Advantages of bicycle aluminum alloy frames

Aluminum alloy frames can be made lightweight. Aluminum is light but not hard enough. To enhance its strength, it is made into an alloy and subjected to heat treatment. The heat treatment uses the aging precipitation strengthening method. Simply put, a substance that hinders metal deformation is formed within the metal. When heat treatment is carried out at a certain high temperature, aging precipitation occurs. If the frame has not undergone this process, it will also undergo aging at room temperature. That is, the frame will gradually become stronger when placed in a room. Many aluminum alloy frames are made of 6061T6 material. The T6 mark indicates that it has undergone heat treatment and aging. Without heat treatment, the strength can only reach 1/2 or 1/5 of the original. Bicycle parts marked with 7075 (such as XTR cranks, etc.) have not undergone strict heat treatment. That is to say, without aging, it undergoes aging at room temperature. 7075 alloy needs to be heat-treated to increase its strength by five times. On the other hand, 7005 alloy is also often used to make frames. Its strength is not as good as 7075, but it can undergo sufficient aging at room temperature. This material can also be made into thin sheets through Padded processing. However, the material itself has low strength and Young's modulus, so the pipe diameter is thickened to increase rigidity. Usually, the so-called thick-walled aluminum frames are of this type. The appearance does not change much over a long period of use. Aluminum itself is a metal that is easily corroded. There is almost no aluminum that has not been oxidized in the air. It is oxidized immediately when placed in the air and forms a very thin oxide film. Why doesn't it rust? The reason is that when the oxide film reaches a certain thickness, it prevents further rusting. The oxide film is almost colorless, so it is not easy to notice the change in appearance (sometimes it turns white). On the other hand, when riding a bicycle made of this material, the more times you ride, the more times stress occurs, and the strength changes significantly. Recently, to achieve lightness, many frames are made of thin materials (to the limit of thinness). These are all made of aluminum alloys without fatigue limits. How will the strength change after long-term use? The SC61-10A made by Dedacciai and others are pipes with shot peening hardening treatment (KET treatment) on the surface. The purpose of this treatment is to extend the fatigue life. According to public data, it can be increased by 140%. KET treatment is: Fatigue failure is caused by cracks on the metal surface, so hardening processing technology is used to increase the hardness of the metal surface. 

Disadvantages of aluminum alloy frames: Aluminum is a material with low elastic modulus and rigidity. Therefore, thick pipes or shape alterations such as cross-over tubes and padded tubes are necessary. Heat treatment is required; otherwise, the strength will be insufficient. Thus, small-scale factories generally lack the ability to purchase heat treatment equipment. Especially for 6000 series aluminum alloy tubes, in most cases, the pipe manufacturers specify the heat treatment conditions. 

Titanium, titanium alloys (Ti, Ti-Al-V) and their advantages: They can be used to make lightweight yet strong frames. Pure titanium has even higher strength. Adding aluminum and vanadium to titanium increases its elasticity (compared to aluminum) and is beneficial for design. It does not rust. Titanium is hardly corroded in normal environments. However, it has another type of corrosion known as dissimilar metal corrosion. For example, when different types of alloys are joined together, they form an electrode state, and the potential difference causes local electrification and corrosion. To prevent this, insulating materials such as grease (grease mixed with metal powder, such as the product name: Ti Plepu, etc.) are used. The metal powder in this grease prevents it from flowing away and also prevents tight contact even if it does. It doesn't matter if this metal powder is conductive; it will be destroyed by a small load upon contact. This phenomenon is called "sticking" and is different from "brazing". "Sticking, biting": The phenomenon where metals join through corrosion. "Brazing": The joining of metals through the heat generated during processing to melt them together. Good ride feel: Titanium is also used in shock absorber springs. Nowadays, titanium frames are suitable for long rides. Some cyclists are fond of titanium frames, believing they have an excellent ride feel. However, opinions vary. Some people, after comparing with chromoly steel frames, think that chromoly steel frames have better shock absorption and ride feel than titanium frames (this is also related to the shape of the chromoly steel frame and the use of thin tubes, which increases stress). If the processing technology of titanium tubes is further improved, perhaps it will have the same ride feel as chromoly steel. Some titanium frames make people feel wobbly, which may be related to the use of plain tubes (tubes without padding, with uniform thickness). Or it may be related to the use of high-strength titanium alloys without thin tubes. 

The disadvantages of titanium alloy frames: high price. Titanium exists in nature in the form of titanium dioxide. The extraction and processing are complex, with high technical requirements and time-consuming, thus the cost is high. Moreover, welding is extremely difficult because titanium has a strong affinity for oxygen and immediately turns into titanium dioxide upon contact with air. Titanium dioxide is hard and brittle, and the strength of this part will continuously decrease. Therefore, inert gas must be used for careful welding. The commonly mentioned Tig welding is: (Tig: abbreviation for tungsten and inert gas) arc welding with tungsten electrodes and argon gas. Titanium welding must be carried out in an air-free environment. Due to the above reasons, titanium frames are very expensive. Recently, some aluminum alloy frames are more expensive than titanium alloy frames, indicating that the price of titanium alloy frames has dropped. The reasons are: after the end of the Cold War, military spending has decreased, and the demand for titanium has reduced; excellent technicians have started to use titanium to make frames; the cost of making titanium frames has decreased, etc. 

In the 1960s, the United States first used titanium alloy to make bicycle frames. Titanium alloy gives the impression of being "light and hard". Early titanium alloy frames, as racing bikes, were insufficiently rigid and were not suitable for long-distance races or all competitions. They were only suitable for specific occasions such as mountain biking, Big Reces, or Time Trials that required lightweight bikes. At first, titanium alloy fascinated some people. Some riders who were passionate about lightweight bikes were not satisfied until they replaced all the screws with titanium alloy ones. Titanium metal gives some people the wrong impression that it is expensive and rare. In fact, titanium is one of the most abundant elements on Earth. It is only because the production cost of alloys is high that the price is relatively high. Titanium is a material that is light in weight, strong, and highly corrosion-resistant. It is generally used in special environments. When aluminum and vanadium (Al, V) are added to metallic titanium, the strength is significantly increased. Heat treatment is also similar to that of aluminum alloys, and the strength can be increased through aging treatment. Mechanical properties of titanium alloys Alloy name Distinction Tensile strength (MPa) Endurance (MPa) Elongation (%) E (GPa) Pure titanium JIS 3 types 480~620 345 18 105 Grade 4 550 480 15 PTT800 800 UTT75 750 3-2.5Ti Annealed 685 590 20 6-4Ti Annealed 980 920 14 110 Aging treatment 1160 1100 10