Soldering

Soldering refers to the random sticking of the molten metal alloy to the casting/tooling material. The molten metal of extremely high temperature forms a metallic compound upon coming into the tool’s contact. Thus, soldering occurs as some of the alloys become a part of the tooling.

It’s a die-casting metal interaction defect known as die sticking and die soldering. Aluminum (Al) alloys are the most susceptible to this defect due to their naturally high affinity to steel. This problem worsens when high-temperature tooling meets with high-temperature molten alloys.

This article briefly looked into the types, causes, effects, and countermeasures of die-casting solder to help address this defect.

Types of Soldering in Die Casting

Soldiers occurring in die casting are primarily of two types based on the molten metal –

1. Aluminum Soldering

Aluminum die-casting alloys create complex inter-metallic compounds with steel at high temperatures. The problem worsens fast if the solder that occurred initially isn’t removed or treated. So, modern die-casting processes practice the removal of aluminum upon a solder buildup event.

The amount of Iron (Fe) in the molten alloy also accelerates solder buildups in die casting. Low Fe content means more affinity to steel, whereas more Fe content relieves the affinity. The condition of the tooling steel is also important in initiating or avoiding solders in aluminum alloys.

2. Zinc Soldering

The mechanical process of zinc soldering differs from that of aluminum soldering. It involves no inter-metallic chemical bonding or automated galvanization with the casting steel. Instead, this defect refers to the eventual deposition of zinc alloys on the die steel surfaces.

Technically, it doesn’t fulfill the generalized definition of soldering or sticking. However, leading die-cast industries still treat it as a solder buildup. Not surprisingly, the deposit may have a merely visible thin layer of aluminum coating to create the metallic bond with steel.

Causes of Soldering

Primary factors that contribute to soldering defect in die casting includes –

• Changes in Draft Angles –A slight draft angle distortion always occurs as no print can predict it accurately. Lowering the maximum limit often sparks affinity between the two metals.
• High Metallic Pressure –More pressure contributes to more sticking and solder buildups. Going beyond the maximum limit will trigger other defects associated with the abnormal pressure.
• Cast Steel Temperatures –Die temperatures are vital in influencing solder buildups. Increasing the steel temps causes more rapid Al-Fe bonding to accelerate the problem across the cast.
• Rough Surface Finishing –Uneven surfaces with undercuts help the solder grow quickly. Allowing the solder to continue will accelerate the buildup, making the recovery or remedy difficult.
• High Metal Temperatures –Excessively high temp at the steel-metal interface provoke soldering. Any unnecessary increment at the contact point can stick the alloys directly to the steel surface.
• Low Iron Content in Alloy –There are different specs of Iron contents in aluminum and zinc alloys. But any less than 0.7% can trigger superfast soldering and sticking of the molten alloy to casts.

Effects of Soldering

Some of the most intuitive effects of solder buildups are –

1. Faulty demolding to distort the desired shape and size.
2. Increase in production costs due to more downtimes.
3. Physical and mechanical damage to metal components.
4. Eventual or gradual malfunctioning of die-cast inserts.
5. Increased abrasion, roughness, and wearing over time.
6. Susceptibility to poor structural/mechanical integrity.

Preventive Measures for Soldering

• Gate Readjustment –Redirecting or setting the gate velocity at lower levels prevents metallic infuse at the entrance. Checking the gate velocity factors (plunger speed, machine pressure, gate size, and plunger size) will help.
• Reduced Filling Time –The molten alloys will heat the tooling steel on contact. But cutting down the fill time allows limited time to reach an extreme level. It diminishes the chances of bonding significantly to reduce solders.
• Limiting Die Temps –The addition of cooling lines along the solder-prone spots can control the flow. Using review spray and steel inserts to transfer the high heat also helps. The spray mostly depends on the die-casting specifications.
• Protective Coating –Introducing an insulating layer made of oxide or PVD (TiC or TiC) hinders direct Al-Fe contact. Nitriding the steel surface in a calculated manner also confines the formation of solders without inducing brittleness.
• Proper Metal Pressure –Setting the minimum metal pressure to get the desired result reduces solders. Intensified and static pressures need precision with calculated readjustment of other dependable parameters.
• Removal of Al-Fe Bond –It features careful dismissal of a thin exterior aluminum layer (preferably a thousandth or some tenth of thousands). This also initiates a slight dimension reduction to alter the shape on several attempts.
• Alkaline or Acid –Inhibited acids or strong alkaline can dismiss the embedded Al-Fe layer. Though it incorporates almost minimal to no change in steel surfaces, extra safety measures are necessary to apply such chemical tricks.
• Applying Spot Cooling –Prevention of instant solder buildup is possible by adopting spot cooling for susceptible spots. It requires a suitable spray to match the high-temp steel characteristics for the best outcome.
• Small Fountain Cooling –Using bubblers or small fountains is a complex yet highly effective spot-cooling method. It reduces soldering to a great extent by requiring the placement of small channels of tubes (1/16”) in certain spots.
• Defining Iron Contents –Any alloy specification comes with a restricted range of Iron constituents in the composition. It’s essential to keep the Fe percentage satisfactorily high (more than 0.7%) to avoid accelerated Al-Fe soldering.
• Polish and Draft –High-quality surface polish with maximum allowable draft angle can effectively reduce zinc and aluminum soldering. Tuftriding and nitriding may deliver some good on zinc alloys with a somewhat uneven result.

Overcoming the soldering problem in die-casting tooling requires proper adjustment of the control parameters. The primary reason behind solder concerns favorably high temps to create metallic bonds. Using the most suitable precautionary measure from the ones above can limit this problem to a null.