Since commercially pure titanium (CP Ti) has many advantages, such as low density, high specific strength, good biocompatibility, and strong corrosion resistance, it is currently being used in numerous medical and heat transfer applications [1]. CP Ti is used for the manufacturing of surgical implants and prosthesis structures because it provides unique combinations of mechanical properties and biocompatibility [2]. A firm adherent oxide layer on the surface of titanium sheets forms as soon as they are placed in the atmosphere, which can protect them from active environments and make them suitable choices for heat exchangers under severe corrosion conditions [3]. Welding is one of the essential processes in products such as plate heat exchangers [4]. Welded plate heat exchangers comprise of two thin sheets with a thickness of 0.5 to 1.5 mm welded together using either lap or edge weld joints. Welded titanium plate heat exchangers are commonly used for highly corrosive media under extremely high pressure conditions up to 60 bar [5]. Therefore, in welded plate heat exchangers a high-strength weld with a specific hardness range is required.
A non-contact and low heat input (HI) welding method such as laser welding has been widely recommended for titanium welding applications. Laser welding is a fusion welding process, in which a laser beam is used as the heat source to construct a metallurgical bond between materials [6]. Significant advantages of laser welding are minimum distortion, heat input and heat-affected zone (HAZ) [7], which result in less residual stress and porosity in addition to improved strength and hardness compared to the conventional TIG welding methods. More specifically, the smaller and more localized heat source reduces the probability of oxygen contamination and subsequently embrittlement in titanium welds. Nonetheless, understanding the role of different hardening and strengthening mechanisms in titanium welds has always been inexplicit in the literature.
The hardness of a titanium weld is an important mechanical characteristic that is in direct correlation with its strength and ductility. Therefore, understanding the governing phenomena behind weld embrittlement is crucial in achieving desired mechanical properties from a titanium welded joint. Pequegnat, et al. [8] have investigated the hardening mechanisms of the laser hole sealing process for grade 1 commercially pure titanium (Ti CP1). They indicated three hardening mechanisms in titanium laser welding, namely microstructure refinement (change in the mode and size of microstructure), interstitial elements, and secondary phases. Their results highlighted the significance of interstitial hardening mechanisms like oxygen contamination, which also result in TixOy secondary phases. Although, these three hardening mechanisms were investigated on the laser hole sealing process to some extent, the results cannot be comprehensively applied to laser welding. Li, et al. [2] investigated the effect of different oxygen concentrations in argon shielding gas on mechanical characteristics of titanium CP1 laser welding. Their results indicated that both grain structure and oxygen absorption of the fusion zone (FZ) are effective on the weld hardness. However, they did not measure the actual amount of oxygen absorbed by the FZ itself, and their findings were based upon the percentage of oxygen contaminated in the shielding gas.
Microstructure refinement of FZ is the first hardening mechanism in titanium fusion welding techniques that is controlled by the amount of process heat input and cooling rate. Kumar, et al. [9] analyzed the effect of fusion zone grain size on laser-welded Ti6Al4V sheets in a butt square configuration. Their study showed that the grain size of the fusion zone increases with increased heat input. The grain’s average size increased from 164 to 246 µm, while increasing the HI from 22.6 to 38.5 J/mm. It was also reported that an increase in grain size was followed by a decrease in weld hardness. In a complementary study, Kumar and Sinha [10] studied the correlation between fusion zone hardness and process HI in Ti6Al4V laser welding. They observed an acicular martensitic ἁ phase within the fusion zone, which resulted in higher hardness for the fusion zone compared to the heat affected zone and base metal. They noticed a reduction in the weld hardness with an increase in the HI, which was attributed to grain coarsening as a result of excessive heat input.
Chattopadhyay, et al. [4] observed the formation of needle-like martensitic phases due to increased cooling rate, leading to higher weld hardness. They also reported a hardness increase, when increasing the welding speed from 2.4 to 4 m/min, because of finer microstructure. Strengthening mechanisms in laser beam welding of titanium were studied by Liu, et al. [11]. They attributed the hardness and strength of the welds to the microstructure and solute solution formation within the fusion zone. A direct correlation was found in this research between a decrease in welding speed and an increase in grain sizes due to the lower cooling rates. While, these studies analyzed the effect of microstructure refinement on hardness, the effects of microstructure secondary phases should also be considered.
One of the most important hardening mechanisms in titanium welding is the formation of secondary phases and interstitial elements. Li, et al. [12] conducted a comparison between laser welding and laser-GMA hybrid welding of CP Ti. They found out that the lower heat input of the laser welding process compared to laser hybrid did not provide a higher hardness of the weld. They observed that in addition to the heat input, oxygen concentration of the fusion zone also plays a significant role on the weld hardness. Although the comparison of laser with laser hybrid processes provides a subtle insight into the effects of heat input and oxygen concentration on hardness, nonetheless this study does not explain the cause behind oxygen contamination and how it can be controlled.
The hardening phenomena occurs in titanium alloys during other laser materials processes as well, including additive manufacturing. Wei, et al. [13] investigated the effect of secondary phases and precipitation elements on the hardness of Ti6Al4V laser additive manufacturing. They observed that augmenting the nitrogen content in the argon shielding gas led to an increase in hardness. Huang, et al. [14] observe similar results, changing nitrogen concentration in argon shielding gas during arc deposition of titanium. Maintaining consistent parameters of power, welding speed, and wire feed rate, they produced specimens with significant discrepancies in hardness owing to the in-situ formation of secondary TiN phases. These studies showed the need for investigating the simultaneous effects of HI and secondary phases on laser material processing of titanium.
Porosity is another highly important factor in defining the strength of a welded joint. Reduced porosity is one of the main advantages of welding titanium alloys with the laser technology. Huang, et al. [15] studied the effect of hydrogen accumulation on the porosity formation during fusion welding of a titanium alloy. They observed that the level of hydrogen in the FZ plays a vital role in porosity formation. However, they didn’t investigate the effect of the FZ geometry on the porosity. Panwisawas, et al. [16] have modeled pore formation in bead-on-plate welding of titanium alloys. They reported that an unstable melt pool flow results in recirculation and increases the chance of porosity formation. Chang, et al. [17] reported the same results based on the numerical model and experimental specimens. Further studies [16, 17] considered the effect of process parameters on unstable flow, but they did not investigate the effect of depth as a geometrical characteristic on pore formation. While, there are numerous empirical and numerical studies on porosity formation in CP Ti laser welds, there are no studies that investigate porosity formation as a function of geometrical characteristics. Such analysis is of importance since the geometrical features of the melt pool in laser welding are different compared to TIG or other conventional welding methods.
While the effect of microstructure refinement and oxygen contamination (interstitial and secondary phase hardening) on titanium weld embrittlement has been studied separately in previous studies, the simultaneous influence of both phenomena has not yet been addressed. Understanding the level of importance of different hardening and strengthening mechanisms is required to control properties of any titanium weld joint. Such understanding is only possible through direct measurement of the oxygen contamination in the fusion zone and its microstructure analyses based upon the process heat input. More importantly, none of the available reports in literature discuss why the fusion zone is contaminated with oxygen and how it can be controlled under similar shielding conditions.
The current research aims to investigate the simultaneous effects of microstructure refinement and fusion zone oxygen contamination on the hardness of titanium laser welding process. Several samples are designed and tested based on different levels of heat inputs. Firstly, the micro-hardness of these samples is measured to observe hardness variations based on the heat input. The microstructure features including grain size and morphology are analyzed accordingly. The fusion zone oxygen content and resultant secondary phases are also measured to compare the combined effect of grain refinement and oxygen contamination on weld embrittlement. Since results show the significant role of fusion zone oxygen contamination, geometrical analysis of the weld cross sections is carried out to understand how oxygen contamination changes under similar shielding conditions. Finally, porosity formation is also studied based on geometrical features of the weld bead, to develop controlling mechanisms for reducing porosity during titanium laser welding.