Page 9 - machining_titanium_05_2019
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                 There a widespread belief that titanium is like austenitic stainless steel in
                 terms of its machinability. This may be true when relating to commercially pure
                 titanium and also, with some assumption, α- or even α-β-alloys; however, it is
                 fundamentally wrong with respect to the treated β- and near-β- alloys.

                 In general, titanium alloys (which we will refer to as titanium and specify their composition, grade    MACHINABILITY OF TITANIUM
                 and properties separately where necessary) are hard-to-machine materials and their machinability
                 depends on various factors: chemical composition, hardness, method of treatment.
                 The main difficulties in cutting titanium are the following:
                • Intensive heat generation leads to excessive adhesive wear of cutting edge.
                • Low heat conductivity results in poor heat transfer and slowing heat dissipation
                 down. Therefore, cutting edge experiences considerable thermal loading.
                • “Springiness” of titanium due to low modulus of elasticity contributes to
                 vibrations and worsens machining accuracy and surface finish.
                 The mentioned factors significantly reduce tool life and affect performance.
                 The averaged data in Table 2 allows estimating machinability of titanium
                 compared with other groups of basic engineering materials.
                 Table 2 - Machinability Of Titanium Vs. Typical
                 Engineering Materials (Averaged Data)
                  Material                                    ISO group            Machinability, %
                  Non-alloy free cutting steel                                  100
                  Low alloy steel, annealed                      P                      60
                  High alloy steel, annealed                                            50
                  Austenitic stainless steel, annealed          M                       40
                  Commercially pure titanium                     S                      43
                  Titanium Ti-6Al-4V, annealed                                          25

                  At the same time, as already mentioned, titanium machinability varies depending on the titanium
                  groups and the grades within the groups.
                  Table 3 provides a comparison of machinability for different titanium representatives;
                  and Fig. 2 shows an appropriate graphic expression.

                   Titanium is a difficult-to-cut-material and its machining is challenging.

                  Table 3 - Machinability Rating Of Titanium Grades (Averaged Data)
                  Group             Designation         Condition*      Hardness     Machinability %
                  Pure titanium     Grade 2                             HB 150-200   170
                  α- and near-      Ti-5Al-2.5Sn        A               HRC 31-34    115
                  α-titanium        Ti-8Al-1Mo-1V       DA              HRC 34-36    110
                                    Ti-6Al-4V           A               HRC 32-36    100
                  α-β-titanium      Ti-8V-5Fe-1Al       A               HRC 34-38    97

                                    Ti-6Al-4V           STA             HRC 39-41    90

                  β- and near-      Ti-10V-2Fe-3Al      STA             HRC 35-42    56
                  β-titanium        Ti-13V-11Cr-3Al     A               HRC 39-41    53
                                     Ti-5Al-5Mo-5V-3Cr  STA             HRC 36-44    51

                 * A - annealed, DA - duplex annealed, STA - solution treated and aged

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