1. Difficulties in grinding and polishing titanium alloys
The preparation of metallographic samples of titanium and titanium alloys is more difficult than steel, and their polishing and polishing efficiency is low. Excessive cutting and polishing processes will produce deformation twins in the α phase. After the deformation twins are generated, the microstructure of titanium The analysis will be disturbed.
Pure titanium is more suitable for cold mounting than hot pressure mounting. Hot pressure mounting may change the content and distribution of hydrogen in pure titanium. Pure titanium is very difficult to remove scratches and plastic rheology during sample preparation.
2. Introduction of titanium and titanium alloy metallographic features
The commercial application of titanium and titanium alloys has a history of more than 50 years. It has the advantages of low density, excellent strength to weight ratio, good corrosion resistance and high mechanical strength. The disadvantage is that the production cost of titanium and titanium alloys is very expensive.
Titanium has an allotropic transformation like iron. Like steel, titanium can also be heat-treated, and alloy elements have a certain effect on the stability of low-temperature α phase and high-temperature β phase.
At room temperature, the stable phases of titanium and titanium alloys exist as alloy-containing α, α-β and β phases, and the other two phases are: near α and near β phases.
3. Development of titanium alloy polishing technology
The early mechanical polishing process was quite time-consuming, and almost all mechanical polishing methods used a polishing solution containing an etchant in the last step or the last two steps of the polishing process.
Electrolytic polishing method can often get a better polished surface, but in the electrolytic polishing process, there is a certain risk of electrolyte, these electrolytes also have some chemical polishing effect.
In the 1970s and 1980s, the mechanical polishing methods of titanium and titanium alloys continued the old polishing methods. Springer and Ahmed first published articles on the polishing methods of titanium and titanium alloys in 1984.
This is the three-step sample polishing method, assuming that 320grit sandpaper is used to complete the smoothing process of the sample, but this may not always be the case. If the sample is cut with an ultra-thin cutting disc or a grinding wheel cutting disc with suitable bonding strength, the cutting surface is flat and the damage layer is minimal, then 320grit sandpaper can be used to complete the sample smoothing process. If the surface after cutting is rough and the damaged layer is large. For example: cutting with a band saw may produce such results. In this case, it is necessary to use coarser sandpaper and take a certain time to remove the damaged layer.
4, Springer and Ahmed titanium alloy three-step polishing technology
- Grind flat, use 320grit sandpaper water cooling, grind for 2 ~ 3 minutes, remove the damaged layer caused by cutting, and make the sample surface flat. Use 320grit SiC sandpaper, water-cooled, speed 240RPM, rotate in the same direction *, pressure: 27N (6lbs) / each sample, until the sample is flat.
Note: The removal of the cutting damage layer is the basic task of polishing. If the removal is not clean, the direct consequence is that the observed phenomenon may be an illusion.
- For rough polishing, apply 9μm of metaDI? Diamond polishing paste to the TEXMET? Polishing cloth with holes in advance, use distilled water as a cooling lubricant, and polish for 10 to 15 minutes. Rough polishing process: 9μmmetaDI diamond polishing liquid + metaDI polishing lubricant, using ULTRA-PAD? Polishing surface, speed 120RPM, reverse rotation **, pressure: 27N (6lbs) / each sample, time 10min.
- For final polishing, use MICROCLOTH? Or MASTERTEX? Polishing cloth, add MASTERMET? Silica suspension polishing solution, and polish for 10 ~ 15 minutes. Final polishing process: on the MICROCLOTH polishing surface, use MASTERMET silica polishing liquid, rotating speed 120RPM, reverse rotation, pressure: 27N (6lbs) / each sample, time 10min.
Note: During the polishing process, the sample needs to be rotated regularly. Most metal alloys have a hard phase. If the direction is not rotated, the polished sample will have a black “long tail” on the hard phase, which affects the quality of the metallography. The key to the “long tail” problem lies in the direction of turning the sample. Each time you can turn 90 degrees or 180 degrees.
5. Müller titanium alloy three-step polishing technology
- P500 sandpaper is water cooled, the speed is 300rpm, the pressure on each sample is 16.7N (3.75lb), and all samples are smoothed out during the preparation time.
- P1200 sandpaper water cooling, speed 300RPM, pressure on each sample 16.7N (3.75lb), preparation time 30S.
Note: The specific time depends on the individual polishing situation. The time parameter is just a reference. Usually, the polishing will be manual polishing. There is no such advanced equipment, so the parameters will be different.
- Use synthetic lint-free polishing cloth + silica suspension polishing solution containing chemical etchant, polishing machine speed 150RPM, polishing time: 33N (7.5lb) on each sample for 10 minutes, 16.7N on each sample ( 3.75 lb) for 2 minutes, with 8N (2 lb) pressure on each sample for 1 minute.
- Polishing agent composition: 260mlSiO2 + 40mlH2O2 (concentration 30%), 1mlHNO3 + 0.5mLHF. The particle size of P500 and P1200 sandpaper of FEPA standard corresponds to 320/360 and 600grit sandpaper of ANSI / CAMI standard, respectively.
6. The difference between manual polishing and machine polishing
- Manual polishing requires experience, and the strength, time, and speed of the hand require experience to work for a long time to master these key parameters, while machine polishing only needs to set the parameters. For novices, they are often irritable and have problems such as insufficient polishing or overthrow.
- The principles of machine polishing and manual polishing are the same. Most of the domestic metallographic work is still dominated by manual polishing. Manual polishing requires metallographic staff to have greater patience, and tossing should be repeated, but as long as you have patience, try and summarize, you can also get good metallographic polishing Effectiveness, therefore, we need to sum up the experience carefully, and we will finally gain by paying the sweat.