Research on improving the mechanical properties of the SLS metal parts.

Pacurar, Razvan ; Balc, Nicolae ; Berce, Petru 等

1. INTRODUCTION

Within the Selective Laser Sintering (SLS) process, there are a number of input parameters that can be controlled and modified to get different characteristics of the sintered parts (Childs et al, 1999). Some of this input factors pertain to the laser (e.g. laser power, laser scan spacing, etc.), while others refers to the metallic powder properties (e.g. particle size, percentage composition of the constituent materials, etc.) or to the sintering parameters (e.g. layer thickness, scanning speed, etc.) (Figure 1) Output parameters of interest might be hardness, density, strength, porosity, etc. (Chatterjee et al., 2003).

A considerable amount of work has been done and reported in this field, some of them mentioned bellow. Miler et al have carried out factorial experiments to express the strength of a sintered sample as a function of laser power, scanning speed and fill scan spacing and their respective interaction terms. The model developed takes into account the variation of small and large beam spot sizes and effect of heat loss on strength of sintered samples (Miler et al., 1997).

The Song paper states the influence of laser parameters like laser beam power and experimental parameters like scanning speed on various properties of a laser sintered bronze product. It is reported that density increases while surface roughness decreases with scanning speed decrease (Song, 1997)

Hardro et al determined the optimal process parameters for SLS of an elastometric polymer using an experimental design approach. Laser power, laser scan spacing and part bed temperature were the factors under consideration, while dimensional accuracy and material strength of the sintered samples were the response characteristics. It was concluded that all the factors as well as their interactions are statistically significant (Hadro et al., 1998).

The current paper is focusing mainly on the porosity issue. Theoretical and experimental methods for estimating the porosity of the SLS parts made by Laserform St-100 powder are revealed. A solution for decreasing the porosity of the SLS metal parts, after post-processing in the oven is also presented.

[FIGURE 1 OMITTED]

2. POROSITY THEORETICAL APPROACH

A number of physical phenomena are possible for the formation of pores inside laser-sintered metal matrices. Three types of pores can be identified in the material structure, as illustrated in Figure 2 and presented below:

--large interconnected pores outside the balls caused by balling phenomenon

--relatively smaller pores inside the balls

--remained pores in the matrix after infiltration in the oven process.

[FIGURE 2 OMITTED]

Powder material always contains a considerable amount of air inside it in between the particles. The non-uniform porosity distribution of the porous material surface has an important influence over its mechanical and technological characteristics. In order to study the porosity, a schematic method is presented as illustrated in Figure 3. A vacuum impregnation equipment is presented. The pump PV that assures the suck of the resin from the porous probe 1, disposed into the pot 2, is creating the vacuum.

[FIGURE 3 OMITTED]

From the theoretical point of view, the porosity of the material can be calculated very simple using the formula presented below:

P := ([m.sub.2] - [m.sub.1]) x [[rho].sub.W]/([m.sub.2] - [m.sub.3]) x [[rho].sub.L] x 100 (1)

where:

* P--is the part porosity

* m1--is the sample mass, weighted in air before resin impregnation;

* m2--is the sample mass, weighted in air after the impregnation

* m3--is the sample mass weighted in water

* [[rho].sub.W] is the water density

* [[rho].sub.L] is the resin density (g/[cm.sup.3]).

3. POROSITY EXPERIMENTAL APPROACH

One case study (a button for lever position adjusting of a grass cutting machine presented in Figure 4) has been developed within a project work, jointly at the Technical University of Cluj-Napoca and Plastor Company (RO).

[FIGURE 4 OMITTED]

Two sets of active elements--punch (illustrated in Figure 5)--were manufactured using the Sinterstation 2000 equipment illustrated in Figure 6 (National Center of Rapid Prototyping--Technical University of Cluj-Napoca).

[FIGURE 5 OMITTED]

The active tools made at the TUC-N on the Sinterstation 2000 machine have been post-processed (infiltrated and sinterization completed) at the TUC-N, as illustrated in Figure 5.

[FIGURE 6 OMITTED]

One punch has been post-processed in classical way by infiltrating with bronze at 1070[degrees]C in the oven, the other one being supplementary impregnated with epoxy resins in vacuum, using the impregnation equipment and dry-oven illustrated in Figure 7.

Both punches were weighed in air before and after the impregnation and in water before impregnation.

[FIGURE 7 OMITTED]

The aim of resin impregnation under the vacuum was to fill all the existing blanks in the model structure. Several steps were followed, as illustrated in Table 1.

The practical tests for injection molding were made within the industrial company that we cooperated with.

4. RESULTS AND CONCLUSIONS

After finishing the experiment, theoretical porosity calculation was made, using formula 1. The obtained results are presented in Table 2.

By the experimental point of view, by using an electronic microscope from TUCN, some estimations were made regarding the porosity in all stages: before infiltration, after infiltration with bronze and after resin impregnation. (Fig. 8)

[FIGURE 8 OMITTED]

ImageJ software was used to analyse the images. The porosity in this case was calculated by using the following formula:

p = [summation] Ai/Atot (2)

where [A.sub.i] represents each granule area and Atot is the entire image area. Obtained results are comparable with the theoretical ones in all stages, revealing the fact that resin impregnation could be a reliable solution when trying to decrease the porosity of the SLS metal parts.

5. REFERENCES

Chatterjee, A. N. et al., (2003). An experimental design approach to selective laser sintering of low carbon steel. Journal of Materials Processing Technology, Vol. 136, No. 1 (2003) 151-157, ISSN 0924-0136.

Childs T. H. C. , et al., (1999). Selective laser sintering of an amorphous polymer--simulations and experiments. Journal of Engineering manufacture, Vol. 213, No. 4, (1999), 333-349, ISSN 0954-4054.

Hardro P. J. et al (1998). A design of experiment approach to determine the optimal process parameters for RP machines. Proceedings of the 5th Int. Conference on Automation Technology, July 1998, Chiao Tung University, Taipei.

Miller D. et al., (1997). Variable beam size SLS workstation and enhanced SLS model. Rapid Prototyping Journal, Vol. 3, No. 1 (1997), 4-11, ISSN 1355-2546.

Song Y. (1997). Experimental study of the basic process mechanism for direct selective laser sintering of low-melting powder. CIRP Annals Manufacturing Technology Vol. 46 No. 1 (1997), 127-130, ISSN 0007-8

Tab. 1. Resin impregnation under vacuum

Mixture Preparation:
 1/2 liter of epoxy resin
 5 % hardener

Mixing the resin and the hardener:
Boiling phenomena occurs after 8 min

Vacuum process cycle (Total time -56 min)

--Realizing the vacuum - 15 m in
--Maintaining at [10.sup.-4] Torr - 10 min
--Releasing the vacuum - 3 m in
--Vacuum process cycle repeat - 2 times

Drying in the oven - 72 hours

Tab. 2. Porosity calculation

 Punch 1

 before after bronze after resin
 Infiltration infiltration impregnation

Aparent 4,84 7,77 8,34
density [rho]

Porosity p 72,053 32,124 25,428

 Punch 2

 before after bronze after resin
 infiltration infiltration impregnation

Aparent 4,776 7,737 8,226
density [rho]

Porosity p 72,087 32,288 25,516