The characteristics of high strength and lead-free machinable
α-β duplex phase brass Cu-40Zn-Cr-Fe-Sn-Bi alloy
Department of Composite Materials Processing, JWRI, Osaka University
Introduction
Brass alloys are used for pipes, valves, and fittings in systems that transport water and other aqueous fluids

Machinable leaded brassAlloys
CAST1
CAST2
CAST3
CAST4

Sn
0.59
0.595
0.6
0.578

(a)CAST1 (without Bi)

Pb-0.005-0.005-0.005-0.005

Cr0.34
0.256
0.26
0.22

Bi--0.994
2.02
2.85

β-phase

Cr-Fe IMC

α-phase110108111
112

Bi(not sphere)

α-phaseα-phase50μm

50μm

Bi(not sphere)

50μm

Hot extrusion
ContainerCu-40Zn alloy
α-phase: 100 Hv
β-phase: 144 Hv

Punch
Brass billet
DieCast ingot(φ41 mm)

Bi in the β-phase formed spherical particles; however, irregularly-shaped Bi particles formed in or aroundthe α-phase. The difference in surface tension between Bi particles and the matrix decreased as the elemental Zn content in the matrix increased,

50μmand spherical Bi particles then formed in the β-phase which contained higher elemental Zn concentration than the α-phase.

(b)EXT2 (0.99mass%Bi)
β-phaseBi35μm35μm

β-phaseBi
Cr-Fe IMC
α-phaseα-phase(c)EXT3 (2.02mass%Bi)

111

Mean grain size

50μm

35μm

50μm

(d)EXT4 (2.85mass%Bi)(c)EXT3 (2.02mass%Bi)(d)EXT4 (2.85mass%Bi)

2.86μm 001

35μm

2.13μm101β-phase

SEIBi

550

45500

40450

35400

30350

25Cr

Fe

300

20Bi

Bi

α-phase
50μm

YS /MPa

50μmExtrusion direction

35μm
35μm
3.10μmTransversal cross section of extruded alloys in extrusion direction

4.30μmof EXT2 (0.99mass% Bi) (a), EXT3(2.02mass% Bi) (b), and EXT4(2.85mass% Bi) (c) by SEM.

Bi particles

Bi particles
Bi particles
100um

100umMatrix
Bi particles

Transversal cross section of extruded alloys in extrusion direction

Average drilling speed (mm/s)

Number of Bi particles in matrix (/mm2)

100um

3000
2500
2000
1500
1000

Cu-40Zn-Cr-Fe-Sn-Bi
Cu-40Zn-Bi

500
01

234

Bi content (mass%)

56

Machinability test

0.4
0.35
0.3
0.25

EXT4(2.85mass% Bi)

0.2
0.15

EXT2(0.99mass% Bi)

0.1
Cu-40Zn-Cr-Fe-Sn-Bi
0.05

Cu-40Zn-Bi

001000

2000

3000

Number of Bi particles in matrix (/mm2)

4000

15(c)EXT3(2.02Bi)(d)EXT4(2.85Bi)

SnThe main strengthening mechanism;♣ Solid solution strengthening with elemental additives♣ Increased area present as the hard β-phase

Conclusion

Back scatteredelectron images(c) EXT4(2.85mass%Bi)(b) EXT3(2.02mass%Bi)

3500

Elongation (%)

10Relationship between distribution of Bi particles and Machinability(a) EXT2(0.99mass%Bi)

UTS /MPa

Extruded (a)EXT1(b)EXT2
Cu-40Zn (without Bi) (0.99Bi)

β-phaseBiα-phase5035μm35μm

Bi

600

200

Cr-Fe IMC

β-phaseof tensile test specimens of EXT4 acquired by SEM-EDS

250

α-phase
Cr-Fe IMC

55

The drilling time to make a holewith a 5 mm depth wasmeasured; after repeating thisdrilling test 10 times, theaverage drilling speed was used
as a machinability parameter forthe extruded specimens.
♣ Rotating speed, N
700 [rpm]♣ Cutting speed, V
164.0 [mm/s]♣ Axial thrust load
100 [N] (10 [kgf])♣Drill diameter, D
4.5 [mm]♣ Drill helix angle
30 [degree]♣ Drill point angle
120 [degree]

The extruded Cu-40Zn-Cr-Fe-Sn-Bi alloys exhibited a better dispersion of Bi particles than that of theextruded Cu-40Zn-Bi alloys. Bi morphology in an extruded specimen depends significantly on that of the cast alloy. As above mentioned
for the cast alloys, because spherical Bi particles are formed in the β-phase domains, these in β-rich brass become finer than those in α-rich brass.

800

Lead-freeHigh-strength
Machinable brass
Cu-40Zn-0.3Cr0.2Fe0.6Sn-2.0Bi

Stainlesssteel

700
UTS /MPa

Cr-Fe IMC

Fractured surfaces

Tensile properties
650

Elongation，ε (%)(b)EXT2 (0.99mass%Bi)
IPF MAP

Tensile properties /MPa(a)EXT1 (without Bi)

EBSD images(a)EXT1 (without Bi)

The extruded CAST1-4 specimens aredenoted as EXT1-4 in this study.

Extruded brassalloy

Microstructural and mechanical properties of extruded specimens
Optical microstructures

2000 kN hydraulic direct press machin
Extrusion speed: 3 mm/s
Diameter of extruded specimen: 7 mm
Pre-heading conditions: 923 K for 180 s
in Ar gas atmosphere

α-phase

Cr-Fe IMC
α-phase

50μmLoadβ-phase
170
165
168
169

Optical microstructures

Bi(sphere)
β-phaseBi(sphere)

Bi(not sphere)

Cu
Bal.
Bal.
Bal.
Bal.

(d)CAST4 (2.85mass%Bi)

Cr-Fe IMC

β-phasePb

Vickers micro hardness /Hv0.025

Fe
0.22
0.229
0.23
0.219

(c)CAST3 (2.02mass%Bi)(b)CAST2 (0.99mass%Bi)

β-phase

Cr-Fe IMC

Zn
40.86
40.81
40.64
40.83

A selection of extruded profiles

α-phasebecause of their excellent corrosion resistance. Pb at levels of 2-5 mass% is also added to traditional brasses toimprove their machinability. However, addition of Pb to these materials is a significant, severe hazard to theenvironment and human health. Thus, material designs must consider safety according to the regulations of theRestriction of Hazardous Substance (RoHS) and Waste Electrical and Electronic Equipment (WEEE) Directives.
New high-strength, Pb-free machinable α-β duplex brass alloys with dispersed Bi particles were produced
using a casting and extrusion process. The effects of Bi dispersoids on the mechanical properties andmachinability of the extruded specimens were investigated.

Materials and methods

Turned brass components

β-phase600

Trade-off balance500

Lead-freemachinable
brass(Si)400

Conventional
machinable brass(with 3.2mass%Pb)
Machinable
brass (Bi, Pb)

300
02468
Machinability ( 1/cutting work time, 10-2/s-1)

10The extruded Cu-40Zn-Cr-Fe-Sn-Bi alloys consisted of α-βduplex phases containing the fine, uniform Cr-Fe IMCs and Bi
particles. Average YS and UTS of the extruded Cu-40Zn-Cr-FeSn-Bi alloys were 288 MPa and 601 MPa, respectively. The
machinability of the extruded Cu-40Zn-Cr-Fe-Sn-Bi also
maintained 75 % of the machinability of a Cu-40Zn-Pb alloy;
thus, the extruded Cu-40Zn-Cr-Fe-Sn-Bi deviated from thetraditionally observed trade-off between hardness andmachinability in conventional machinable brass materials.