Properties of Cu-based Alloys-powders for Brazing Prepared by Water Atomization
Lastnosti prahov-zlitin na osnovi bakra izdelanih z vodno atomizacijo
B. Šuštaršič*, B. Breskvar, V. Leskovšek, A. Rodič, Institute of Metals and Technologies, Ljubljana, Slovenia
Cu based alloys-powders are currently used as solder and brazing agents in many fields of application. Cu-Ag based alloys-powders are the most common brazing agents used in electrical engineering and electronics. Similarly, Au-Cu based ailoys-powders find many applications in electrical engineering and electronics as vvell as in jewellery. Vacuum brazing of tool and high speed steels as vvell as hard metals (cemented carbides) on the structural steel base vvith Cu-Ni or Cu-Cr base alloys-powders is also increasingly used in many fields of application. Fluid and centrifugaI atomization are the most frequent and therefore the most important manufacturing methods for the producfion of Cu based alloys-powders. Among the fluid atomization processes, gas and vvater atomization are the most common and the most important ones. The stream of liquid metal is disintegrated by high pressure jets of mert gas (N,, Ar or He) or vvater. Gas atomization makes it possible to produce high quality. on the surface non-oxidized spherical povvder particles. l/l/afer atomization is a simpler and cheaper povvder manufacturing method. The metal povvder produced by vvater atomization usually has irregularly shaped particles vvhich are coated vvith a thin oxide film. In spite of that, the gas atomized povvders could be replaced by vvater atomized povvders in many fields of application. We therefore investigated the applicability of vvater atomization for the preparation of Cu based alloys-powders. The present arficle introduces the morphological and microstructural characteristics of prepared Cu-based alloys-powders and possibilifies for their application.
Key vvords: povvder manufacturing, povvders for brazing. vvater atomization, properties of Cu-based alloys-powders.
Prahovi-zlitine na osnovi bakra se uporabljajo na mnogih področjih kot sredstva za mehko in trdo spajkanje. V elektrotehniki in elektroniki so najpogostejše spajke na osnovi Cu-Ag. Podobno se zlitine na osnovi Au-Cu uporabljajo v elektrotehniki, elektroniki in tudi zlatarstvu. Vakuumsko spajkanje orodnih jekel ali karbidnih trdm na osnovo iz konstrukcijskega jekla s prahovi-zlitinami na osnovi Cu-Ni ali Cu-Cr se tudi vse bolj uveljavlja v praksi. Prahovi-zlitine na osnovi Cu se najpogosteje izdelujejo s tekočinsko in centrifugalno atomizacijo. Med postopki tekočinske atomizacije se najpogosteje uporabljata plinska in vodna atomizacija. Curek raztaljene kovine pri teh postopkih razpršimo v hitro strjene delce prahu s pomočjo visokotlačnih curkov inerfnega plina (He, Ar ali N,.) ali vode. Plinska atomizacija omogoča izdelavo visoko kakovostnih neoksidiranih prahov kroglične oblike. Vodna atomizacija je enostavnejši in cenejši postopek, vendar so tako izdelani kovinski prahovi običajno površinsko oksidirani in imajo zato tudi nepravilno oblikovane delce. Kljub temu bi lahko vodno atomizirani prahovi na mnogih področjih zamenjali dražje plinsko atomizirane prahove. Zato smo raziskovali možnost uporabe vodne atomizacije za izdelavo kovinskih prahov-spajk na osnovi Cu. V pričujočem delu so predstavljene morfološke in mikrostrukturne značilnosti izdelanih prahov ter posledično možnosti njihove praktične uporabe. Ključne besede: izdelava kovinskih prahov, prahovi za trdo spajkanje, vodna atomizacija, lastnosti prahov zlitin na osnovi Cu.
1. Introduction
Metal povvders are used extensively as filler material in the brazing and soldering industries'. Powders offer the most con-venient method of appl ving filler metal to parts which have to be joined (bonded) together. although alternative filler metal forms (wire, rod, sheet. foil, etc.) are also used. Filler metals (alloys) for brazing have liquidus temperature above 450°C and below the solidus of the base metal, and filler metals for soldering have liquidus temperature belovv 450°C and below the solidus of the
: Mag. Borivoj ŠUŠTARŠIČ. dipl. inž .
IMT Ljubljana, Lepi pot I I. 61 (Kit) Ljubljana
base metal. For brazing and soldering applications metal alloys-povvders either as pure povvder vvithout additions or in flux-pow-der paste form are used.
Cu based alloys-powders are currently used as brazing agents (filler material) in many fields of application. Cu-Ag based alloys-powders are the most common brazing agents used in electrical engineering and electronics. Similarly, Au-Cu based alloys-powders find many applications in electrical engineering and electronics as vvell as in jewellery. Vacuum brazing of tool and high speed steels (HSS) as vvell as hard metals (cemented carbides) on the structural steel base vvith Cu-Ni or Cu-Cr as vvell
as Ni-Cr base alloys-powders is also increasingly used in many fields of applicatiotr
Fluid and centrifugal atomization are the most frequent and therefore the most important manufacturing methods for the pro-duetion of Cu based alloys-powders. Among the fluid atomization processes. gas and water atomization are the most eommon and the most important ones4 \ The stream of liquid metal is dis-integrated either bv high pressure (comntonly 10 - 30 bars) jets of inert gas (N,. Ar or He) or by high pressure (commonlv 100 -300 bars) jets of vvater. Fig. 1 represents sehematically a gas at-omizer and the disintegration of a free falling molten metal stream by high pressure jets (d' inert gas.
Gas atomization makes it possible to produce high quality. on the surface non-oxidized spherical powder particles. Water
vacuum	vakuumska
induction	indukcijska
melting furnace ( talilna peč
spheres kroglice
coliec+ion zbiralna chamber, posoda
metal powder kovinski prah
Figure 1: Schematie presentation of gas atomizer4 Slika I: Shematični prikaz postopka plinske atomizacije4
atomization is a simpler and cheaper powder manufacturing method. Fig. 2 shows schcmaticallv the production of metal powder by water atomization.
The metal povvder produced bv vvater atomization usuallv has irregularlv shaped particles vvhieh are coated vvith a thin ox-ide film. In spite of that. the gas atomized povvders could be re-placed by vvater atomized povvders in manv fields of application. We therefore investigated the applicabilitv of vvater atomization for the preparation of Cu based alloys-powders.
2 Experimental procedure
Most of the experintents and investigations have been done at the Institute of Metals and Technologies, Ljubljana.
Tvvo different vacuum brazing allovs (Cu vvith 2% of Ni and Cu vvith 1,5% of Cr) and tvvo typical Cu-Ag based brazing allovs (L-Agl5P and L-Ag40Cd) vvere selected for our e\periments and investigations. For the comparison some other types of al-loys-powders (Ni and Co-based povvders for vvelding, HSS povvders. Alnico hard magnetic povvders) of previous investigations7^"111 vvere also used. The povvders of selected allovs vvere prepared by vvater atomization (pilot atomizer Davv McKee, typc D5/2) installed at IMT Ljubljana (see Fig. 3).
5CSS«
Figure 3: PM&RST laboratorv al the IMT Ljubljana vvith vvater atomizer in the foreground.
Slika 3: PM&RST laboratorij na IMT Ljubljana z vodnim atomizerjem v ospredju.
Figure 2: Schematie presentation of vvater atomization" Slika 2: Shematični prikaz postopka vodne atomizacije"
Table 1: Chemical compositions of prepared Cu-Ag based vvater atomized povvders in comparison vvith ASTM standar-dized1 brazing allovs-povvders.
Tabela 1: Kemične sestave z vodno atomizacijo priprav ljenih prahov na osnovi Cu-Ag v primerjavi z zlitinami standardiziranimi po ASTM standardih
Material	B CuP-5	L Agl5P	B Ag-2	L Ag40 Cd
	nominal	IMT	nominal	IMT
Elements	I mass 'i )			
Cu	>79,85	balance	26.0	19.0
Ag	15,00	14,9-15,60	35.0	40.0
P	5,00	6,0-6,20	-	
Zn	-	-	21.0	21.0
Cd	-	-	18.0	20.0
Other elements	0,15max.	not determined	0.15max.	not determined
Melt Talina
Tundish Vmesna posoda
High pressure water jets Visokotlačne vodne šobe
Induction melting
furnace Indukcijska talilna peč
Metal stream Curek tekoče kovine
vessel
komora
Slurry Gošča (water/powder)
To pressure devvatering and vacuum drying Vakuumsko sušenje
Molten metal Raztaljena kovina
Tundish nozzle Izlivek
High pressure pump Visokotlačna črpalka
Table 2: Process parameters of water atomization for the preparation of Cu-2<7f Ni and Cu-Ag based alloys-powders.
Tabela 2: Procesni parametri postopka vodne atomizacije za pripravo kovinskih prahov Cu-2%Ni in Cu-Ag.
Process parameter (vvater atomizer D5/2 Davv McKee	Cu-2'SNi allov	Cu-Ag allovs	Remarks
Temperature of superheating (°)	123C)±20°C	810+31) C	pyromcter
Tundish temperature ( 1	1210120 C	850±30"C	Pt-1'lRli 13
Tundish nozzle diameter (mm)	04.0/4.5	04,0/4.5	fused quarl/
Metal flou rate (kg/min.l	4.5-5,5	5,6-7.3	
Water nozzle diameter i mm) lwo (2) main nozzles tvvo (2) side nozzles	12 x 1.05 1,1 x 0.85	1.2 x 1.05 1.1 x 0.85	150.3 tvpc 1502tvpc
Jet apex angle (°) betvveen main vvater streams betvveen side vvater streams	50 41)	50 40	original manifold
Water/metal ratio	11-12	7,3-9,6	
Watcr pressure range I bari	120	180-215	manometer
Protective atmosphere (nitrogen)	O.hni Vb	0.65m'/h	flovv meter
The initial chemical compositions (ingots for melting) as well as the final chemical compositions of prepared povvders vvere determined and controlled bv classical chemical analvsis. No significant difference betvveen initial and final compositions vvas observed. In Table 1 the chemical compositions of selected Cu-Ag brazing allov s are presented.
The process parameters of vvater atomization are vvater pres-sure, tundish nozzle diameter, apex angle and diameter of vvater jelš. superheating of the melt. etc. The main influent povvder par-ticle size parameter of these is vvater pressure" and vvater veloc-itv. respecliv el v. In the first stage of our experiments the process parameters of vvater atomization vvere established and optimized in order to get an optimal mean particle size and optimal particle size distribution. For brazing, first of ali the povvder fractions betvveen 45 and 125 um are commonly used. In Table 2 the main controlled process parameters of vvater atomization during our experiments are presented.
Ali prepared povvders vvere then sieved and their main mor-phological properties (particle shape, particle size distribution, mean particle diameter, apparent densitv and flowability) vvere determined. Besides the chemical composition of the alloy. these povvder properties are the main applicabilitv criteria of metal povvders for brazing.
3 Results and discussion
Ali prepared povvders vvere examined by optical and scanning electron microscopv (SEM). The micrographs shovv mi-erostruetures vvhich stionglv depend on the particle size and the cooling rate. Fig. 4 and 5 shovv the tvpical microstructure of prepared povvders. Some internal porositv of povvder particles vv as noticed. Fig. 6 shovvs the SEM micrograph of a vvater atomized Cu-2CNi allov-povv der vv ith nearlv spherieal particles. Fig. 7 shovvs internal porositv of a vvater atomized Cu-Ag (I. Agl5-P) based allov-povv der. Particularlv Cu-Ag based alloys-powders shovv a large amount of povvder particles vvith internal porosity.
The standard sieving analvsis. as vvell as the Silas Alcatel laser granulometry of povvders vvere also perlormed'"l:. The
povvders have a regular and partially irregular particle shape. The mean particle diameter of prepared povvders stronglv depends on the vvater pressure used. The portion of irregularlv shaped particles raises vvith inereasing vvater pressure (inereased eollision betvveen droplets of molten metal as vvell as partially solidified particles during atomization). The relatively high portion of regularlv shaped particles confirms the sometimes forgotten fact that vvater atomization can also produce nearlv spherieal povvders. vvhich is a condition for high apparent density and good flowability of povvders.
Figure 4: Ccllular solidification of Cu-2% Ni allov--povvder particle. Slika 4: Celična strjevalna mikrostruktura delca Cu-2'/r Ni prahu.
Figure 5: Cellular solidification of Cu-Ag (L Agl5-P) alloy-powder particle.
Slika 5: Celična strjevalna mikrostruktura delca Cu-Ag prahu (L Agl5-P).
The mean particle diameter of prepared Cu-2%Ni povvders is approximately 50 pm at atomizing pressure 120 bars vvith a relativen regular particle shape and narrovv particle size distribution (standard deviation cr = 2.2). The mean particle diameter of the prepared Cu-l,5%Cr povvders is approximately 55 um at atomizing pressure 150 bars vvith a higher amount of irregularlv
Figure 6: Particle shape of prepared Cu-29f Ni alloy-powder. Slika 6: Oblika delcev pripravljenih Cu-2'/< Ni prahov.
Figure 7: Internal porositv of nearlv spherical Cu-Ag allov -povvder particles.
Slika 7: Notranja poroznost delcev Cu-Ag prahov.
shaped particles (high melt ing point Croxide formation) in cotrt-parison vvith Cu-Ni povvders but a relativelv narrovv particle size distribution (a ~ 2.1) is obtained. The preparation of Cu-Ag based povvders vv as performed al considcrablv higher atomizing pressures ( 1 SO to 220 bars) in order to get a higher amount of particle size fractions belovv 125 um. The mcan particle diameter of tltc prepared Cu-Ag povvders is approximately 7(1 um al I SO bars and approximately 45 pm at the atomizing pressure 215 bars vv ith a higher amount of incgularlv shaped particles (inereased colli-sion) in comparison vvith Cu-Ni povvders and a rclativclv narrovv particle size distribution (o = 2,25) is also obtained. Tvvo infor-malive atomizing cxpcrimcnts for the preparation of Au-Cu based allovs-povvdcrs (alloy vv ith 5S mass '/r of Au - 14 carat gold and allov vvith 75 mass 9! of Au - IS carat gold) vvere also
Figure S: SEM micrograph of Au-Cu u ater atomizcd povv der v\ ith irregularlj shaped (tear drop and ligamental) particles.
Slika 8: REM posnetek vodno atomiziranega prahu na osnovi Au-Cu / delci nepravilne (vlaknaste in kapljičaste) oblike.
performed. The mcan particle diameter of the prepared Au-Cu povvders is approximately 55 um al atomizing pressure 140 bars vvith a rclalivclv high amount of incgularlv shaped particles i see Fig. 8).
Unlortunatelv our pilot vvater atomizer is too large (15 kg/bateh) for the preparation of small quantities ofthe verv cx-pensive Au-Cu based allov s-povvdcrs vvhich are normallv used in practice. For these types of alloys-powders a small atomizer vv ith the capacilv of up lo 0.5 kg/bateh max. is suitable. We therefore dccidcd lo give up further cxpcrimental vvork until vve purchase a small gas/vvater atomizer suitable for the preparation of these allov s-povvdcrs.
For other constant process parameters ol atomizat ion. the re-lationship betvveen vvater pressure and the mcan particle stzc shovvs a good. from the literature4vvcll knovvn exponential correlation (DSI = k . P,,,,,") for ali prepared povvders. The con-stants k and n depend on the allov composition. the geometrv of the atomizer and other process parameters of atomization. For the Cu-Ag based allov (L Agl.vP type) the constants are esti-matcd as k = 8900 and n = -0.96 for the tundish nozzlc diameter 4.0 mm and the other process parameters of atomization given in Table 2. Fig. 9 shovv s the experimental correlation: mean part i -
Co-base Milit F alloy * Co zlitina Milit F
Cj-Ag braz 'g alloy Cu-Ag zlitina za spaikan|e
Ql----------------------
0	50	100	150	200	250 300
Water pressure (bars) Tlak vode (bari)
Figure 9: Mean particle size vs. vvater pressure for Cu-Ag brazing-allov povvder and Co-based vvelding-allov povvder. Slika 9: Povprečna velikost delcev v odvisnosti od tlaka vode za prahove na osnovi Cu-Ag in prahove za navarjanje na osnov i Co.
E E
~ 3 250 o
g J 200
cle size vs. water pressure for the Cu-Ag brazing alloy-powder and the Co-based welding-alloy powder. The influence of chem-ical composition is evident.
Ali prepared powders have the bimodal particle size distribution with two maxima at approximately 40 to 60 um and ap-proximately 100 to ISO pm virtuallv independent of the applied water pressure and other parameters of water atomization. This confirms the statement1' that the disintegration of the molten metal stream during water atomization is carried out in two steps (the so-called primary and secondary disintegration). Fig. 10 presents the sieving analvsis of Cu-AI powders with notieeable bimodal particle size distribution and the influence of chemical composition. Fig. 11 shows the bimodal particle size distribution of prepared Co-based water atomized powders determined by laser granulometrv". Bimodal particle size distribution vvith tvvo maxima is clearlv evident.
In the Wosthoffs apparatus the oxygen content of prepared povvders vvas determined. The prepared vvater atomized povvders have a relativelv high oxygen content in spite of the fact that for pure metals (Cu. Ag. Ni and Au) Gibbs free entalphy for the re-action Me+H:0=VIe0+H is positive"14 and therefore theoreti-cal possibilitv for oxidation of pure metals vvith vvater steam is relativ elv lovv. Obviously. the alloying elements and impurities vvith higher affinitv to oxygen (Cr. Mn. Cd. Zn, P. etc.) have an important influence on t h in oxide film formation on the surface of povvder particles as vvell as other factors vvhich can inerease oxygen content (diseussed later in the article). Fig. 12 shovvs that from the theoretical point of vievv, for some pure metals as vvell as allovs smaller oxygen contents in vvater atomized povvders could be expected, and vice versa. In spite of that a rough esti-mation of oxygen content in vvater atomized povvders is possible from this diagrammatic presentation. The diagram is based on literature" as vvell as our ovv n cxperimental data.
<45 45/63 63/75 75/90 90/125 125/250 250/1000 >1000 Particle si2e fractions (pm) Velikost delcev po frakcijah (pm)
Figure 10: Sieving analvsis histograms of Cu-AI vvater atomized povvders. Slika 10: Histogrami sejalnih analiz Cu-AI vodno atomiziranih prahov.
25
20
Ji	<D
■S ^ 15
g	8
g	E ..
Water pressure
- 275 bars
........ 200 bars
--- 100 bars
---40 bars
50
Particle size ( pm) Veli kost delcev ( p m )
500
Figure 11: Bimodal particle size distribution of Co based vvelding alloy-powders performed bv laser granulometry.
Slika 11: Dvogrba velikostna porazdelitev delcev prahov za navarjanje na osnovi Co dobljena z laserskim granulometrom.
ji
o o
-600
Ag *						
Ni	Cu					
Au-NI * «......- Cu-Sn i	*			Co Fe	t	
	Fe-C	Fe-13Cri	t		Fe-	80Mn
* Ni-Cr-B-Si			i Fe-45S			
						Al*
3000
3500
0 500 1000 1500 2000 2500 Average oxygen content (ppm O2)
Povprečna vsebnost kisika (ppm O2)
Figure 12: Gibbs free entalphy for the reaetion Me+H,0=Me0+I 1, vs. oxygen content for different vvater atomized alloys-powders. Slika 12: Gibbsove proste entalpije za reakcije Me+Il,0=Me0+H v odvisnosti od vsebnosti kisika za različne vodno atomizirane prahove-zlitine.
The plot oxygen content vs. particle size fractions exhibits, for most povvders, the minimum oxygen content for particle fractions betvv een 60 and 00 pm. The expIanation of this phenome-non is in the combination of the effects of the particle size and the cooling rate (specific surface area - degree of oxidation) of particles during vvater atomization1. Exceptionally, povvders vvith a high degree of particle shape irregularity exhibit a direct pro-portional inereasing degree of oxidation vvith inereased specific surface area of the povvder1. Fig. 13 shovvs the plot oxygen content vs. particle size fractions of different prepared povvders vvith
0
<45 45 /63 63/75 75/90 90/125 125/250 Particle size fractions (pm) Velikost delcev po frakcijah (pm) Figure 13: Oxygen content of prepared povvders. Slika 13: Vsebnost kisika v izdelanih prahovih.
0.6
'—o— PM-23/180 bars —#- BRM-2/150 bars 0.5 -I—:AlN1C0 1500L/200 bars Milit F/40 bars Cu-2% Ni/120 bars
the minimum oxygen content for partiele size fractions between 60 and 90 um.
The average oxygen content of prepared Cu-2%Ni povvders is 0.23 mass ''/, O,, for Cu-l,5%Cr povvders 0.15 mass % of O, and for Cu-Ag povvders 0,15 mass % of O,. Our opinion is ihat a considerable reduction of these values is possible by an addi-tional optimization of vvater atomization (melting time reduction, superheating temperature as lovv as possible, inert gas blovv-ing or protective slag used during melting, reduced vvater/povvder partiele interconnecting time, etc.). For example, the average oxygen content 0,075 mass % for Cu-Ag alloy-povv-der vvas already obtained during our experiments.
The relativcly high oxygen content found in prepared Cu-based allovs-povvders requires the consideration in vvhat forms the oxygen can be found in povvder particles. It is usually found in metal particles in the follovving forms: dissolved oxygen, sur-face oxides, surface adsorbed molecular oxygen and diserete ox-ides. The Ni and Cu-based povvders can contain several thousand ppm of dissolved oxygen if not adequately deoxidised prior atomization"*. The free energy for the reaction of Ni and Cu vvith vvater steam is positive. vvhich suggests that the amount of surface oxides present in the Cu-Ni based povvders is insignificant. The origin of diserete oxides is usuallv slag and refractory particles. The surface adsorbed molecular oxygen, vvhich results from povvder handling after the atomization, niust not be neglected, ei-ther. In our future experiments, it vvould therefore be necessary to determine precisely the individual contribution of the above mentioned oxides to the total oxygen content of Cu-Ni povvders.
The flovvabilitv and the apparent density of povvders vvere determined vvith the HalFs apparatus1'. The flovvabilitv raised vvith inereased sphericity of povvders. The fractions betvveen 45 pm and 125 um of ali prepared povvders have relatively good flovvabilitv but the partiele size fraetion belovv 45 pm has no flovvabilitv. except Cu-2%Ni and Cu-Ag povvders prepared at the lovvest (120 bars) vvater pressure. Fig. 14 shovvs the correlation: flowability of prepared povvders vs. partiele size fractions. The povvder fractions betvveen 63 pm and 125 pm have the best flowability. The prepared Cu-2%Ni povvders have the best flowability in comparison vvith other prepared povvders. This is in accordance vvith the highest amount of regular (spherical) povvder particles obtained for this type of alloy.
In accordance vvith high amounts of internal partiele porositv of prepared L Agl5-P alloy-powders a relativelv poor apparent as vvell as tap densitv of these povvders is obtained.
o S
"S o
	i				X
	- + ■■Cu-2V.NI/2; 120 bars —°— Cu-1,5 V. a, 150 bars -■a— L Ag 15-P; 120 bars				
					^ A
	-X— L Ag15-P; 180 bars				C
				---	
					
					
					
63/75	75/90	90/125	125/250
Log partiele size fractions (pm) Log velikosti delcev po frakcijah (pm)
Figure 14: Flowability of prepared povvders. Slika 14: Tekočnost izdelanih prahov.
Fig. 15 shovvs the correlation: apparent density of prepared povvders vs. partiele size fractions. The highest apparent as vvell as tap densities are obtained for the finest fractions of povvders.
—X— Cu-t.5V.Cr, 150 bars —x— L Ag 15- P , 180 bars --+••- L Ag 15-Pi 120 bars —o— Cu - 2 7. Ni, 120 bars
0 — <45
45/63	63/75	75/90	90/125 125/250
Partiele size fractions (pm) Velikost delcev po frakcijah (pm)
Figure 15: Apparent densities of prepared povvders. Slika 15: Nasipne gostote izdelanih prahov.
The determined morphological properties of prepared al-loys-powders shovv that vvater atomized povv ders could be useful for different brazing applications, vvhere relativelv high oxygen content is not harmful (vacuum brazing, brazing in reduetive atmosphere, brazing vv ith allov additions of deoxidizers or fluxes that produce light lovv melting point protective slags). Therefore vve also made some brazing experiments: lx"\ Vacuum brazing of high speed steel (circular savv segments and paper knifes) on the structural steel base vvith a Ni-Cr based allov (Nicrobraz Wall Colmonov type 30 and LM filler metals) as vvell as vvith the prepared Cu based brazing allov-povvder gives high strength. non porous and other defect free. vvell diffusion bonded joints. Fig. 16 shovvs a high temperature vacuum brazed high speed steel (circular savv segments) on the structural steel base vvith simul-taneous heat treatment performed at IMT Ljubljana. Fig. 17 shovv s the microstructure of a high temperature vacuum brazed tool steel/structural steel joint produced at IMT Ljubljana vvith a Cu-based brazing povvder.
Vacuum brazing of sharp edged WC-Co particles on the steel plate also gives good results. Fig. 18 shovvs vacuum brazing of sharp edged WC-Co particles on a steel plate (grinding vvheel)
Figure 16: Vacuum brazing of high speed steel (circular savv segments) on the structural steel base. Slika 16: Segment krožne žage iz orodnega jekla vakuumsko spajkan na osnovo iz konstrukcijskega jekla.
Figure 17: Microstructure of high temperature brazed tool steel/struetural steel joint.
Slika 17: Mikrostruktura vakuumskega spoja orodnega in konstrukcijskega jekla.
with a prepared Cu-2'<Ni brazing alloy-powder performed at IMT Ljubljana. Cu-Ag based alloys-powders also give good brazing results and the above mentioned brazing alloys are al-ready produced on the level of small scale production for Slovenianelectrical engineering industrv (ISKRA Autoelektrika Nova Gorica, Metalflex Tolmin, etc.). Fig. 19 shows industrial vacuum brazing of heating elements on the steel plate carrier performed at IMT Ljubljana. Fig. 20 shows electrical components industriallv brazed vvith vvater atomized Cu-Ag based alloys-povvders.
Figure 19: Vacuum brazing of boiler heating elements on the steel plate carrier.
Slika 19: Vakuumsko spajkanje grelnih elementov na jekleno nosilno osnovo.
4 Conclusions
The preparation of Cu based brazing alloys-powders by vvater atomization vvas investigated. The main morphological properties of prepared povvders vvere then determined. The deter-mined morphological properties of prepared alloys-powders, as well as practical brazing experiments shovv that vvater atomized povvders could be successfullv used in many brazing applica-tions. especially vvhere relatively high oxygen content in vvater
		
ifJH m w		
k A i	•jgC*	lwmm ■ I
l&f ^'•"»••'-SjU	mm	■BHSMr?*'' f M '",/ • M
Figure 18: Vacuum brazing of WC-Co particles on the steel plate. Slika 18: Vakuumsko spajkanje ostrorobih delcev karbidne trdine na jekleno osnovo.

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7 8 9 20

!!l|llll|llil|llll|!ll'|li'lpl:,i>.<| 2 3 t S E
Figure 20: Electrical components for automotive applications industriallv brazed vvith vvater atomized Cu-Ag based alloys-powders manufactured at IMT Ljubljana.
Slika 20: Električni sestavni deli za avtomobilsko industrijo spajkani / vodno atomiziranim Cu-Ag prahom izdelanem na IMT Ljubljana.
atomized povvders is not harmful. Hovvevcr. practical brazing ex-periments supported by metallographical and mechanical inves-tigations of each individual brazing application are always nec-essary in order to confirm the usability of the selected vvater atomized Cu based alloy-powder.
An important fact that has to be considered is the y ield of us-able metal povvder (fraction +45 -I25pm up to +45 -75um) for brazing applications. Water and gas atomization give approxi-mately the same but relatively poor povvder yield (=25+35%). Centrifugal atomization gives a better yield (= 40% ) but seems to present some practical difficulties in engineering and reliabil-itv1 . Therefore, from this point of vievv, it can be concluded that vvater atomized povvders for brazing are comparable vvith gas atomized povvders.
References
1	E. Klar et al.: Part 3: Production of Metal Povvders and Part 7: Povvder Systems and Applications - Metal Povvders Used for Brazing and Soldering, Metal Handbook, 9lh edition. Volume 7. Powder Metallurgv.
2	V. Leskovšek, D. Kmetič, J. Gnamuš, G. Rihar: High Temperature Vacuum Brazing of HSS on Construction Steel vvith Simultaneous Heat Treatment, Vuoto. 20, 1990, 2, 515-518.
M. Stock. K. Hack: Thermochemical Aspects of Multiphase Diffusion during Brazing of Hard Metal, Z. Metallkd. 84, 1993, 11, 759-766.
4 R. M. German: Povvder Metallurgv Science, Metal Povvder Industries Federation (MPIF), Princeton, Nevv Jersey, 1984. F. V. Lenel: Powder Metallurgv - Principles and Applications, MPIF. Princeton, Nevv Jersey. April 1980.
6 J. J. Dunkley: The Production of Metal Povvders by Water Atomization, Povvder Metallurgv International. Vol. 10. No. 1/78.
B. Šuštaršič, S. Beseničar. J. Holc, Z. Lengar, S. Tašner: Water Atomized Fe-Co-based Povvders for AlNiCo magnets, S'h International Confercncc on Povvder Metallurgv in Czeclioslovakia, October 1992, Pieštany, ČSFR. Conference Proeeedings Vol. 3. pp. 261-264.
s M. Torkar, [i. Šuštaršič: Microstructural Characteristics of Water Atomized Povvder of Ni-superalloy, Praktische
Metallographie. Sonderbande 24. Carl Hansen Verlag. Miinchen-Wien. pp. 79-85. " B. Šuštaršič, A. Rodič, M. Torkar. F. Vodopivec: Preparation of Cobalt Based Povvders bv VVater Atomization, Metallurgv. 33, 1994, 4. 147-150; Croatian Metallurgical Societv. Facultv of Metallurgy Sisak, Universitv of Zagreb.
10	B. Šuštaršič, M. Torkar: Manufacturing of High Speed Steel Povvders by Water Atomization (Slovenian language), VAKU-UMIST - Slovenian Societv of Vacuum Teehnology, Ljubljana, May 1991, No. 23-24. 1991/2-3, 9-15.
11	J. J. Dunkley, J. D. Palmer: Factors Affecting Particle Size of Atomized Metal Povvders, Powder Metallurgv International. 29. 1986.4.
12	MPIF: Standard Test Methods for Metal Povvders and Povvder Metallurgv Products. Metal Povvder Industries Federation, Edition 1985/1986. Princeton, Nevv Yersey.
" M. Biirger. E. V. Berg. S. H. Cho, A. Schatz: Fragmentation Processes in Gas and VVater Atomization Plants for Process Optimization Purposes, I. Discussion on the Main Fragmentation Processes. Povvder Metallurgv International. 21. 1989, 6. 10-14.
14	C. Jorgensen, I. Thorngren: Thermodvnamic Tables for Process Metallurgists. Almqvist & VViksell. Stockholm. 1969.
15	D. Kmetič et al.: Bonding ofTool Steels vvith Structural Steels in Vacuum Heat Treatment Furnace. IMT Ljubljana R&D reports (Slovenian language). November 1988, No. MI 87-032.
"' D. Kmetič et al.: High Temperature Brazing vvith Simultaneous Heat Treatment in Vacuum. IMT Ljubljana R&D reports (Slovenian language). November 1991. No. IMT 89-064.
J. J. Dunklev: The Atomisation of Electronic Grade Solder Povvder. Povvder Metallurgv World Congress PM'94. Pariš, Conference Proeeedings Vol. 2. 353-356.
K J. J. Dunkley: The Factors Determining the Oxygen Content of VVater Atomized 304L Stainless Steel Povvder. Reprint of Paper Presented at the National P M Conference. Philadelphia, U.S.A., May 1981. Davy-Loewy R&D Centre.