Aluminum mold repair: how to choose the right aluminum welding wire and welding technology

Aluminum welding technology

The use of aluminum tools is becoming more and more common. Choosing the correct welding wire and the correct welding technique will effectively repair the mold.

Once strictly regarded as a prototype tool material, aluminum has gradually become an option for the production of molds. When considering this option, it is important to first look at the difference between steel and aluminum and then determine how to use the increased thermal conductivity of aluminum without reducing the quality of the part. It is also important to consider issues related to mold maintenance. It is relatively easy to produce high-quality parts with aluminum molds, but repairing these molds may pose challenges. To ensure successful repair of aluminum molds, it is necessary to understand the differences between steel and aluminum and aluminum alloy options, as well as knowledge about welding wires and proper welding techniques.

Materials: Aluminum and Steel

The high-strength aluminum alloys available today are a viable alternative to tool steel. Aluminum is primarily used to produce prototypes due to its low cost, so aluminum has other advantages, making it a viable option for producing molds.

First, due to various key material characteristics, the cost of aluminum molds is only about half of the cost of steel molds, and can be delivered in about half the time. For example, although steel is much stronger than aluminum, aluminum weighs only a third of aluminum, and certain aluminum materials can be processed to produce strength comparable to steel. Because aluminum is softer than steel, it is easier to cut. The combination of high strength and low weight makes aluminum have a wide range of uses, which can be widely used in aerospace, automobiles and industry.

Another advantage of aluminum over steel is its high corrosion resistance. When aluminum is exposed to air, a thin layer of aluminum oxide is formed, which enhances corrosion resistance. In addition, as the temperature decreases, aluminum does not become brittle like steel. In fact, it can increase the tensile strength and maintain toughness.

As we all know, the thermal conductivity and electrical conductivity of aluminum is six times that of steel. The thermal conductivity of aluminum plays a vital role in welding and mold repair, so that the welding will cure faster. This also helps to achieve more "practical" welding, better fix the metal in place and simplify on-site maintenance. High thermal conductivity means that the heat energy applied to one part of the metal will be quickly transferred to other parts. This keeps the material stable while maintaining a high temperature. However, in order to avoid the deterioration of parts, aluminum needs to be welded with more intense heat at a faster rate, because the heat will be quickly dissipated.

Choice of alloy

If you choose to use aluminum in the next mold project, it is very important to choose the right aluminum alloy. Adding elements including copper, magnesium, and zinc to the aluminum matrix produces alloys. Each added element will help improve the unique beneficial properties of aluminum.

There are two main categories of aluminum alloys: forging and casting. Forged alloys are first cast into steel billets, and then mechanically processed by hot or cold rolling methods, such as rolling, forging, extruding, and forming, to achieve the desired shape. Rolling is used to produce aluminum sheets, foils or plates; forging is used to produce complex shapes with excellent properties, and extrusion is used to produce tubes or rods. The casting alloy is directly cast into the desired shape, making it very suitable for the application of complex shapes.

Forged and cast alloys are further classified as heat treatable and non heat treatable. Heat-treatable alloys contain alloying elements that increase the strength and solubility of the material through heat treatment, but the heat-affected zone (HAZ) is usually not fully annealed, which affects the strength of any weld. Non-heat treatable alloys are strengthened by cold working methods.

These classifications are important considerations regarding the weldability and maintainability of aluminum alloy molds. Some materials are more suitable for specific welding processes, so when the mold manufacturer determines the welding material, the welding method must also be considered.

Some commonly used aluminum alloys are produced from the 7000 series family. These alloys are attractive to molders because they usually have high strength comparable to steel and are more durable than other series. Zinc is the main alloying element in these aluminum alloys. It is used in combination with other elements (such as magnesium and copper) to help it become a strong alloy at ambient temperature. The addition of zinc also makes aluminum heat treatable, thereby hardening the precipitate, a heating technique used to increase the yield strength. Through precipitation hardening, 7000 series alloys can reach a tensile strength of up to 700 MPa (megapascals), the highest of any aluminum alloy. After dispersing the alloying elements by rolling and forging, heat treatment is more effective. The combination of these three processes will establish the properties required for high-strength aluminum. Although the 7000 series alloys have good fatigue strength and machinability, they have poorer corrosion resistance than other aluminum alloys, so they are susceptible to stress corrosion cracking and are difficult to weld.

Other popular choices for production mold applications are alloys of the 2000, 5000 and 6000 series. By adding copper, those in the 2000 series can be precipitation hardened as in the 7000 series, giving them similar strength to steel, but the alloys in the 2000 series have lower corrosion resistance, so compared to those, They are more susceptible to stress corrosion cracking. Because of this fragility in the 7000 series, many 2000 series alloys are considered non-weldable.

Magnesium is added to 5000 series alloys. Magnesium can provide solid solution strengthening and improved strain hardening properties, so that they have high non-heat treatment strength in aluminum alloys. Because of these characteristics, 5000 series alloys are very difficult to extrude and expensive. They are mainly made into sheets and plates and are only occasionally used as molded parts.

Aluminum in the 6000 series is alloyed with magnesium and silicon to make a heat-resistant, strong and easy-to-extrude alloy with good corrosion resistance. Although 6000 series alloys are one of the commonly used general-purpose alloys, they cannot reach the high strength of 2000 and 7000 series alloys.

These other alloy series are usually produced by the same factory that manufactures the 7000 series alloy series billets, but they are not rolled, forged and heat treated to increase strength. Instead, cast these alloys directly into their final form, so that they can obtain their properties, including strength and weldability, from the alloy composition rather than through the manufacturing process. Since the cast alloy does not need to be rolled or forged into a plate or sheet shape, it can be used more economically for complex product shapes. However, they will not have the same strength as rolled or forged similar products. Cast alloys are a popular choice for prototype production because they cost about half the cost of 7000 series alloys. For prototypes, weaker alloys are acceptable. Modified 2000 series alloys in rolled or forged form can also be provided to achieve a good balance between strength and welding characteristics. Both types of alloys are easy to weld according to their respective characteristics.

The ideal quality inherent in each series of alloys will determine their suitable production applications. Because of its high strength, the 7000 series alloys are commonly used in high-performance applications such as aerospace, armored vehicles and sports equipment. The 2000 series alloys of comparable strength are commonly used in aircraft or aerospace applications, while the 5000 series alloys have higher weldability and non-heat treatable strength, making them useful for various structural applications such as bridges, buildings, trucks, etc. Shipbuilding and pressure vessels. 6000 series alloys are simple and economical alloy extrusion methods, making them suitable for both welding and various extrusion shapes.

Aluminum alloy welding wire selection

Different aluminum alloys require different welding wires to achieve successful welding. The characteristics of the welding wire should match the characteristics of the specific alloy to be welded, and the color matching ability, strength and influence on the heat affected zone of the welding wire should also be considered. In addition, the welding wire must have a melting temperature similar to that of its base material in order to perform effective welding. For example, an aluminum alloy with a high magnesium content should be welded with a filler alloy that also contains a high magnesium content. Each type of welding wire has its own dynamic chemical composition, which will produce more effective results for aluminum alloys with similar characteristics. In addition to improving production efficiency, welding distortion should also be reduced.

The maintenance of production molds made with several 7000 series alloys is challenging because their sensitivity to thermal cracking or stress corrosion cracking prevents them from welding using arc welding technology. The exceptions to this rule are 7003 and 7005 extruded alloys and 7039 plate alloys. Qualified 7000 series alloys can be welded using 5356 or 2319 alloy welding wire, both of which can produce non-porous welds of acceptable strength, which can be matched to the integrity of the aluminum alloy. 5356 is commonly used in these welding wires because it is rigid and can provide acceptable strength and continuous feed, while 2319 is heat treatable and has high strength and good ductility. 5356 welding wire also has a filler with 5% magnesium content, which can reduce the sensitivity to welding cracks. The higher the magnesium content, the lower the risk of cracking.

Two factories producing 7000 series aluminum alloys have helped develop proprietary alloy welding wires designed for 7000 series aluminum alloys. These products have not been widely used, but all early test results show that the color matching is improved compared to 2319 and 5356 welding wires. However, for these new wires, the heat-affected zone around the weld repair area is common to all heat-treated alloys in all repair processes.

2319 welding wire is suitable for welding molds made of 2000 series alloys. The forged 2000 series alloys can be heat treated to achieve a higher tensile strength, which can be as high as 448 MPa. They form alloys with copper and help produce better welding characteristics. Tests have shown that 2319 welding wire has excellent welding quality and color matching. When color matching is very important, it becomes a popular choice. 2000 series alloys have higher aging and tempering properties, so they have higher thermal fatigue resistance than 7000 series alloys, which means that the heat generated by welding will not cause the strength of the welded material to decrease.

6000 series alloys are heat treatable wrought alloys that show a heat affected zone around the welding area. The suitable welding wire for welding 6000 series aluminum alloy is 4043, which is one of the easy-to-handle welding wires. It has a lower melting point and better fluidity, so it is less sensitive to welding cracks. This welding wire is suitable for critical applications where the strength and color of the welding area are matched. However, if the parent material will be anodized after repair, 5356 welding wire is a better choice and will produce a closer color match, because 4043 will become dark gray after anodizing.

Because cast alloys provide a stronger grain structure, and forged alloys have higher porosity, they are more likely to exhibit stronger weld repairability. The ideal choice for 5000 series aluminum alloy is 5356 welding wire, while 2319 welding wire is suitable for 2000 series alloy. These welding wires produce excellent color matching with the alloy because of their low silicon content, which maintains the anodized silver mixing effect instead of the obvious and unattractive black. As a non-heat-treated product, the cast alloy has no heat-affected zone around the weld, so the weld area is not visible on the finished surface of the mold.

Aluminum mold repair skills

Once the appropriate welding wire is selected, the focus must be shifted to the appropriate technology of the mold to be welded. This is an important factor for successful weld repair. Suitable technology involves not only actual welding, but also more technologies. It must also consider the environmental conditions and preheating of the material, which will eliminate condensation and improve its weldability.

Generally, alternating current (AC) is used to weld aluminum, but direct current (DC) can produce better results when repairing mold alloys. AC and DC refer to the polarity of current when it flows through the electrodes. Choosing the electrode with the proper polarity will have a significant impact on the strength and quality of the weld.

As mentioned earlier, when aluminum is exposed to air, a thin layer of aluminum oxide is formed. If the oxide layer is not removed, it will adversely affect proper welding fusion and fluidity. The temperature required to remove this layer is much higher than the temperature required to melt the base aluminum. Alternating current flows half the time in one direction and half in the other direction. Since alternating current often changes polarity (the polarity is changed 120 times per second at a current of 60 Hz), the oxide layer is first removed, allowing the base metal to melt and melt more quickly. In order to obtain good results, an equal and balanced current is required in both directions.

The direct current only flows in one direction, forming a constant polarity. DC pulsed welds will produce arc pulses between high peak and low background currents, which will narrow the heat affected zone and the heat applied to the parent material, thus retaining the original performance of the metal. The use of AC pulse welds results in greater porosity in the welded area and poor color matching. When a welding current that is too low is used, when the gas is entrained by the welding turbulence, porosity may occur. On aluminum, welding should be performed quickly to prevent heat from penetrating into the mother block. If the execution is too slow, the risk of overburning will increase.

As the use of aluminum tools increases, the cost of parts will decrease, which in turn will make the use of aluminum tools more common. Choosing the right welding wire and technology will make potentially difficult processes easier and provide excellent welding results.