grenoble, France b
UTBM, Site de Sévenans 90010 Belfort Cedex, France
* Corresponding author: e-mail : [email protected] Tel: +33 4 76 88 12 44 Fax: +33 4 76 88 10 01
Abstract In this work, several copper alloy deposits were manufactured by cold spray with helium as accelerating and carrier gas. Electrical conductivity was measured to establish the potential of cold spray as a manufacturing process for high strength (> 500 MPa) and high conductivity (> 90 % IACS) copper alloys. The deposits which are characterized by a low oxygen content (< 200 ppm) and a low porosity level (< 0.1 %) present yield strength values up to about 700 MPa and electrical conductivity values up to 58.2 MS/m (100.3 % IACS). Results show that, even if a compromise has to be made between the properties according to the objectives of the application, this additive manufacturing route appears suitable for the production of large copper alloys parts with high mechanical properties and high electrical and thermal conductivity. The role of alloy composition and post heat treatments on the strength and
conductivity of the deposits was especially considered in this work. Cold spray deposits properties were finally compared with those obtained with other manufacturing routes.
Keywords: Cold spray, Cu alloys, Second phase precipitation, Work hardening, Yield strength, Electrical conductivity
1. Introduction Availability of high strength copper base alloys with high electrical or thermal conductivity is a key point for the industrial development of a large number of applications. Cooling of electronic products, lead frames, connectors [1], wiring for a bullet train, sheet and wire conductors [2], combustion chambers [3], first wall and diverter of tokomaks, rocket and space shuttle engines [4], high magnetic field magnets [2,5,6] can be cited among others. Pure copper presents high electrical (58 MS/m) and thermal (λ = 380 W.m-1.K-1) conductivity but rather poor mechanical properties (tensile strength limited to 225 MPa [7]). Several alloying elements like Ag, Cr, Fe, Nb, Zr, Be, … allow reinforcing the mechanical properties of the material thanks to precipitation hardening. Thus, copper alloys composition has been widely studied in the past to improve the strength of copper alloys by precipitation hardening, phase boundary hardening and solid solution hardening. However, the nature, the percentage and the distribution of those elements often limits the conductivity. Generally, the amount of additional elements has to remain below 5 wt.% to maintain a good conductivity. Silver which also presents as a pure metal a high conductivity (about 106 % IACS) is often used as a precipitation hardener to enhance the mechanical properties of copper as its solubility in this metal is very low. In this case, discontinuous precipitation (i.e. selective grain boundary precipitation) occurs that can be clearly distinguished in the micrographs [8]. It is generally considered that discontinuous precipitation is mainly linked to
the difference between the atomic radii of elements in the considered alloys [9,10]. In the case of Cu-Ag alloys, already widely studied in the past [8,11], precipitation mechanisms were mainly attributed to structural transformation and related to the Fournelle and Clark’s mechanism [12]. Other elements like zirconium can also be added to promote a homogeneous precipitation of the Ag rich phases which results in higher mechanical properties [13]. Indeed, zirconium acts as a grain refiner [6] and inhibits the discontinuous precipitation thus enhancing the continuous precipitation mode [5]. Another well-known way to enhance the hardening of copper alloys is to introduce a large number of dislocations into the grains. Several processes such as forging [14], Equal Channel Angular Pressing (ECAP) [7,15] and also cold rolling, drawing [6] or even spraying [16-18] have already been used to obtain the cold working hardening effect of the matrix. In summary, the requirements to produce parts with high mechanical properties and high conductivity can be listed as follow: - Precise management of the precipitation mechanisms to ensure high thermal and electrical conductivities, - Strong cold work effect to enhance the mechanical properties, - Moderate temperature during manufacturing operations to avoid oxygen contamination, - High productivity to fit with industrial requirements, - Ability to work at industrial scale and to produce larges items. Considering these requirements, an additive manufacturing process working below the melting point of the material such as Cold Spray appears as a good candidate to reach these goals. Cold Spray [19] is a technique consisting in spraying small particles (10 to 100 µm) at temperature well below the melting point of the material thanks to a high velocity gas jet obtained via the discharge of a compressed gas inside a De Laval nozzle. Indeed, ductile
materials like copper and copper alloys are particularly suitable for deposition by cold spray as the critical velocity (corresponding to the minimal velocity of the particles requested to obtain adhesion on a substrate or previously deposited layers) is relatively low (between 350 and 650 m/s) versus that requested by other materials [17,20-23]. Deposition efficiency is also a factor depending on the particles velocity but may reach very high values, close to 100 %, when the velocity is situated in the upper half of the deposition window. In addition, a cold work effect (i.e. dislocations pile-up) also results from the high plastic deformation of the powder particle impacting the substrate. Indeed, each particle sprayed at high velocity is severally deformed and then cold work hardened by the following impinging particles. Previous works concerning copper have demonstrated that the dislocations stacking effect associated with a fine microstructure (average grain sizes of few hundreds nm) allows obtaining fairly high mechanical properties [24]. It is well-known that the yield strength increases as the mean grain size decreases, following the classical Hall-Petch relation. The rate increase of the yield strength relative to the grain boundary strengthening can be evaluated by dividing the Hall-Petch constant of the considered alloy by the square root of the mean grain size [25]. Generally cold spray is performed with air or nitrogen, but gases with lower atomic weight like helium can be used favorably to enhance the velocity of the gas stream and thus the particle velocity [26]. Meanwhile, as a small penalty, with helium the drag force exerted on the particles will be lower. Therefore, the distance required for the particles to reach a given fraction of the velocity of the driving gas (which is the limiting velocity) will be greater for helium [21]. Another problem may arise from the cost of the gas but recycling allows solving that problem. Some researchers also considered the increase of the gas temperature [17] in order to enhance the deposition rate (gas speed increases due to the thermal expansion). In addition, working at
higher temperature leads to thermal softening of particles that causes a decrease in critical velocity for many materials [27]. Higher particles temperature at impact can be reached by several means: preheating the powder particles, increasing the gas temperature or increasing the distance between the particle injection and the nozzle throat using an elongated chamber [17]. Meanwhile, increasing particles temperature induces a high risk of oxygen contamination. With the use of helium gas, the system can be operated at lower temperature with high particles velocity and thus oxidation occurring during the quenching of the particles from high temperature to ambient [28] can be lowered. A great advantage of this low temperature is also to reduce the residual stresses usually encountered for deposits obtained with conventional thermal spray processes working at temperature higher than the melting point of the deposited material (1083.4°C for copper and 961.9°C for silver), given the high thermal gradients. In the same line, Schmidt et al. reported a lower hardness value (125 HV0.3) for copper deposited at 900°C when compared to deposits performed at 800°C (137 HV0.3) due to the thermal treatment of the deposit by the hot gas stream as well as by the higher particle impact temperature [29]. In this work, a Cold Spray system developed in the laboratory allowed to work at relatively low temperature (below 600°C) with high particles velocities (> 700 m/s) thanks to the use of helium gas. Deposition was performed in a chamber equipped with recycling loop to avoid costly helium losses and also possible oxygen contamination. Copper alloys were deposited and characterized before and after heat treatment regarding their mechanical properties and their conductivity. The influence of alloys composition and heat treatments was especially considered to seek for high mechanical properties without degrading conductivity. Finally, the potential of the cold spray process to produce large parts consistent with industrial needs was evaluated.
2. Experimental details 2.1 Feedstock powder In this work, we studied the influence of the Ag and Zr alloying content. The initial feedstock materials were copper, silver and zirconium granules with a purity of 99.99, 99.95 and 99.2 % respectively. The alloys powders were obtained in the laboratory by high pressure argon atomization (Nanoval process). The raw atomized powder was sorted between + 10 µm and - 63 µm using first a Walther&Cie Type 150 classifier machine operated with argon and then a sieving mesh. The particle’s size distribution was measured by laser light scattering (Malvern Mastersizer particle size analyzer). The exact composition and the oxygen content of the deposits were then determined by Atomic Emission Spectrometry (ICP-AES) and with an oxygen analyzer (LECO TC436).
2.2 Cold spray parameters The cold spray deposition process allows the manufacture of large parts with complex shapes thanks to the use of a removable mandrel. Given the use of a spray chamber, the size of the part is of course limited by the size of the chamber but nevertheless it can reach in our system a size comprised in an envelope of about 1×1×1 m3. In this work, Cu alloys deposits were produced using a CGT K-2000 Cold Spray gun equipped with a MOC 24 accelerating nozzle. The cold spray system is equipped with a closed loop circulating device which drastically limits the amount of helium necessary to operate the gun (about 0.5 m3 for 1 hour of operation). This device as well as deposition conditions were reported in previous papers [18,30].
An electrical heater is used to preheat the main gas at about 600°C to further enhance the gas expansion and thus the deposition efficiency. The distance from the particles injector exit to the nozzle throat is about 15 mm ensuring a low temperature of the deposited particles [17]. The temperature of the gun (about 550°C) and in the region close to the deposited particles on the removable mandrel (about 200°C) stays well below the melting point of copper. Temperatures were monitored by a thermocouple and by a thermal camera (FLIR Systems SC3000). This low temperature was chosen firstly to avoid the softening of the deposited particles which would lead to a limited cold work effect and thus to unsufficient mechanical properties and secondly to avoid oxygen pick-up by the deposited alloy because, even if the oxygen level in the deposition chamber is low (lower than 1000 ppm), a higher temperature could enhance the reactivity with residual oxygen. Thus, high purity copper alloys with oxygen contents lower than 200 ppm were obtained during this work with deposition parameters summarized in Table 1. A fairly high deposition rate (> 140 g/min) was also achieved without significant degradation of deposits properties [31]. During the course of the experiments, different nozzle pressures were selected for a sake of comparisons. But in fact the effect of this parameter in the selected range being limited, the deposition velocity remained in all cases in the middle of the deposition window (about 750 m/s as determined with the Kinetic Spray Software of KSS GmbH for particles of 30 µm in diameter). Therefore we assume that the different nozzle pressures used during the experiments presented here had no significant effect on the particles deformation rate at impact. Thus, in the following sections, comparisons will be made between the several deposited alloys not taking into account the pressure parameter of the gun during spray deposition.
2.3 Samples preparation and heat treating conditions
Deposits were prepared on AISI 4130 steel cylindrical mandrels (49.7 mm in diameter x 150 mm long). Before deposition, these mandrels were degreased and grit-blasted. After deposition, the steel mandrels were removed by machining thus leaving copper alloys cylinders. To characterize the tensile properties, Naval Ordnance Laboratory (NOL) rings samples [32], which are easy and cheap to manufacture, were machined from the sprayed cylinders. First, the cylinders (between 3 and 4 mm thick) were machined from the bulk. Then 10 mm high rings were cut and then milled on both sides. A schematic of the tensile test specimens was displayed in a previous paper [33]. Some tensile test specimens were kept in the as-sprayed condition while others were heat treated at several temperatures between 180 and 500°C. Heat treatments were carried out in an electrical furnace (SCFEB) under laboratory atmosphere and with a temperature accuracy of ± 5°C. Specimens were held at treatment temperature for 4 h before cooling under quiet air. To characterize the conductivity, rectangular pieces (about 30 × 20 × 3.5 mm3) were cut from the as deposited cylinders. Then, these pieces were polished (using P1000 grit paper) to obtain a plane surface suitable for conductivity measurements. Concerning alloys composition, pure copper was used as the base material (and also the reference). Then, two-phase binary Cu-Ag [8,34,35] and ternary (Cu-Ag-Zr) copper alloys were manufactured, the Ag and Zr contents of which were respectively varied between 0.1 and 23.7 wt.% and 0.05 and 0.5 wt.%.
2.4 Measurements Microstructure observations were carried out on samples cross sections by scanning electron microscopy (JEOL JSM 7800 F, equipped with X-ray energy dispersive spectroscopy, EDS). Image analysis techniques (NIH Image J software) were used to quantify the porosity level.
The accuracy of this technique is certainly questionable as it depends often on the skill of the operator but considering the objective of the measurements and our experience of the type of material it was considered of sufficient performance. Tensile tests on NOL Ring specimens were performed at room temperature using a Lloyd Instrument testing machine (LR50K) with a constant traverse speed of 2 mm/min. Electrical conductivity which easily gives a good insight on the thermal conductivity was measured using a Verimet M4900C (Testwell) conductivity meter. The probe was applied on a large polished (P1000) surface to avoid deleterious edge effect. Meanwhile, due to the limited size of the samples, a correction factor was applied on the measured values. Thus, the precision of the measurements made at room temperature is about ± 0.5 %. Some measurements were also performed using an eddy current-based coating conductivity gauge (Sigmatest 2.069, Forester, USA) for a cross check of the results. Properties of the deposits were determined in some cases along two directions on deposits made at a larger scale (cylinders about 50 mm high, internal and external diameters of 100 and 150 mm respectively) in order to consider a possible anisotropy in the structure.
3. Results and discussion 3.1. Microstructure A typical cross section of the atomized powders particles is shown in Figure 1. The rapidly solidified copper alloy powder particles show a typical two phase microstructure: the Cu-rich solid solution (black area) and the eutectic phase consisting of the Cu solid solution and Agrich solid solution lamellas (white area, Figure 1 a and b). The deposits cross sections (Figure 1 c) show that during the cold spray process, the powder particles do not undergo melting and quenching and hence no visible change in the phase repartition occurs. Heavily deformed particles and dense microstructure (porosity measured below 0.1 % per image analysis) can be
observed. However, following heat treatment at high temperature, a change in the Ag repartition in the matrix (Figure 1d) is observed as a spheroidisation of the second phase alloy occurs. The trend was already presented in more details previously for the binary Cu-Ag alloy [18] and the ternary Cu-3Ag-0.5Zr alloy [36]. Quite similar observations related to phase separation and decompositional process between Ag and Cu were also reported by Chen et al. in case of Ag-Cu alloys deposited by magnetron sputtering [37] and by Labisz et al. for long term aged Ag-Cu alloys [38].
3.2. Mechanical properties and electrical conductivity of the as-deposited samples 3.2.1. Mechanical properties As stated before, for cold spray deposits, cohesion results from adiabatic shear instabilities caused by the high strain rate deformation during impact [22,23,29]. The high hardness is due to the high density of dislocations formed during the high velocity impact of particles [20]. At first, the influence of the alloy composition on the mechanical properties was evaluated on as-sprayed samples manufactured with similar cold spray parameters (as reported in Table 1). Yield strength, ultimate tensile strength and elongation measurements made on the asdeposited samples are presented in Table 2. Among the various contributions to the increase in strength of the material, precipitation hardening is the most significant one directly linked to the alloy composition. In the same way, solid solution hardening occurs when solute atoms of different sizes are introduced in the matrix. The results presented in Table 2 clearly indicates that when the Ag content is increased (from 0.1 to 5.7 wt.%), the yield strength is increased (from 438 to 643 MPa) while the elongation at break is decreased (from 4.04 to 1.25 %). Ag-rich phase precipitation acts strongly as reinforcing phase for the Ag depleted copper matrix. However, this effect reaches
a plateau since there is no further increase of the yield strength when the silver content reaches 23.7 wt.%. On the contrary, ductility is further decreased coming down to nearly zero. The ternary alloy Cu-0.1Ag-0.1Zr (wt.%), presents strength values similar to the binary Cu0.1Ag (wt.%), however, the elongation at break is multiplied by 1.7 and reaches a value close to 7%. As could be expected, the Cu-3Ag-0.5Zr (wt.%) alloy exhibits a yield strength (576 MPa) situated between the results obtained with the Cu-0.1Ag and the Cu-5.7Ag alloys. However, the elongation at break obtained for this alloy manufactured with higher zirconium content is nil. Thus, there is a strong and combined effect of the Ag and Zr contents on ductility, with optimal values that remain to be determined. Those preliminary results thus indicate that even if the increase of the silver content in the copper matrix enhances the mechanical properties by hardening precipitation, there is a limit. Introduction of Zr in the matrix seems beneficial to the strength when the amount remains low (in the range of 0.1 wt.%) because Zr acts as a grain refiner and thus the yield strength is increased. Meanwhile, over this approximate value ductility is severely affected.
3.2.2. Electrical conductivity The electrical conductivity of the as-deposited samples is presented in Table 2. The high conductivity of those alloys can be explained by the extremely low solubility of the additional elements (i.e. Ag or Zr) in copper [39]. Meanwhile, despite the high electrical conductivity of Ag, the electrical conductivity of Cu-Ag alloys decreases when the percentage of silver increases. This is clearly observed here as the electrical conductivity decrease from 95.4 ± 0.5 to 62.4 ± 0.5 % IACS when the silver content is increased from 0.1 to 23.7 wt.%. When Zr is added, the continuous precipitation mode is enhanced and the electrical conductivity is further decreased. For example, as reported by Gaganov et al. in the case of Cu-7Ag (wt.%), the addition of 0.02 wt.% Zr, reduce the corresponding electrical
conductivity of 2.5 % IACS. When the Zr addition reaches 0.05 wt.% Zr, the electrical conductivity is reduced by 10.5 % IACS [5]. In this work, an addition of 0.1 wt.% of zirconium to the Cu-0.1Ag (wt.%) alloy is observed to reduce the conductivity by about 7.6 % IACS. A calculation based on reasonable estimations indicates that an electrical conductivity of about 81 % IACS could be expected for a Cu-3Ag alloys manufactured with our cold spray system conditions. Thus, the decrease of the electrical conductivity could be of about 16.6 % IACS for a 0.5 wt.% zirconium addition, confirming the severe deleterious effect of this element.
3.3. Mechanical properties and electrical conductivity of the heat-treated samples 3.3.1. Mechanical properties Numerous studies on copper alloys manufacturing are concerned with the use of ageing treatments to modify the mechanical properties [13,34,35,40]. Concerning this work, typical ultimate tensile strength, yield strength and corresponding elongation values at break as a function of aging treatments carried out at several temperatures ranging from 180 to about 500°C are presented in Figure 2, 3 and 4 respectively. Loss in strength associated with elongation increase normally occurs during the ageing process. Three different steps can be observed here. First, the strength level practically does not change after annealing at low temperature (i.e. below 200°C). The higher the Ag content, the higher the heat treating temperature necessary to see a change in the mechanical properties (i.e. heat treatment during 4 hours at about 230°C for the Cu-0.1Ag (wt.%), 300 °C for the Cu-5.7Ag (wt.%) and 325°C for the Cu-23.7Ag (wt.%)). This effect can be related to the diffusion rate which depends directly on concentration [41].
Then, annealing above the previously mentioned temperatures causes a decrease of the strength level which becomes more and more pronounced. This drop with the temperature increase can be related to the spheroidisation of the second phase and some changes at the grain boundaries (Figure 1 c). Hamana et al. have also observed the rearrangement and formation of new grain boundaries when annealing is performed on highly cold worked materials [41]. This phenomenon can be explained by the rapid atom transport path provided by the grain boundaries [42]. Diffusion along grain boundaries leads to mass transport and spheroidisation of the second phase. Thus, following heat treatment, cell formation occurred at both side of the grain boundary. This mechanism is consistent with the one proposed by Fournelle [43]. Finally, heat treatments at higher temperature (i.e. above 250°C for the Cu-0.1Ag (wt.%) and 350 °C for the Cu-5.7Ag (wt.%) and for the Cu-23.7Ag (wt.%)) dramatically decrease the strength level due to the further coarsening of the silver rich precipitates as well as the recrystallisation of the Cu matrix. Evolution of the precipitates shape and composition as a function of the aging time has also been widely studied in the past. On a macroscopic scale, heat treatment acts mainly on the reorganization of the matrix and thus mainly affects the gain in mechanical properties previously obtained by the cold work effect. When 0.1 wt% of Zr is added to the Cu-0.1Ag (wt.%), the strength is increased but also the material is more resistant to heat treating transformation. Between 300 and 375°C, the ternary Cu-3Ag-0.5Zr (wt.%) presents similar strength with the binary Cu-5.7Ag (wt.%). However, even if the ductile behavior is more prevalent for copper alloys with a low content of additional elements, the Cu-5.7Ag alloy present a higher ductility (about 9 % instead of 3 % for Cu-3Ag-0.5Zr (wt.%)). It can also be noticed again that a higher temperature is required to change the microstructure of the ternary alloy.
3.3.2. Electrical conductivity Electrical conductivity is also strongly affected by thermal treatments. As shown in Figure 5, the electrical conductivity of the deposited copper alloys tends to increased with the heat treatment temperature. This observation is more prevalent when the amount of second phase is higher. This behavior can also be related to the spherodisation of the precipitates. Indeed, due to the spheroidisation, the precipitates also have a lower surface to volume ratio. At higher temperature, the second phase formation is promoted and consequently the depletion in additional element (i.e. Ag) of the Cu solid solution in the matrix leads to a further increase of the electrical conductivity.
3.4. Isotropic properties In the case of sheet-conductors manufactured by cold-rolling, Sakay et al. reported anisotropy in strength with respect to the rolling direction with a decrease of 10% but no anisotropy regarding the electrical conductivity [2]. Typically, for cold rolling the strength in the perpendicular direction is higher than in the rolling direction and the rate of anisotropy is increased when increasing the reduction ratio. In the case of cold spray deposits, homogeneous properties concerning strength were observed and reported in previous paper [31] for the Cu-0.1Ag alloy. The electrical conductivity measured along the 3 direction gives values closed to 95.5 % IACS (i.e. 95.8, 95.5 and 95.1 %IACS) for the as-sprayed Cu-0.1Ag alloy. Following heat treatment at 225°C, the electrical conductivity stays about 96.1 ± 0.5 % IACS for any direction. Measurements performed on large parts revealed that no anisotropy in electrical conductivity existed within an experimental error of 0.5 % IACS. Isotropic properties (both in strength and conductivity) measured on large parts confirmed that cold spray can be advantageously used
for the manufacturing of large items. However, strength following the deposition direction was not yet measured but will be evaluated in further works.
3.5 Comparison with literature data To evaluate the work hardening effect obtained by cold spray, our results were compared with other data reported in literature. Table 3 presents respectively the ultimate tensile strength and the electrical conductivity of copper alloys manufactured using several processes. Generally, the ultimate tensile strength (UTS) is higher for the samples manufactured with high percentage of alloying elements. As example, in case of Cu-7Ag (wt.%), a UTS value higher than 1000 MPa could be obtained by hot forging, rotary swaging and drawing [6]. The work hardening effect could be very high on the severely deformed material obtained by drawing, ECAP or cold rolling (see Table 3). However, the electrical conductivity is lower than 80 % IACS and the size of the manufactured part is generally limited. In case of cold rolled and heat treated Cu-Cr alloys with and without Zr and Ag, UTS values close to 500 MPa and electrical conductivity close to 90 % IACS are reported [40]. In this case, a high electrical conductivity is maintained thanks to the low percentage of alloying elements. Table 4 focuses on results obtained for copper alloys deposited by cold spray. The best performances reported by Schmidt et al. is about 90-95 % IACS for pure copper. Calculated particles velocities corresponding to spraying with nitrogen at pressure of 30 and 40 bars and temperatures of 800 and 900°C respectively indicate that the impact velocity is exceeding in that case the critical velocity by more than 200 m/s [29]. Indeed, an increase in particle impact velocity or particle impact temperature results in significant increase of the metallurgically bonded areas. Increasing temperature seems more effective for particles of larger size which retain higher temperature upon impact [29].
Watanabe et al. presented results where cold spray copper deposits sprayed with nitrogen (gas temperature of 800°C and pressure of 30 bars), exhibited an electrical resistivity much higher (about 1.9 Ω.m) than that of bulk copper [16]. This effect was attributed to the micro texture. In the present work, the electrical conductivity of pure copper deposits in the as-sprayed condition reaches 96.9 % IACS. This obviously shows that the main effect regarding electrical conductivity is rather linked to the bonding between lamellas and to the porosity level as in our work the porosity level was below 0.1 % and the particles deformation rate ensuring cohesion was higher.
4. Conclusion In this work, a specific cold spray system allowing the use of helium without significant losses was used to manufacture several copper alloys. Helium gas was used to enhance the kinetic energy of the particles, necessary to produce a solid state bond, and avoid high temperature that could have an effect on the oxygen content. High velocity (about 750 m/s) i.e. in the middle range of the deposition window, and low temperature (impact temperature below 200°C) of the particles induced a high cold work effect. The results show that a fine microstructure, a low porosity rate (< 0.1 %) and a low oxygen content (about 150 ppm) of the deposits could thus be obtained using the cold spray manufacturing route. Mechanical properties and electrical conductivity were measured on numerous cold sprayed copper alloys samples. In the as sprayed condition, the copper alloys exhibit relatively high mechanical properties when compared with forged available copper alloys. The hardening effect resulting from the process was clearly observed with two phenomena: - a fine second phase precipitation initiated in the particles manufactured by inert gas atomization - a significant cold work effect with a flattening ratio of the particles close to 0.5.
The nature and distribution of the additional elements in the copper matrix play an important role on the deposit performances. An increase of the silver content enhances the strength due to the larger amount of second phase but reduces the conductivity. Heat treating (aging) may afterwards be used to precisely tailor the mechanical properties (strength versus ductility) selecting adequately the temperature. Changes in microstructure lead to a progressive decrease in strength linked with an increase in ductility and conductivity. An optimal composition and heat treating process could thus be defined to reach a good compromise between mechanical properties conductivity and cost of copper alloys. Such properties are interesting for applications demanding both high mechanical properties and high electrical or thermal conductivity. Moreover, cold spray allows high freedom concerning free form manufacturing and the production of large parts consistent with the industrial demand.
Acknowledgments Authors are grateful to Lucas Dembinski (UTBM) for assistance in powder atomization.
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Figure 1 : Cu-5.7 Ag (wt.%) cross sections SEM micrographs of powder particles (a) and (b), of deposit as-sprayed (c) and following heat treating at 600°C for 4 hours (d) - “ white “ particle ” in picture a) is only an artifact ” Figure 2 : Ultimate tensile strength (± 10 MPa) versus heat treating temperature (4 hours) for cold sprayed copper alloys (full symbol for binary alloys Cu-0.1 Ag (♦), Cu-5.7 Ag and (♦) Cu-23.7 Ag (♦) and open symbol for ternary alloys Cu-0.1Ag-0.1Zr (□) Cu-3Ag-0.5Zr (Δ))
Figure 3 : Yield strength (± 10 MPa) versus heat treating temperature (4 hours) for cold sprayed copper alloys (full symbol for binary alloys Cu-0.1 Ag (♦), Cu-5.7 Ag and (♦) Cu23.7 Ag (♦) and open symbol for ternary alloys Cu-0.1Ag-0.1Zr (□) Cu-3Ag-0.5Zr (Δ)) Figure 4 : Elongation (± 0.5 %) versus heat treating temperature (4 hours) for cold sprayed copper alloys (full symbol for binary alloys Cu-0.1 Ag (♦), Cu-5.7 Ag and (♦) Cu-23.7 Ag (♦) and open symbol for ternary alloys Cu-0.1Ag-0.1Zr (□) Cu-3Ag-0.5Zr (Δ)) Figure 5 : Electrical conductivity (± 0.5 % IACS) versus heat treating temperature (4 hours) for cold sprayed copper alloys (full symbol for binary alloys Cu-0.1 Ag (♦), Cu-5.7 Ag and (♦) Cu-23.7 Ag (♦) and open symbol for ternary alloys Cu-0.1Ag-0.1Zr (□) Cu-3Ag-0.5Zr (Δ))
Table 1 : Main deposition parameters for the cold-sprayed copper alloys Table 2 : Properties of the copper alloys deposits in the as-sprayed conditions Table 3 : Comparison of tensile strength and electrical conductivity obtained with copper alloys manufactured by different processes Table 4 : Comparison of tensile strength and electrical conductivity for pure copper manufactured by the cold spray process
Table 1 Chamber pressure (mbars) 476-945
Nozzle pressure (bars) 16.0-29.5
Nozzle temperature (°C) 488-540
Carrier gas Powder feed Spray flow rate distance (L/min) (g/min) (mm) 53-108 35-110 18-25
Table 2 Copper alloys
Oxygen content (wt.%) (ppm) Cold sprayed (This work)
Yield Strength (MPa)
Ultimate Strength (MPa)
Tensile Elongation Electrical conductivity (%) (% IACS)
Pure copper Cu-0.1Ag Cu-5.7Ag Cu-23.7Ag Cu-0.1Ag-0.1Zr Cu-3Ag-0.5Zr
<200ppm <200ppm <200ppm <200ppm <200ppm <200ppm
306 ± 10 438 ± 10 643 ± 10 646 ± 10 442 ± 10 576 ± 10
320 ± 5 466 ± 5 701 ± 5 646 ± 5 483 ± 5 576 ± 5
3.0 ± 0.5 4.04 ± 0.5 1.25 ± 0.5 0 ± 0.5 7.03 ± 0.5 0 ± 0.5
96.9 ± 0.5 95.4 ± 0.5 74.3 ± 0.5 62.4 ± 0.5 87.8 ± 0.5 64.4 ± 0.5
Table 3 Material
[5]
[6]
[44] [2]
(wt.%) Cu-7Ag-xZr Draw wire pieces Cu-7Ag Cu-24Ag Cu-7Ag-0.05Zr Cu-0.5Cr
[45]
Cu-6Ag Cu-8Ag Cu-12Ag Cu-24Ag Sheet-conductor Cu-Ag Wires Cu-0.44Cr-0.2Zr
[7]
Cu-0.5Cr
[40] [40]
Cu-0.5Cr-0.15Zr0.1Ag Cu-0.5Cr-0.03Zr
[40]
Cu-0.5Cr-0.1Ag
[16]
Bulk copper C1020
[35]
Deformation process Hot forging, profile rolling and drawing Hot forging, rotary swaging and drawing Equal channel angular pressing Cold rolling cold drawing and heat treatment Drawing and heat treatment Equal channel angular pressing Equal channel angular pressing and cold rolling Cold rolling and heat treatment Cold rolling and heat treatment Cold rolling and heat treatment /
Ultimate Tensile Strength (MPa) 1416
Electrical conductivity (% IACS) 59.9
1080 ≈ 1100
68.7 68
620
74.9
912 947 989 1050
/ / / 75
1000
80
700
81
570 – 579 554
35 84
570
86
490
88
500
95
225
98,5 ± 4
Table 4 Material
[46] [47] [17] [29] [48] [29] [49] [17] [29] [29] [16] [This work]
Pure Copper Pure copper Pure copper Pure copper Pure copper Pure copper Pure copper Pure copper Pure copper Pure copper Pure copper Pure copper
Gas
Gas temperature
Gas pressure
Electrical conductivity
(bars) 20
Ultimate Tensile Strength (MPa) /
Air
(°C) 450
Air
450
22
/
35 ± 3
Nitrogen
350
30
57
50
Nitrogen
300
30
90
60
Nitrogen
305
30
/
63
Nitrogen
600
30
240
80
Nitrogen
600
27
325
/
Nitrogen
800
30
391
80
Nitrogen
800
30
375
80 - 90
Nitrogen
900
40
450
90 - 95
Nitrogen
800
30
360
90.7 ± 5
Helium
600
30
300
96.9
(% IACS) ≈ 31