The Microstructure and Hardness of ZrO2

einforced 24CrNiMoY

Alloy Prepared by SLM

Chaofan Shi

, Suiyuan Chen

, Qing Xia

and Zhuang Li

Key Laboratory for Anisotropy and Texture of Materials, Ministry of Education, Key Laboratory for Laser Application

Technology of Liaoning Province. School of Materials and Engineering, Northeastern University, Shenyang, Liaoning,

China

Keywords: Selective laser melting, Brake disc, 24CrNiMoY steel, Microstructure, Hardness.

Abstract: In this paper, 1.5% ZrO

was added in 24CrNiMoY alloy steel powder to form a composite powder, and the

alloy steel sample was prepared by selective laser melting technology. Effects of different laser power and

scanning speed on the microstructure and hardness of the fabricated sample were studied by OM, XRD,

SEM and hardness test. The results show that when the optimized laser power is 1700 W and scanning

speed is 10mm/s, the phase structure is composed of martensite, bainite, ferrite and carbide. The

microstructure is uniform and fine, there are no cracks in the fabricated sample, the porosity is 0.96%. The

average hardness is 467 HV, ZrO

plays a role on improving the hardness. The alloy steel sample prepared

by SLM has a fine microstructure and high hardness, which provides a basis for laser additive

manufacturing of high-speed brake disc.

1 INTRODUCTION

High-speed rail brake discs are one of the key parts

that guarantee the safe operation of high-speed rails.

Many studies have focused on the manufacturing of

brake discs [1, 2]. At present, the rail brake discs are

mainly prepared by traditional methods of casting,

forging and heat treatment. This method has some

problems such as high manufacturing cost,

complicated heat treatment process, limited

mechanical processing equipment [3]. Selective

laser melting is a new technology for the preparation

of complex metal parts. It has the characteristics of

rapid manufacture, and the structure and

performance can be designed and controlled [4, 5].

Therefore, SLM is one of the main research

directions to prepare the brake disc with high

hardness.

Zirconia is a good reinforcement phase, and its

strengthening mechanism has been explored in a

variety of papers. Lei et al. [6] used plasma spraying

to prepare HA-30% ZrO

coating layer. Compared

to the pure coatings, microhardness and wear

resistance were improved. Gao et al. [7] studied the

effect ZrO

content on the properties of Cu-ZrO

composites. The results show that when the ZrO

content was 7%, the hardness and tensile strength

are significantly improved. However, the

strengthening and toughening mechanism of

zirconium oxide in 24CrNiMoY alloy steel has not

been studied.

24CrNiMo alloy steel is the main material for the

preparation of high-speed rail brake discs, which has

the advantages of stable friction characteristics and

good wear resistance at high temperature. In our

previous studies, the hardness of the 24CrNiMo

alloy steel fabricated by SLM technology is 346 HV

[8], its hardness is not enough to meet the surface

material requirements of brake discs. So we used the

vacuum induction melting atomization method to

prepare 24CrNiMo steel powder with yttrium added

firstly. Then, the laser power and scanning speed

was optimized to fabricated 24CrNiMoY steel with

ZrO

. The purpose is to provide a theoretical basis

for the preparing of high-speed rail brake discs via

SLM.

Table 1: The alloy composition of each element.

Table 2: Parameters of the fabricated sample process.

2 EXPERIMENTAL

MATERIALS AND METHODS

2.1 Experimental Materials

The substrate material was Q235 steel. Before the

experiment, the surface of Q235 steel plate with

dimensions of 100 mm × 100 mm × 10 mm was

polished to a roughness of Ra 6.5. The composite

powder used for SLM is 24CrNiMoY powder

containing 1.5% ZrO

. The composition of

24CrNiMo powder is shown in Table 1. Composite

powder were ball milled using the QM-5 Roller Ball

Mill.

2.2 Experimental Methods

The prepared alloy steel powder is uniformly spread

on the substrate, and the laser scanning path is

controlled to completely melt the powder layer.

After each layer is melted, the forming cylinder is

lowered by one powder layer thickness, re-powdered

and the laser is then completely melted according to

a present path. In this experiment, different layers of

laser scanning paths were perpendicularly

intersected to reduce defects in the laser cladding

process. The specific process parameters are shown

below.

Figure 1: Fabricated sample morphology under different scanning speed. (a) S1 Sample, (b) S2 Sample, (c) S3 Sample, (d)

S4 Sample, (e) S5 Sample.

element Fe C Cr Ni Mo Mn O Si Y

wt.% 94.74 0.24 0.89 0.937 0.503 0.989 0.02 0.382 1.3

Sample

Numble

Overlap

ratio

(%)

Powder

thickness

(mm)

Spot

diameter

(mm)

Scanning

speed(mm/s)

Power

(W)

S1 35 0.4 4 7 1800

S2 35 0.4 4 8 1800

S3 35 0.4 4 9 1800

S4 35 0.4 4 10 1800

S5 35 0.4 4 11 1800

B1 35 0.4 4 10 1600

B2 35 0.4 4 10 1700

B3 35 0.4 4 10 1900

B4 35 0.4 4 10 2000

(e)

200μm

(a)

200μm

(b)

200μm

(d)

200μm

(c)

200μm

Figure 2. SEM figures of specimens at different laser power. (a) B1 Sample, (b) B2 Sample, (c) S4 Sample, (d) B3 Sample,

(e) B4 Sample.

3 EXPERIMENTAL RESULTS

AND ANALYSIS

3.1 Effect of Scanning Speed on

Microstructure of the Fabricated

Sample

Figure 1 shows the metallographic images of the

cladding layer at different scanning speeds. It can be

seen from the metallographic image that when the

power and other process parameters are kept

constant, with the change of scanning speed, the

number and size of pore defects in the fabricated

layer have changed.

3.2 Effect of Power on SEM Pictures of

the Fabricated Sample

Figure 2 shows the SEM images of the alloy steel

cladding layer under different power. The SEM

images show that the phase composition of the

cladding layer under different power conditions are

martensite, lath bainite and ferrite. During the

cooling process of high-temperature austenite,

proeutectoid ferrite (PF) grows first at the austenite

grain boundary nucleation. Since laser cladding is a

rapid cooling process, when austenite is subcooled

to the bainite and martensite transformation

temperatures, most of the carbon is dissolved in

alpha ferrite, so bainite and martensite grows inside

austenite crystals.

3.3 Effects of Laser Power and

Scanning Speed on Porosity and

Hardness of Fabricated Sample

It can be seen from Figure 3 that when the laser

power of 1800 W. When the scanning speed is

10mm/s, The porosity of the coating reached a

minimum of 1.07%. And at the scanning speed of

10mm/s, when the laser power is 1700 W, the

porosity of the fabricated sample reached a

minimum of 0.96%.

Figure 3: Porosity rate under different scanning speed and

laser power.

7891011

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

Porosity(%)

Scannin

eed

(

mm/s

)

Porosity

16001700180019002000

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

Porosity (%)

Laser power (W)

Porosity

(a)

lath Bainite

Martensite

10μm

(d)

lath Bainite

Martensite

10μm

(e)

lath Bainite

Martensite

10μm

(b)

10μm

(c)

Martensite

10μm

lath Bainite

Lath Bainite

Martensite

Figure 4 shows the hardness of the forming layer

at different scanning speeds and laser powers. When

the laser power is 1800 W, the fabricated sample has

the highest hardness when the scanning speed is 10

mm/s, and when the scanning speed is determined as

10 mm/s, With the power of 1700 W, the fabricated

sample has the highest hardness.

Figure 4: Microhardness of the fabricated sample at

different laser power and scanning speed.

Based on the above analysis of the porosity and

hardness of the fabricated sample, the optimum

power for the SLM process is 1700 W, and the

optimal scanning speed is 10 mm/s. The porosity of

the fabricated sample under optimal parameters is

0.96% and the hardness is 467 HV.

Figure 5 shows the XRD pattern of the fabricated

sample when the laser power is 1700 W and the

scanning speed is 10mm/s. It can be seen from the

pattern that the main phase of 24CrNiMoY alloy

steel is α-Fe(M), and M represents the element C,

Cr, Ni, Mo, etc. in the alloy steel. Pre-eutectoid

ferrite PF is α-Fe phase with a solid solution carbon

content of 0.0218% or less. lath bainite is composed

of bainitic ferrite BF phase and carbide phase M

Martensite is a supersaturated solid solution of C in

α-Fe.The peak of zirconia can also be seen, zirconia

is dispersed in the alloy steel as a compound.

20 40 60 80 100

100

200

300

400

500

600

ens

(

a.u.

)

(deg)

•⎯

♦⎯

⎯

♦

•

(b)

(a)

Figure 5: XRD pattern of cladding layer with laser power

of 1700 W.

3.4 Analysis and Discussion of

Experimental Results

The main process parameters that affect the quality

of the fabricated sample include laser power P,

scanning speed V, spot diameter D, overlap ratio

W, and powder coating thickness H. The energy

absorbed per unit volume of the cladding layer is

generally expressed according to formula (1).

VHWD

)1( −

(1)

When the laser power is 1600 W, the laser

radiation energy is too low, and the energy volume

density per unit area of the molten pool is not

enough, resulting in insufficient melting of the

powder and increasing the probability of generating

porosity in the fabricated sample. When the laser

power increases to a certain threshold (1700 W in

the experiment), as the energy density increases,

the depth of the molten pool increases, and the

surrounding metal liquid flows to the porosity so

that the number of pore is reduced. When the

output power of the laser is is too high, a large

amount of matrix material will be melted so that

the dilution of the alloy of the cladding layer will

increase, the stirring of the molten pool will

intensify, and the elements of the matrix element

and the fabricated sample will mutually diffuse.

Resulting in increasing chance of causing

porosity.The XRD diffraction peak width can be

used to determine the grain size. If the grain size is

small, the diffraction direction will be diffused,

which cause a broadening of the diffraction peaks

[9]. Therefore, the crystal grain size can be judged

by the broadening analysis of XRD diffraction

peaks.

7891011

300

325

350

375

400

425

450

475

Hardness (HV)

Scanning speed (mm/s)

Hardness

1600 1700 1800 1900 2000

300

325

350

375

400

425

450

475

500

Hardness(HV)

Power(W)

Hardness

(

)

The value of k is generally taken as 0.89. It can

be seen from formula (2) that the larger the half-

height width hkl, the smaller the grain size Dhkl.

Figure 5(b) shows the analysis of the XRD

diffraction peaks at a power of 1600 W cladding. In

accordance with the same method for the analysis of

XRD diffraction peaks at other powers, the results

are shown in the table. It can be seen from the table

that as the power increases, the half-width of the α-

Fe diffraction peak first increases and then

decreases, indicating that when the power is from

1700 W to 1800 W, the grain refinement is more

obvious.

Table 3: XRD diffraction peak half-height widths of the

fabricated sample at different power.

The grain size under different powers was

calculated by Image software. The grain size first

decreased with increasing power and then increased,

the grain size reaches a minimum at 1700 W. When

the laser power is low, the absorbed heat in the

molten bath is relatively reduced, the dilution of the

fabricated sample is small, and the Fe element of the

substrate is less in the cladding layer, so the content

of (NiFe) in the cladding layer is low, and the

carbide is hard, the higher content of the phase and

the tendency to agglomerate to form larger needle-

like and strip-like hard phases, as a result of which

the grain size of the microstructure increases.

Therefore, in a certain range of increasing the laser

power, the microstructure of the fabricated sample

tends to be dense, and the crystal grains are finer.

When the laser power is too large, the solution in the

molten pool absorbs more energy and the cooling

rate decreases, so that the tissue particles will

increase compared to the tissue particles at the

moderate scan speed. Which also verifies the the

results of statistical analysis using XRD diffraction

peak broadening. It can also explain that when the

laser power is 1700 W, the grain size refinement is

most obvious, and the hardness of the forming layer

is the highest.

4 CONCLUSIONS

(1) The 24CrNiMoY alloy steel with ZrO

added

cladding layer obtained by SLM has good

formability, no obvious crack defect, and a small

amount of porosity defects. And the defects has a

tendency to change as the process parameters

change.

(2) When the scanning speed of 10 mm/s and the

laser power of 1700 W, the defects of the cladding

layer are minimal and the microstructure is uniform,

the grain size refinement is most obvious. The grain

size is 9.6 μm and the hardness value can reach a

maximum value of 467 HV.

(3) By analyzing the SEM images of the cladding

layer, it can be concluded that the phase composition

of the cladding layer under different power is

martensite, bainite and α-ferrite, and the phase

composition is uniform.

ACKNOWLEDGEMENTS

This work was financially supported by National

Key R&D Program of China (2016YFB1100201),

Green Manufacturing System Integration Project of

the Industry and Information Ministry of China

(2017), Project 201710145047 Supported by

National Training Program of Innovation and

Entrepreneurship for Undergraduates.Research and

development, plan for the future emerging industries

in Shenyang(18-004-2-26).

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Sample

serial

numbe

B1 B2 S4 B3 B4

Laser

owe

160

170

1800

190

200

Half-

width

0.22 0.25 0.24 0.23 0.22

hklhkl

hkl

θβ

cos