In the Artificer section that talks about Artificer infusions it says "you gain the ability to imbue mundane items with certain magical infusions". So if I was to use an Artificer infusion on a normal weapon (Greatsword in my case) and then can I use the pact blade ability on that weapon, would it keep the infusion because the weapon does turn magical and the infusion says it can be used only on nonmagical items
A mundane weapon can be Infused and then bonded into a pact weapon
The Artificer's Infuse Item feature states:
At 2nd level, you gain the ability to imbue mundane items with certain magical infusions. The magic items you create with this feature are effectively prototypes of permanent items. [...]
Notably, you can only imbue mundane items and they become magic items. Meanwhile the Pact of the Blade feature states:
[...] You can transform one magic weapon into your pact weapon by performing a special ritual while you hold the weapon. [...]
Note, this requires a magic weapon, where Artificer Infusions require a mundane weapon.
Thus, you must first have a mundane weapon, turn it into a magical weapon by applying an Infusion and then you can make it into your pact weapon. If you have a magical weapon that is already your pact weapon, you have no way of Infusing it because it is already magical.
A non-bonded pact weapon is not magical, so it can be infused
Note that if you are just summoning a non-specific pact weapon using the Pact of the Blade feature, this are what the rules states (emphasis mine):
You can use your action to create a pact weapon in your empty hand. You can choose the form that this melee weapon takes each time you create it (see the Weapons section for weapon options). You are proficient with it while you wield it. This weapon counts as magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage. [...]
This weapon counts as magical only for the purposes of overcoming resistance and immunity to nonmagical attacks and damage, it does not count as magical for any other purpose. Thus, if you summoned, say a Longsword, you could then Infuse that Longsword because it is not magical.
Note that any Infusion used in this way would be lost once you shunt the pact weapon away, unlike what happens when you make a magical weapon into your pact weapon.