1. Difficulties in grinding and polishing titanium alloys
The preparation of metallographic samples of titanium and titanium alloys is more difficult than steel, and the polishing and polishing efficiency is low. Excessive intense cutting and polishing processes will produce deformation twins in the α phase, and the microstructure of titanium after deformation twinning. The analysis will be disturbed.
Titanium alloy rod
Pure titanium is more suitable for cold inlay than hot pressure setting. Thermal pressure setting 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 metallographic characteristics of titanium and titanium alloys
Titanium and titanium alloys have been used in commercial applications for more than 50 years, with low density, excellent strength and weight ratio, good corrosion resistance and high mechanical strength. The disadvantage is that the production costs of titanium and titanium alloys are very expensive.
Titanium rods
Titanium and iron have the same allotropic transformation. Like steel, titanium can also be heat treated, and alloying elements have a certain effect on the stability of low temperature α phase and high temperature β phase.
At normal temperature, the stable phase of titanium and titanium alloy exists in the phase containing alloy α, α-β and β, and the other two phases are: near α and near β phase.
3. Development of titanium alloy polishing technology
Early mechanical polishing processes were quite time consuming, and almost all mechanical polishing methods used a polishing fluid containing an etchant during the final or final two-step polishing process.
Electropolishing methods tend to give better polished surfaces, but there is a certain risk of electrolytes during electropolishing. These electrolytes also have some chemical polishing effects.
In the 1970s and 1980s, the mechanical polishing methods for titanium and titanium alloys continued the old polishing method. Springer and Ahmed first published articles on polishing methods for titanium and titanium alloys in 1984.
This is the three-step sample polishing method, assuming that 320grit sandpaper is used to complete the sample smoothing process, but this may not always be the case. If the sample is cut with an ultra-thin cutting piece or a grinding wheel cutting piece with suitable bonding strength, the cutting surface is flat and the damage layer is the smallest. At this time, 320grit sandpaper can be used to complete the smoothing process of the sample. If the surface after cutting is rough and the damaged layer is large. For example, the use of band saw cutting may produce such results, in which case it is necessary to use coarser sandpaper and take some time to remove the damaged layer.
4, Springer and Ahmed titanium alloy three-step polishing technology
1) Smooth, use 320grit sandpaper for water cooling, grind for 2~3 minutes, remove the damage layer caused by cutting, and make the sample surface flat. Use 320 grit SiC paper, water cooled, speed 240 RPM, rotate in the same direction*, pressure: 27 N (6 lbs) per sample until the sample is flattened.
Note: Removing the damage layer is the basic task of polishing. If the removal is not clean, the direct result is that the observed phenomenon may be an illusion.
2) Rough polishing, 9 μm of metaDI? diamond polishing paste was pre-applied to a perforated TEXMET® polishing cloth, and distilled water was used as a cooling lubricant for 10 to 15 minutes. Rough polishing process: 9μmm etaDI diamond polishing solution + metaDI polishing lubricant, using ULTRA-PAD? polished surface, speed 120RPM, reverse rotation**, pressure: 27N (6lbs) / each sample, time 10min.
3) Final polishing, using MICROCLOTH? or MASTERTEX? polishing cloth, adding MASTERMET? silica suspension polishing solution, polishing for 10 to 15 minutes. Final polishing process: On a MICROCLOTH polished surface, MASTERMET silica polishing solution was used at 120 RPM, reverse rotation, pressure: 27 N (6 lbs) per sample, time 10 min.
Note: During the polishing process, the sample needs to be rotated in a regular direction. The hard phase exists in most metal alloys. If the direction is not rotated, the polished sample will have a black “long tail” on the hard phase, which will affect the quality of the metallographic phase. The key to the "long tail" problem is the direction in which the sample is rotated. You can turn 90 degrees or 180 degrees each time.
5, Müller titanium alloy three-step polishing technology
1) P500 sandpaper water was cooled at a speed of 300 rpm, and the pressure on each sample was 16.7 N (3.75 lb). All samples were smoothed until the preparation time.
2) P1200 sandpaper water cooling, speed 300RPM, pressure on each sample 16.7N (3.75lb), preparation time 30S.
Note: The specific time is based on the individual polishing conditions. The time parameter is only a reference. Usually the polishing will be hand polished. Without such advanced equipment, the parameters will be different.
3) Use synthetic synthetic fluff polishing cloth + silica suspension polishing liquid containing chemical etchant, polishing machine speed 150RPM, polishing time: pressure on each sample is 33N (7.5lb) for 10 minutes, pressure on each sample is 16.7N ( 3.75 lb) for 2 minutes, each sample was pressured 8 N (2 lb) for 1 minute.
4) Polishing agent composition: 260 ml SiO 2 + 40 ml H 2 O 2 (concentration 30%), 1 ml HNO 3 + 0.5 mL HF. FEPA standard P500 and P1200 sandpaper sizes correspond to ANSI/CAMI standards 320/360 and 600 grit sandpaper, respectively.
6, the difference between manual polishing and machine polishing
1) Manual polishing requires experience. The strength, time and speed of the hand need to be experienced for a long time to master these key parameters, and the machine polishing only needs to set the parameters. For novices, they are often upset and have problems with insufficient polishing or throwing over.
2) The principle of machine polishing and manual polishing is the same. Most of the metallographic work in China is still based on manual polishing. Manual polishing requires metallurgical staff to have greater patience. It is necessary to repeat the throwing when throwing over the head, but as long as you have patience, try and sum up, you can also get good metallographic polishing. The effect, therefore, needs to carefully sum up the experience, and the sweat will eventually gain.
