ENFOQUES DE CONTEXTUALIZACIÓN, DISCIPLINARIEDAD, INTERDISCIPLINARIEDAD,

From the literature, it has been mentioned that the Ti-6Al-4V is classified under α/β

group of titanium alloy. Titanium alloy which fall either within α/β or β groups is very responsive to heat treatment and this affect its resultant microstructure, as compared to α group alloy [Polmear (2006)]. It was reported that different heat treatment process produces different strength and properties of Ti-6Al-4V alloy. This is due to phase transformation of β at high temperature, and different routes of subsequent cooling rate affects β to α phase transition, thus produces different composition of α+β phase as well as different microstructure morphology and properties in final product. Schematic diagram of microstructure range for α/β alloy heat-treatment can be referred in literature [Polmear (2006)] and will be shortly shown in the following section. Research specifically in the SLM Ti-6Al-4V material [Facchini et al. (2010)] suggested a post-manufacture heat-treatment to transform the metastable martensite into dual phase α+β, which resulted in an improvement in ductility, but a reduction in strength values. Besides traditional heat- treatment, hot isostatic pressing (HIP) has also been mentioned to improve porosity in powder manufactured titanium alloy products [Polmear (2006)], or to improve final microstructure in cast titanium alloy products [Wu and Hu (2005)].

In Shen’s study (2009), HIP process has been done but the results showed that the process gave no significant improvement on specific strength of SLM Ti-6Al-4V micro-lattice cores with high manufacturing parameters i.e. 200 W laser power and 1000 µs laser exposure time. Only low set manufacturing parameters of HIP micro- lattice cores i.e. 180 W laser power and 500 µs laser exposure time, showed more stable deformation during compression test as compared to un-HIP cores. These findings were taken into consideration in the current study on microstructure of SLM Ti-6Al-4V micro-strut; where due to high cost of the HIP process which involved sophisticated facilities, basic post-manufacture heat-treatment using simple facilities was suggested and will be discussed in this thesis. The objective of the introduction of heat-treatment process was to control the final microstructure of the specimen, and hence for improvement in the strength of material.

In ASM Handbook [Gilbert and Shannon (1998)], it was mentioned that titanium alloy reacts with oxygen, water and carbon dioxide normally found in heat-treating atmospheres and also with hydrogen formed by decomposition of water vapour. A brittle oxide layer which is commonly called α case, as a result from interaction between oxygen and titanium surface, need to be removed before the heat-treated product can be used. A table was given in the handbook, which suggested minimum layer thickness of metal removal after thermal exposure of titanium alloys in an oxidizing atmosphere. It was found that minimum of 145 µm surface removal is suggested for 1 hour heat-treatment at temperature around 1000°C.

In this study, micro-struts of 200 W and 1000 µs parameters with build angle of 90° and 35° were subjected to a heat-treatment process using simple available facilities, in order to compare the effects on the SLM Ti-6Al-4V microstructure transformations and its mechanical properties. The process was expected to introduce some improvement in the ductility and behaviours of material. Extra care was taken since the micro-struts were having cross sectional diameter of not more than 400 µm. Oxidized surface removal after the heat-treatment was thought to be not practical, since it will leave less than half of the diameter, which would mean that at the end only around 110 µm would remain. Therefore, a vacuum atmospheric condition (0.1 mbar) with the usage of sealed quartz tubes was introduced in order to avoid the oxidizing environment in a normal heat-treatment furnace oven. Figure 2.7(a) shows a photograph of the furnace oven facility which was used in the heat- treatment process. In this study, each micro-strut was sealed inside a vacuumed quartz tube as shown in Figure 2.7(b), prepared from a glass-blowing process. The glass-blowing process [Barbour (1978)] was done at the Chemistry Department, University of Liverpool.

(a)

(b)

Figure 2.7: The facilities involved in heat-treatment process (a) a furnace oven; (b) an example of a vacuum sealed quartz tube with a 23 mm length micro-strut inside

[S1-90-200-1000-AR]

In order to compare the microstructure developments, two types of heat-treatment processes were done. The first process or Process A involved solution treatment at 920°C for 1 hour and water quench, which resulted more α than β phase microstructure. In the second process or Process B, the vacuum sealed micro-strut was solution treated at 1000°C for 1 hour, water quenched and precipitation heat treated at 540°C for 4 hours. The resulted microstructure showed a more balanced

α+β phase, and was found comparable to the schematic illustration of double solution treated α/β titanium alloys that can be found in Polmear (2006). Table 2.2 summarizes the heat-treatment processes used in this study. Meanwhile, Figure 2.8 and 2.9 illustrate schematics of temperature versus time plots of two processes that were done for both heat-treatment processes, with the resultant microstructure images (observed under an optical microscope) respectively.

Table 2.2: Summary of both heat treatment processes Materials identification Micro-strut parameters First solution treatment Second solution treatment As-received [S1-35-200- 1000-AR-M] 200 W X 1000 µs; 35° b.a. - - Process A [S1-90-200- 1000-HT(A)-M] 200 W X 1000 µs; 90° b.a. Heat up to 920°C for 1 hour and

water quench - Process B [S1-90-200- 1000-HT(B)-M] and [S(1-9)-35-200- 1000-HT(B)] 200 W X 1000 µs; 90° and 35° b.a. Heat up to 1000°C for 1 hour and

water quench

Heat up to 540°C for 4 hours and air cooled

(a) (b)

Figure 2.8: (a) A schematic of temperature versus time plot for solution heat treatment on the SLM Ti-6Al-4V micro-strut (Process A) and; (b) the resultant microstructure from the process (strut with 200 W X 1000 µs parameters) [S1-90-

200-1000-HT(A)-M]

(a) (b)

Figure 2.9: (a) A schematic of temperature versus time plot for both solution and precipitation heat-treatments on the SLM Ti-6Al-4V micro-strut (Process B) and; (b)

the resultant microstructure from the process which shows balance α+β phase (strut with 200 W X 1000 µs parameters) [S1-90-200-1000-HT(B)-M]

1000°C 

Temp(°C)  _{Solution heat} 

treatment 

Water 

quench 

1 hour  4 hours Time(hour)

Precipitation  heat treatment 540°C  Air cooled   920°C 

Temp(°C)  _{Solution heat} 

treatment  Water  quench  1 hour  Time(hour) 100 µm  100 µm