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Use of Selective Laser Melting on Titanium

Question:

Discuss About The Arguments Regarding Appropriate Technique?

Transition materials like titanium are advanced metal having unique mechanical properties. However, there is lot of arguments regarding the appropriate technique that can be applied to manufacture the metal due to the high melting point and high strength of the materials. Although many alternative technique are available for the fabrication of intermetallic structure of titanium, however their use is often limited by high cost and lack of its application at the industry level (Leuders et al. 2013). As fabrication of metal is also being done by the use of selective laser melting, this literature review aims to find the role of selective laser melting technique on the microstructure and properties of the metal. This technique has been mainly chosen for evaluation because it is several advantages of the technique such as lesser steps in production and high efficiency of the material used. The outcome of the review can give clear idea about the strength and limitation of the technique and the scope of its application in the manufacturing of titanium materials.

Transition metals like silicides and carbides have advanced properties such as low density, stiffness and environmental resistance. However, one challenge in manufacturing intermetallics is their high melting point  (Tsai and Yeh 2014). Many well-developed methods are available for the fabrication of metallic parts and the research by (Krakhmalev and Yadroitsev 2014) proposed manufacturing titanium substrate by means of selective laser melting. The researcher tried fabricating the titanium substrate with titanium and silicon carbides by selective melting of the Ti-SiC powder mixture. This was done by Ytterbiums fibre laser and the structure and mechanical properties of metal was examined. The main purpose of adapting the selective laser technique was to find the correlation between microstructure and composition of the precursor powder. Mechanical properties were assessed by the parameters of hardness, indentation fracture toughness and abrasive wear resistance. The researchers also gave a promising approach to achieve structural homogeneity of the material.

The examination of the top surface and longitudinal structure of metals by scanning electron microscopy (SEM) showed crack networks on the surface and dark stripes were also found in areas enriched with Si and C. Gradient zone was also formed in all coating due to partial melting of the substrate. Hence, the overall conclusion from the study results was that composite structure was successfully formed by selective melting however the irregularities that was observed includes agglomerates of SiC at interlayer  and unequal distribution of elements in the track. The study adequately presented the reasons for this anomaly. The agglomeration occurred due to change in phase whereas the heterogeneous distribution was seen due to the penetration of laser radiation on powder base. In the area mechanical properties, hardness of the metal was influenced by coatings containing high amount of SiC (Krakhmalev and Yadroitsev,  2014). The strength of the study is that the research provided innovative technique to produce metals with high melting points. Secondly, the researcher also provided appropriate approaches to improve any anomaly in the process and enhance the homogeneity of the coating.

Development of Microstructure of Ti-6Al-4V by Selective Laser Melting

Another research study highlights the advantage of selective laser melting (SLM) on the development of the microstructure of Ti-6Al-4V. Selective laser melting has emerged as an effective additive manufacturing technique as it facilitates development of complex parts by means of selectively melting successive layers of powder (Read et al. 2015). Thijs et al. (2010) focused on the development of the microstructure of Ti-6Al-4V and determining the impact of scanning strategy on the microstructure of metals. The main advantage of the SLM process in this research is that the technique promotes development of complex geometrical features, which is not possible through conventional methods.

Compared to the research by Krakhmalev and Yadroitsev, (2014) which showed the limitation of SLM due to building up of thermal stress and metal segregation, Thijs et al. (2010) study moved further to deal with the problem by analyzing the impact of different process parameters on the SLM process. The finding of research showed that top view of the structure revealed the scanning strategy. Based on the movement of the laser beam, different patterns and orientation were seen. Secondly, energy density also had an impact on the results and the variation in the scanning velocity resulted in different band patterns. The research also gave proper explanation of the impact of scanning strategy on the microstructure. The main strength of this journal article is that the researcher adequately explained the reason for martenistic structure of Ti-6Al-4V. High correlation was also found between the orientation of the grains and scanning velocity and scanning strategy. The scanning strategy can act as a promising tool in managing the grain orientation in metals maintaining the microstructural orientation. The study is also consistent with the study by Leuders et al., (2013) which showed development of fatigue resistance and crack growth performance following the development of TiAl6V4 by SLM technique.

There is an argument that optimization of mechanical treatment via heat treatment gives contrasting results when the metal is produced by selective laser melting (SLM) compared to conventional method. With this insight, Vrancken et al. 2012) investigated about the effect of heat treatment on microstructure and mechanical properties of Ti6Al4V. The focus was also on evaluating the effect of time, temperature and cooling rate on the microstructure of titanium. The researcher used Ti6Al4V powder as a base material for the SLM process and the examination of the microstructure after heat treatment revealed columnar grains in the building direction. Secondly, the impact of temperature on the microstructure was that the martensitic structure changed into the combination of α and β. The columnar grain was visible at the sides of the metal. In addition, the residence time at high temperature was found to affect grain growth. However, one significant outcome was that heating rate was not regarded to have major effect during the heat treatment. The performance of the microstructure on the basis of different cooling rates revealed low cooling rates promoted growth of grains. Different types of heat treatment was also found to change the mechanical properties of the metal. The results of the research also pointed out to limitation of heat treatment on reducing the yield stress. Therefore, desired results were not obtained from heat treatment. However, the study was useful in pointing out the improvement in fracture strain of the material after heat temperature. The effect of heat temperature on yield stress was also presented graphically which gave better clarity to the research. The key implication of this research study is that since heat treatment has not given expected results, there is a need for further research to determine other combined treatment that would optimize the tensile properties of titanium.

Effect of Heat Treatment on Microstructure and Mechanical Properties of Ti6Al4V

The research by Murr et al., (2009) is significant as it focused on manufacturing Ti-6Al-4 V for biomedical application. As there is rise in knee joint replacement surgery, there is a need to improve the quality of bone plates and joint replacement. This is because most of the components do work well for patients who have abnormal autonomy. Another challenge in the fabrication of the implant components is that sometimes manufacturing complex shapes becomes difficult (Zalnezhad et al. 2014). Murr et al., (2009) compared the microstructure and mechanical behavior of simple geometric products by selective laser melting and electronic beam melting compared to wrought and cast products. The review gave the idea about thermo-mechanical processing, microstructure and mechanical property of wrought and cast Ti-6Al-4V and compared it with electron beam melting and selective laser melting. For biomedical application, the main intention is to produce cast parts with crack propagation and creep resistance properties. Secondly, porosity is eliminated by means of heat treatment. Wrought products particularly have the advantage of tailoring it to desired mechanical properties. In addition, the comparison parts revealed that powder metallurgy is beneficial for manufacturing medical devices. The advantage of layer manufacturing was supported by the ability of SLM to fabricate complex scaffold products. Despite this information, there is need for future research in the area of process parameters needed to produce specific microstructures. The study also pointed out that cost difference can be maintained by recycling waste powder.

As the SLM process provides flexible approach to manufacturing metals, there is great scope of exploring their role in creation of alloys and metal-metal composites. Vrancken (2014) took Ti-6Al-4V with 10% MO powder to examine its unique microstructure following its creation by the SLM technique. Past studies have also focused on combining different powders to create desired products and hardness. In case of the combination of the Ti-6Al-4V with MO powder, columnar grains were found after the SLM.  However, no such grains was found if 10% MO was added to it. The MO particle was distributed homogenously in the matrix. The mechanical properties were in also explained in Ti-6Al-4V+10 MO through the stress/curve. The study is considered important because MO phase was found to be successful in disrupting transformation from β phase to α martensite which occurred in Ti-6Al-4V alone. Therefore, the choice of combining Mo was found useful in this research due to its property to stabilize the β phase and high melting temperature. This further promoted the retention of the melted Mo particle in the titanium matrix. The content of the alloy can be modified too to achieve the desired microstructure and physical properties. The key strength of this literature is that the capability of SLM in processing powder mixtures was demonstrated. The study proved the potential of SLM on adding alloy elements and reducing stress issues.

Comparison of Microstructure and Mechanical Behavior of Ti-6Al-4 V by Selective Laser Melting

The above research focused on the performance of Ti-Mo alloy and the Chlebus et al. (2011) conducted a research to examine the mechanical properties and microstructure of Ti-AL-Nb alloy. The rational for studying the Ti-AL-Nb alloy is that it is often used as implant materials and has been found to have beneficial mechanical properties such as biotolerance and corrosion resistance.  Three versions of the alloy were produced and its mechanical characteristics were tested by the parameters of tensile strength, compression testing and hardness. The characterization was done through optical and scanning electron microscopy and the X-ray diffraction alloy. After the manufacturing of the alloy, the examination of its microstructure revealed the structure is dependent on quenching at β phase and tempering at high temperature. The thermal history also determined the structural changes occurring in layered alloy. Different orientation in solidification  observed and the columnar grains pattern was determined by laser movement. On the parameter of hardness too, it was found that hardness property was proportional to the specimen area. In the area of tensile properties too, specimen building strategies determined the distribution of residual stress and tensile strength. The scanning strategy was found to influence above parameters, however no changes were seen in the compression strengths due to scanning strategies. On the examination of fracture surface of the alloy, many quasi cleavage fracture surfaces were detected. Distinction between the smooth and flat structure and the rough structure was adequately presented. On the whole, the alloy was found to be susceptible to fatigue cracks. The study proposed strategies to reduce the property of the material by means of thermal treatment process. Hence, more extensive research will be required on the role of thermal treatment in reducing porosity.

Conclusion

The literature review focused on evaluating the microstructure and mechanical properties of titanium by means of SLM. The review of studies revealed limitation in approach and how the limitation was address by means of adjusting different process parameters. The improvement in structure was also proposed by combination with other metals. Secondly, heat treatment was found to be an effective approach to improving the microstructure and increasing the resistance of the titanium material. As the titanium material is increasingly used in knee implants, there is a need to further study the effect of heat treatment on microstructure of the metal.

Reference

Chlebus, E., Ku?nicka, B., Kurzynowski, T. and Dyba?a, B., 2011. Microstructure and mechanical behaviour of Ti?6Al?7Nb alloy produced by selective laser melting. Materials Characterization, 62(5), pp.488-495.

Krakhmalev, P. and Yadroitsev, I., 2014. Microstructure and properties of intermetallic composite coatings fabricated by selective laser melting of Ti–SiC powder mixtures. Intermetallics, 46, pp.147-155.

Leuders, S., Thöne, M., Riemer, A., Niendorf, T., Tröster, T., Richard, H.A. and Maier, H.J., 2013. On the mechanical behaviour of titanium alloy TiAl6V4 manufactured by selective laser melting: Fatigue resistance and crack growth performance. International Journal of Fatigue, 48, pp.300-307.

Murr, L.E., Quinones, S.A., Gaytan, S.M., Lopez, M.I., Rodela, A., Martinez, E.Y., Hernandez, D.H., Martinez, E., Medina, F. and Wicker, R.B., 2009. Microstructure and mechanical behavior of Ti–6Al–4V produced by rapid-layer manufacturing, for biomedical applications. Journal of the mechanical behavior of biomedical materials, 2(1), pp.20-32.

Read, N., Wang, W., Essa, K. and Attallah, M.M., 2015. Selective laser melting of AlSi10Mg alloy: Process optimisation and mechanical properties development. Materials & Design (1980-2015), 65, pp.417-424.

Thijs, L., Verhaeghe, F., Craeghs, T., Van Humbeeck, J. and Kruth, J.P., 2010. A study of the microstructural evolution during selective laser melting of Ti–6Al–4V. Acta Materialia, 58(9), pp.3303-3312.

Tsai, M.H. and Yeh, J.W., 2014. High-entropy alloys: a critical review. Materials Research Letters, 2(3), pp.107-123.

Vrancken, B., Thijs, L., Kruth, J.P. and Van Humbeeck, J., 2012. Heat treatment of Ti6Al4V produced by Selective Laser Melting: Microstructure and mechanical properties. Journal of Alloys and Compounds, 541, pp.177-185.

Vrancken, B., Thijs, L., Kruth, J.P. and Van Humbeeck, J., 2014. Microstructure and mechanical properties of a novel β titanium metallic composite by selective laser melting. Acta Materialia, 68, pp.150-158.

Zalnezhad, E., Baradaran, S., Bushroa, A.R. and Sarhan, A.A., 2014. Mechanical property enhancement of Ti-6Al-4V by multilayer thin solid film Ti/TiO2 nanotubular array coating for biomedical application. Metallurgical and Materials Transactions A, 45(2), pp.785-797.

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