S.L.S Metal powder& Test Results

1. Introduction
Selective laser sintering (SLS) is one of layered manufacturing technologies that can directly create component from powder-based materials layer by layer according to CAD model , and . It is much significant for metal parts to be manufactured via SLS process without the aid of any tooling because metal parts are always widely used and highly valued in the world. In present, it is competent for indirect SLS to fabricate metal parts through several steps. Firstly, a metal green shape composed of polymer binder and metal powders is usually formed via SLS process. Then polymer binder is burned out of the green shape and the shape is sintered at high temperature to be a porous metal part. Pores still in it, the sintered part should be infiltrated with low melting point materials such as resin, copper, brass and bronze to be a fully dense object finally. Thus, manufacturing metal parts via indirect SLS process is usually composed of two cycles which are SLS forming cycle and post furnace densification cycle, respectively.
As for steel parts, singular alloy powders such as 316, 316L, 420 stainless steel, 1080 steel and alike powders are used as indirect SLS processing powders previously. But not all types of steel powders can be purchased in the market, and it is also inconvenient for them to be produced due to the trouble of content control. Hence it is not an appropriate method to apply singular alloy powders to produce metal parts in SLS process.
2. Experimentation
2.1. Materials
Several alloy elemental powders are selected in SLS process, and they are electrolytic iron powder, deoxidized copper powder, carbonyl nickel powder and graphite powder (Table 1. Polymer powders with the diameter within 12 μm are used as a binder whose softening point is about 44 °C in the experiment. The powder materials listed in Table 1 are all irregular and they are blended with polymer binder in a 3D movement container to compose CEPS admixture. The weight ratio of polymer binder is 3% in each admixture.


Table 1.
The size and compositions of CEPS
Element
Size (μm)
1 (%)
2 (%)
3 (%)
4 (%)
5 (%)

Cu ≤45 8 8 8 8 8
Ni ≤7 0 0 4 4 0
C ≤45 0.5 1 0.5 1 0
Fe ≤45 Balanced Balanced Balanced Balanced Balanc
2.2. Experimental procedures
Metal green shapes were formed on the HRPS-II A SLS system manufactured by Huazhong University of Science and Technology (HUST) in China. The system is assembled with a continuous wave CO2 laser (wavelength = 10.6 μm). The power of CO2 laser is continuously adjustable from 0 to 50 W, and the laser beam spot is 0.2–0.25mm in diameter. To determine the appropriate SLS process parameters including laser power, scan speed, scan spacing, layer thickness and powder bed temperature, six terms of specimens (100 mm × 10 mm × 5 mm) were formed. There were five specimens in each term. Also provides the building orientation and the layer structure of a specimen. During SLS forming process, scan speed, scan spacing, layer thickness and powder bed temperature were kept 2000 mm/s, 0.1 mm, 0.1 mm and 35 °C, respectively, except laser power which was varied from 14 to 20 W. But as for a certain term of specimens, laser power was adjusted to a fixed value between 14 and 20 W. Then three points bending tests were carried out on a universal test machine after the specimens had been formed completely. The mean value of the bending strength of five specimens in one term was considered as the final result corresponding to a given laser power. Then the highest strength corresponding to one certain combination of parameters is obtained after the bending test, and the parameters in this combination were regarded as optimal ones. Metal green shapes (240 mm × 70 mm × 50 mm) of different composition in Table 1 were built on the HRPS-II A using optimal parameters subsequently. Meanwhile, the binder material was examined for the thermal gravity (TG) behavior in N2 at the heating rate of 10 °C/min to about 750 °C. Then the green shapes were degreased in a furnace in H2 environment according to the TG behavior of the binder material. After the binder had been burnt out, the green shapes were heated in a vacuum furnace at a heating rate of 15 °C/min from room temperature (RT) to 900 °C and 5 °C/min from 900 to 1200 °C, and then they are cooled naturally in the furnace after being sintered at 1200 °C/2 h. After the examination of the porosity of the sintered parts using Archimedes’ law, appropriate copper powder compact in which the volume of full dense copper is equal to the total volume of the pores in the sintered parts were placed on them before infiltration process. All the sintered parts were then infiltrated with molten copper in a sintering furnace in H2 environment at 1200 °C/1 h with a heating rate of 15 °C/min from RT to 1200 °C, and naturally cooled to RT. After infiltration process, the whole alloys except no. 5 alloy in Table 1 were thermally treated following the processes shown in Table 2 to modify their mechanical properties. Five tensile specimens (ASTM E8) were made out of each copper infiltrated alloys through linear cutting and then they were tensile loaded on a universal testing machine with the loading rate of 3 mm/min. Likewise, the average value of five data of a term of alloys was accepted for the final result. The copper infiltrated alloys of different composition were also examined for Brinell hardness using a 10 mm tempered steel ball with the load of 3000 kg and the loading time was 30 s. The final result was also the mean value of five data. Then samples were prepared from copper infiltrated alloys for examination by X-Ray diffraction (XRD), scanning electron microscope (SEM) using standard techniques.

The thermal treatments processes
Quenching temperature (°C)
Tempering temperature (°C)

1 850
2 800 200
3 890
4 830
3. Results and discussion
SLS parameters except laser power and powder bed temperature are universal ones in this SLS system. While both laser power and powder bed temperature are connected with powder materials, it is appropriate to keep the powder bed temperature at 35 °C which is close to the softening point of binder material . Then laser power can be determined by the green shapes’ maximum strength which is a vital factor in SLS forming cycle. This method is feasible because the strength of metal green shapes is only associated with laser power on the condition that other parameters are kept constant.
Polymer viscosity flow is formed when the polymer binder is heated by laser beam, and then it wets metal particles’ surfaces to bind them finally. Furthermore, the higher the laser power is, the higher the temperature of the flow is to lower its viscosity, which makes many more surfaces of the metal particles be wetted and bonded. Polymer binder even vaporizes and decomposes if laser power increases highly enough to cause binder flow temperature to rise up to its vapor or even decomposition point. But the bending strength of metal green shapes decreases simultaneously due to the excessive vaporization and decomposition of polymer binder. The relationship between the bending strength of metal green shapes and the laser power is shown in Fig. 2. Seen in Fig. 2, the strength reaches its peak value (2.41 MPa) when laser power approach 19 W. The strength drops if the laser power exceeds 19 W. Therefore, the appropriate value of laser power is 19 W and then the optimal SLS parameters are: laser power, 19 W; scan speed, 2000 mm/s; scan spacing, 0.1 mm; layer thickness, 0.1 mm; powder bed temperature, 35 °C.

4. Summary and conclusions
Metal green shapes composed of polymer bonded composite elemental powders (iron, copper, nickel and graphite) are formed via indirect SLS process. The operating parameters are as follows: laser power, 21 W; laser scan speed, 2000 mm/s; laser scan spacing, 0.1 mm; layer thickness, 0.1 mm; powder bed temperature, 35 °C. The bending strength of these green shapes is 2.41 MPa. After a series of post densification processes such as decomposition until 900 °C, sintered at 1200 °C and infiltrated with molten copper at 1200 °C, hybrid alloys composed of iron based alloy and copper are obtained finally.
Alloy elements (copper, nickel, graphite) can diffuse into γ-Fe during sintering and infiltration processes, and they distribute homogeneously after infiltration process. Quenched at different temperatures higher than 800 °C and then tempered at 200 °C, tempered martensite and precipitated α-Cu are the main RT microstructures of copper infiltrated Fe–8Cu–4Ni–0.5C and Fe–8Cu–0.5C alloys. Fe–Cu eutectoid still remains in copper infiltrated Fe–8Cu–4Ni–1C and Fe–8Cu–1C alloys to worsen the
mechanical properties of the two hybrid alloys. On the condition of the same content of carbon and copper, the addition of nickel enhances the strength of hybrid alloys due to the solution strengthening of nickel. The elongation of all the hybrid alloys is lower than 3%, the yield strength of them is over 400 MPa, the Brinell hardness of them is over 200 and the elastic modulus around 100 GPa. Compared to other hybrid alloys, copper infiltrated Fe–8Cu–4Ni–0.5C alloy has better mechanical properties.
A typical CEPS of iron based alloy system and steel parts fabricated with these CEPS via indirect SLS process are successfully presented in this paper. In addition, many more alloy elements such as molybdenum, titanium, vanadium and chromium can also be used as constituents of CEPS besides those powders mentioned in this experiment. With the introduction of CEPS to SLS process, the trouble of the preparation of powder materials is eliminated, and manufacturing steel parts via indirect SLS process becomes convenient either. The mechanical properties of these hybrid alloy parts can also be varied conveniently though changing the category and proportion of alloy element powders among CEPS. Therefore CEPS break a new direction for indirect SLS process to fabricate alloy parts.