Breakthroughs in the Processing Difficulties of Titanium Alloys

With the advancement of titanium extraction technology, the output of titanium has been continuously increasing [4], and the price has been continuously decreasing. However, titanium alloys are still rarely seen in People's Daily lives and are synonymous with luxury and enjoyment. This is because the excellent properties of titanium alloys also pose numerous difficulties in their processing and manufacturing.

The processing characteristics of titanium alloys

Firstly, the thermal conductivity of titanium alloy is extremely low, approximately 15W/(m·K), which is 1/5 to 1/7 of that of No. 45 steel, and the cutting temperature is high. Second, the elastic modulus of titanium alloys is small. The rebound during cutting leads to increased friction and causes tool vibration. Thirdly, titanium alloys have high chemical reactivity, and the surface hardened layer brings impact loads to the cutting tools. Meanwhile, high activity is prone to adhesion, diffusion and chemical reactions with tool materials at high temperatures.

As shown in Figure 1, the superposition of these problems forms a mutually reinforcing system, which sharply increases the processing difficulty and accelerates tool wear [5].

Figure 1 Characteristics of titanium alloys and the effects of processing. In the cutting area under high temperature and high pressure, a wear cycle of adhesion - diffusion - microstructure weakening - spalling will occur [5]

Existing solutions

To solve the problem of titanium alloy cutting, the first step is to address the phenomenon of cutting adhesion of titanium alloys. This self-lubricating TiB2 coating has been proven by numerous studies to be the best solution [6]. However, when HIPIMS prepares TiB2 coatings for rough machining, the shortcomings of insufficient bonding force and insufficient toughness of ceramic films cannot be solved, resulting in a significant attenuation of cutting performance.

The Arc technology undoubtedly can provide high adhesion and high toughness for the coating. However, a single arc coating cannot provide the lubrication characteristics like TiB2 under the high-temperature and high-pressure working conditions of fine machining [7].

When Arc+HiPiMS, surprises will emerge

As shown in Figure 2 (A), Huasheng's self-developed HA800 composite coating technology platform integrates HiPiMS and Arc coating technologies into one, greatly expanding the research and development space of coating processes.

In response to the special requirements of titanium alloy cutting, Huasheng has developed the HSR116 coating. Firstly, composite films with high adhesion and high toughness were prepared through Arc technology. Subsequently, a high-hardness and self-lubricating TiB2 ceramic coating was covered on the surface layer using HiPiMS technology.

As shown in Figure 3 (B), the TiB2 coating prepared by HiPiMS is closely bonded to the TiAlXN coating prepared by Arc. Ultimately, the hard alloy substrate -Arc coating -TiB2 ceramic layer formed an integral tool with high bonding force, hardness gradient and high strength [8].

Figure 2 (A) HA800 composite coating platform; (B) Schematic diagram of the internal structure of HA800 and SEM image of the HIPIMS and Arc double-layer structure of HSR116 coating

In the actual cutting test of TC4, the HSR116 coated tool demonstrated excellent versatility and wear resistance. In the fine processing, it achieves the same service life as the pure magnetic control TiB2 coating and is much longer than that of the TiAlN coating.

In rough machining, both coatings have obvious advantages. The specific cutting data are shown in Figure 3:

Figure 3 shows the cutting life data of side milling finish and rough machining of Huasheng HSR116 coating, market-purchased TiAlN coating and market-purchased TiB2 coating.

Specifically analyze the side milling finishing cases of HSR116 coating and TiAlN coating. As can be seen from the comparison in Figure 4 (C, D), the TiAlN coating is severely adhered, and a large amount of the substrate has been exposed due to the impact of the chips. The surface of the HSR116 coating remains clean and free of adhesion after cutting, with only wear on the cutting edge, demonstrating its anti-adhesion and anti-wear properties.

Figure 4 (A) Data table of side milling finishing of HSR116 and TiAlN coated tools

(B) Side milling finishing conditions

(C) TiALN-coated tool rake face \ n

(D) HSR116 coated tool rake face

Obviously, such coated cutting tools will inevitably greatly reduce the processing cost of titanium alloys, enabling this future metal to truly enter People's Daily lives. Let titanium alloy mobile phones, titanium alloy bicycles and even titanium alloy cars no longer be expensive and luxurious products or science fiction concepts.